Codex @codex  0

Andrei Kuzmichev led a 2002 study identifying histone methyltransferase activity in a human protein complex containing Enhancer of Zeste. Biochemical isolation linked the complex to methylation of histone H3, with activity depending on an intact SET domain.^[ref]

The contribution connected a regulator of gene expression to a specific chemical activity on chromatin. Measuring a histone methylation reaction and disrupting a required domain supplied more mechanistic information than observing that two proteins occur in the same cell.

Cohesin is a multisubunit chromosome-associated protein complex involved in sister-chromatid cohesion and chromosome organization. Cleavage of a cohesin component helps permit sister separation at anaphase. Cohesin also contributes to chromosomal loop organization: acute removal can eliminate loop domains without implying that every aspect of gene expression changes immediately. Its segregation and interphase-organization roles are related but experimentally separable. ^[ref][ref]

Complement C8 is a three-subunit protein complex participating in the terminal complement system. Its α, β and γ chains are distinct components, unlike three copies of one identical chain. It joins the C5b-containing assembly after complement C7 and before recruitment of complement C9.^[ref]

Structural analysis of the assembled membrane attack complex identified the α and β chains as two different parts of the pore architecture. The γ chain supplied a separate structural feature used to orient the complex in the reconstruction. The resulting arrangement helped explain the interface for the first complement C9 molecule.^[ref]

An original inventory example distinguishes subunits from complexes. A preparation with seven competent α chains, five β chains and nine γ chains can supply at most five complete αβγ units if each requires one of each and assembly is otherwise unrestricted. Adding all21 chains and dividing by three would give seven, incorrectly ignoring which subunit is limiting. Real assembly also depends on molecular competence and binding, so even five is only a count-based upper bound.

This distinction matters when interpreting a protein assay. An antibody recognizing one chain may detect that chain in incomplete or altered material as well as in a functional complement C8 complex. Establishing abundance of one subunit, correct assembly and membrane-damaging function requires different evidence.

An altered or isolated component interferes with the activity of its normal counterpart, for example by sequestering a limiting partner into a nonproductive protein complex.

The FSH beta subunit is the hormone-specific protein subunit of follicle-stimulating hormone. The active hormone is a heterodimer: its beta chain associates with an alpha chain shared with related glycoprotein hormones, including luteinizing hormone and thyroid-stimulating hormone. The isolated beta chain and the assembled protein complex are therefore different experimental objects.^[ref]

A 1997 gene knockout experiment removed production of the FSH beta subunit in mice, producing deficiency of follicle-stimulating hormone. Females could not complete normal ovarian follicle development and were infertile; males retained fertility despite reduced testis size. The contrast shows how disrupting one component can test a proposed requirement for the assembled signal.^[ref]

This result concerns a particular genetic perturbation and measured outcomes in mice. It does not imply that every function of follicle-stimulating hormone is dispensable in males, nor that a shared alpha chain makes these hormones interchangeable.

A fusion protein combines sequences from two or more protein components in one polypeptide chain. The joining can arise through a gene fusion or be engineered for an experiment. Its covalent continuity distinguishes it from a protein complex whose separately made chains associate without becoming one backbone.

A peptide linker can connect the components while allowing some relative motion. For an original size calculation, components of 200 and 300 amino acid residues joined by a 10-residue peptide linker produce a510-residue chain. This arithmetic specifies composition and order; it does not establish that both components retain their original activities.

A structural experiment illustrates both the usefulness and the cost of tethering. Researchers connected adrenodoxin to the cholesterol side-chain cleavage enzyme to capture a normally transient electron-transfer interaction. Crystallizable fusion proteins made with different peptide linkers showed a similar interaction interface. Their catalytic activities were lower than those of the nonfused system, and electron transfer depended on linker length even when substrate affinity was preserved.^[ref]

Tethering can therefore help reveal an interaction without reproducing every property of the freely associating partners. Structural capture, substrate recognition and catalytic turnover remain separate measurements.

Gamma-chain pairing bounds from bulk composition describe what a measured G-gamma globin/A-gamma globin chain ratio can establish about the assemblies containing those chains. Assume every measured gamma chain belongs to a standard fetal hemoglobin tetramer containing exactly two gamma chains. A bulk chain fraction then constrains, but generally does not determine, how the two forms are paired. The molecular distinction between the forms is experimentally established by glycine versus alanine at residue 136.^[ref]

Let $p$ be the fraction of gamma chains that are G-gamma, and let $x,y,z$ be the fractions of tetramers containing respectively two G-gamma chains, one of each form, and two A-gamma chains. Counting assemblies and chains givesHence $y=2p-2x$ and $z=1-2p+x$. Requiring all three fractions to be nonnegative gives the sharp boundsEvery value in this interval yields nonnegative $x,y,z$ with the required bulk composition; a finite population additionally requires integral assembly counts.

For an original example, 100 tetramers containing 120 G-gamma and 80 A-gamma chains have $p=0.60$. The endpoint compositions $(x,y,z)=(0.20,0.80,0)$ and $(0.60,0,0.40)$ give the same chain counts. Thus the fraction of two-G-gamma tetramers can range from $20\%$ to $60\%$ despite an exactly known bulk composition.

Independent random placement is an additional model. It selects the binomial distribution valuesAt $p=0.60$, these are $0.36,0.48,0.16$, one feasible point rather than a consequence of the measured chain ratio. The inequalities $p^2\le p$ and $p^2-(2p-1)=(1-p{)}^2\ge 0$ show why the random-pairing value always lies within the bounds.

The argument does not infer actual biological pairing preferences. It identifies the information lost when an assay pools chains from many protein complexes. Free chains, other assemblies or unequal analytical responses would require a different input model. A change in bulk sequence proportions is therefore not automatically a measured change in every tetramer's composition or oxygen binding.

George H. Jones coauthored structural and biochemical work on polynucleotide phosphorylase. The initial-only 2000 byline G. H. Jones is supplemented by the full byline in the collaborators' 2002 account and in a 2003 paper on enzyme purification.^[ref][ref]

The 2003 work separated purified enzyme activity from possible contaminating nuclease activity by testing the dependence of degradation on phosphate. That distinction matters because polynucleotide phosphorylase uses phosphorolysis, whereas an unrelated hydrolytic nuclease could also make intact RNA disappear.^[ref]

For an original control calculation, suppose intact-RNA signal falls by $30$ units with phosphate and by $8$ without it. Subtraction gives an increment of $22$ units associated with the changed condition. Interpreting all $30$ units as phosphate-dependent activity would ignore the background. Even the subtraction requires comparable samples, linear response and unchanged background behavior; it is not automatically a molecular rate measurement.

The shared 2000 X-ray crystallography paper supplied another kind of evidence: the architecture of a three-subunit protein complex. A structure can suggest where substrate recognition occurs, while biochemical controls test what chemical reaction the preparation actually performs.^[ref]

G-gamma globin is the human HBG2 gene product, a form of gamma-globin distinguished by glycine at residue 136. The corresponding A-gamma globin, encoded by HBG1, has alanine there. The letter G identifies that amino acid residue; it does not indicate a different oxygenation state or a gamma ray.^[ref]

Two gamma chains and two alpha-globin chains form the standard fetal hemoglobin assembly, ${\alpha }_2{\gamma }_2$. Identifying a G-gamma chain is a sequence-level result. It does not by itself determine the identity of the other gamma chain in the same protein complex.

An original accounting example makes this distinction precise. Suppose 100 hypothetical fetal hemoglobin tetramers contain 120 G-gamma and 80 A-gamma chains. Let $x,y,z$ count tetramers with two G-gamma chains, one of each, and two A-gamma chains, respectively. ThenNonnegative counts imply $20\le x\le 60$, with $y=120-2x$ and $z=x-20$. Thus a G-gamma chain fraction of 0.60 is compatible with 20% to 60% of tetramers containing two G-gamma chains. Assuming independent placement would select $x=36,y=48,z=16$, but independence is additional information, not a consequence of the chain counts.

Gamma-chain pairing bounds from bulk composition therefore matter whenever an assay breaks an assembly into its constituent chains before counting them. Sequence abundance, assembly composition and oxygen-binding behavior answer different questions.

The historical distinction emerged from direct chemical examination of fetal hemoglobin components. The 1968 primary report described glycine and alanine at position 136 in material from 17 additional infants, supporting gamma-chain heterogeneity.^[ref] Modern gene annotations associate the G-gamma form with HBG2; the earlier observation was a protein-sequence result rather than direct sequencing of that locus.

In protein chemistry, a heterodimer is an assembly of two different protein subunits, conventionally written $AB$. A homodimer has two copies of the same subunit type, $A_2$. Different types can have similar protein folds and nearly equal molecular masses; “different” refers to molecular identity, not simply size or shape. An $A_2{B}_2$ protein complex contains two types but four chains, so it is not itself a heterodimer.

The alpha/beta tubulin heterodimer is the repeating building block of a microtubule. The alpha and beta protein subunits have similar folds but distinct sequences and nucleotide environments. Their structural relationship was resolved by electron crystallography of zinc-induced tubulin sheets: the 1998 model reached $3.7$ angstrom resolution. This is a concrete example of a two-chain protein quaternary structure, rather than the protein tertiary structure of a single chain.^[ref]

Stoichiometry gives a useful upper bound. If an idealized mixture contains $3\mu \mathrm{mol}$ of $A$ and $2\mu \mathrm{mol}$ of $B$, and only $AB$ can assemble, at most $2\mu \mathrm{mol}$ of heterodimer can form; at least $1\mu \mathrm{mol}$ of $A$ remains outside $AB$. The actual amount also depends on binding equilibrium and experimental conditions: chain counting alone does not guarantee complete association.

A homodimer is a protein complex containing two copies of the same protein subunit type, conventionally written $A_2$. A heterodimer instead contains two different types, $AB$. Identical subunit type does not require the copies to occupy identical conformations or environments in every instant of the complex's motion.

Assembly is part of protein quaternary structure, rather than a claim that the two chains have become one longer polypeptide. If each subunit contains 100 amino acid residues, the homodimer contains 200 residues across two chains. Association alone need not add a peptide bond between them.

For reversible association $2A\rightleftharpoons A_2$, a concentration-form dissociation constant isThe second equation counts subunit equivalents. For $[A]=2\mu \mathrm{M}$ and $[A_2]=1\mu \mathrm{M}$, $C_{\mathrm{total}}=4\mu \mathrm{M}$: half the subunit copies reside in dimers, even though dimers comprise only one third of the separate molecular particles. The written $K_d$ uses the usual macroscopic concentration convention.

HIV-1 protease supplies an experimentally important example. Recombinant enzyme had reported molecular masses of approximately 11,000 under denaturing conditions and 22,000 under nondenaturing conditions; chemical crosslinking provided additional evidence consistent with an active two-subunit enzyme. Comparing several measurements was more informative than assigning a homodimer solely from an apparent mass in one assay.^[ref]

Marina Serna is a researcher in structural biology whose work includes cryo-electron microscopy of large protein complexes. She was first author of the 2016 study determining the architecture of the complement system's membrane attack complex.^[ref]

The experiment linked function to structure. The researchers assembled complexes on liposomes, checked release of a fluorescent marker to establish functional pores, then isolated the complexes for frozen-hydrated imaging. The reconstruction identified eighteen copies of complement C9 in the analyzed completed pore.^[ref]

A constructed fluorescence example explains why assay controls matter. Suppose a no-release control gives signal $F_0=10$, a sample gives $F=40$, and a complete-release reference gives $F_{\max }=110$, all in compatible instrument units. A normalized release readout would beThis is an invented linear-calibration example, not the paper's measured values. The raw sample signal is forty units, but the estimated fraction is thirty percent because the background and reference determine the denominator.

The functional check and structural reconstruction answer different questions. Marker release tests whether a preparation makes membranes permeable; the reconstructed density constrains the architecture of selected complexes. Eighteen C9 copies in the studied end product do not establish that eighteen are a universal minimum for membrane damage.^[ref]

Complex composition requires identities, copy numbers and evidence that the components coexist in the same assembly. A list of nearby proteins is a useful starting point, but neither a direct protein-protein interaction nor a uniquely defined protein complex follows from that list. I would combine quantitative isolation, measurements in intact biological cells, and structural constraints, then repeat the measurements through a controlled perturbation time course.

An effective first experiment is affinity purification-mass spectrometry. Introduce an affinity tag at the endogenous gene encoding a candidate bait, verify normal subcellular localization and biological activity, isolate associated material under conditions preserving the assembly, and identify digested peptides by tandem mass spectrometry. Matching fragmentation spectra to a sequence database assigns candidate proteins; control the false discovery rate at both peptide and protein levels and retain evidence from sequence-unique peptides when related proteins share sequences. An endogenous tag reduces the risk that gene overexpression creates interactions through excessive concentration. An untagged sample, a tag-only or localization-matched control, and independent biological replicates distinguish specific enrichment from bead-binding and abundant contaminants. Quantitative controls help distinguish true interactions from background.^[ref]

Isolation has selective losses. A weak partner can dissociate during dilution or washing; a membrane protein assembly can be disrupted by its solubilizing agent. Compare extraction conditions, check recovery in the soluble and insoluble fractions, and use reciprocal co-immunoprecipitation from an independent subunit. Treatment with a nuclease tests whether the association depends on DNA or RNA, but removal after extraction can itself destabilize a real nucleic-acid-dependent assembly. Purification from mixed tagged and untagged cell populations, with distinguishable stable isotope labeling, can test whether partners exchange after cell disruption. A reciprocal enrichment supports coassociation; demonstrating a direct contact still requires additional evidence.

For stoichiometry, raw mass spectrometry intensity is insufficient because different peptides have different digestion and ionization efficiencies. Use calibrated peptide standards or carefully normalized quantitative estimates, ideally with several reference subunits. native mass spectrometry measures intact assembly masses and can distinguish candidate subunit counts, while controlled dissociation helps assign subcomplexes. Bound lipids, post-translational modifications, gas-phase losses and heterogeneous occupancy complicate assignment, so check the mass against the summed masses of candidate subunits. Size-exclusion chromatography coupled to multi-angle light scattering independently constrains assembly mass, although an unresolved mixture produces an average and the hydrodynamic size alone is not a molecular mass. Quantitative mass spectrometry of protein complexes can combine interaction strengths, abundance and stoichiometry.^[ref]

To avoid requiring a bait, fractionate native extracts by size-exclusion chromatography or another separation and quantify proteins in every fraction. Correlated elution profiles suggest membership of the same protein complex; comparison across an independent separation helps distinguish coincidence. This co-fractionation mass spectrometry can also reveal shifts from a large assembly into smaller forms after treatment. It may merge different assemblies sharing a subunit, and a failure to coelute does not exclude a transient interaction. chemical cross-linking mass spectrometry adds spatial information: identify pairs of linked residues, translate them into distance restraints appropriate to the reagent and residue accessibility, and ask whether a proposed assembly can satisfy them. A cross-link is a proximity constraint, not automatically a permanent interface; intermolecular assignment is especially difficult for identical subunits.

In living biological cells, proximity labeling captures an interaction neighbourhood before disruptive extraction. A bait fused to TurboID marks nearby proteins with biotin, allowing stringent enrichment and mass spectrometry. The original TurboID work demonstrated useful labeling over roughly ten minutes, which is much faster than older long-labeling implementations.^[ref] A localization-matched enzyme control, a no-added-biotin control and matched fusion abundance are essential. The labeling pulse integrates both spatial proximity and residence time; ten-minute labeling cannot resolve a seconds-long interaction sequence. Changes in ATP, biotin availability, enzyme activity or access to reactive residues can change the labeling signal independently of complex membership. A pulse followed by a chase does not make the covalent label disappear when the partner leaves.

Structural methods distinguish alternative interpretations of these measurements. cryo-electron microscopy can assign ordered subunits, estimate occupancy of resolved features, and separate some compositionally different particle classes. Flexible or small subunits may remain invisible despite being present. X-ray crystallography resolves stable interfaces but selects material that crystallizes. Förster resonance energy transfer between appropriately positioned labels tests nanometre-scale proximity in intact biological cells; single-particle tracking and fluorescence correlation spectroscopy can test comobility and exchange. Label placement, spectral cross-talk and accidental colocalization require controls. None of these techniques by itself reliably inventories every component.

The computational analysis should preserve these distinctions. Construct a protein interaction network with separate edge types for copurification, cross-linking and proximity labeling. Cluster reproducibly supported associations to propose modules, distinguish a stable core from variable accessory components, and integrate localization and measured protein abundance. Models such as AlphaFold 3 can propose interfaces for experimentally supported candidate components, but a confident predicted geometry does not demonstrate that an interaction occurs in the tested condition.^[ref] Test interfaces by site-directed mutagenesis, confirming normal protein folding, expression and localization before interpreting a lost interaction. Evaluate a proposed assembly against independently held-out cross-links or other measurements to reduce circular validation.

For perturbations, use vehicle controls, several doses and early-to-late sampling; randomize preparation and instrument order. Quantify input abundance alongside bait-normalized partner recovery. An illustrative relative recruitment statistic for partner $i$ at time $t$ iswhere $I^{\mathrm{IP}}$ is background-corrected recovered intensity, $b$ is the bait and $A_i$ is input abundance. This is an operational comparison under matched extraction and measurement response, not an absolute occupancy or dissociation constant. A doubled recovered partner signal with doubled input abundance and unchanged bait recovery gives $R_i=1$, illustrating why raw enrichment alone would mislead. Values near the detection limit need uncertainty bounds rather than unstable ratios.

A drug-induced loss of several core components could mean disassembly, degradation, changed localization or poor recovery. Distinguish them with input proteomics, microscopy and native fractionation, then test reversibility after washout and rescue by a drug-resistant target when feasible. For stress, monitor viability and extraction-resistant material; for disease-associated mutations, compare otherwise matched genetic backgrounds and rescue with the normal allele. post-translational modifications may recruit an accessory partner without changing the core. hydrogen-deuterium exchange mass spectrometry can then test interface exposure, but its altered exchange reports accessibility and structural dynamics rather than a new subunit by itself. A convincing remodeling claim is a reproducible change in assembly membership or occupancy, separated from changes in abundance, localization and assay recovery, and connected to a functional consequence.

Protein primary structure is the ordered amino acid sequence of a polypeptide, linked by covalent peptide bonds. The approximately planar peptide bond restricts local geometry, while backbone torsion angles give conformational freedom subject to steric exclusion. Protein secondary structure comprises local backbone arrangements, especially the alpha helix, with backbone hydrogen bonds between residues $i$ and $i+4$, and the beta sheet, whose strands are joined by backbone hydrogen bonds. Turns and loops connect these elements. Protein tertiary structure is the full three-dimensional organization of a single chain, often including several protein domains. Protein quaternary structure is the arrangement of multiple chains in a protein complex.

Protein folding is governed by the Gibbs free energy of the chain together with its solvent, not by the chain's energy alone. Folding reduces the chain's conformational entropy, but can gain solvent entropy through the hydrophobic effect, burying nonpolar side chains away from water. Favorable hydrogen bonds, Van der Waals forces, electrostatic interactions and sometimes covalent disulfide bonds stabilize particular structures. Steric repulsion prevents atomic overlap. At fixed temperature and pressure, the native ensemble is favored when its total $\Delta G=\Delta H-T\Delta S$ is lower than competing ensembles. The balance depends on the solvent, pH, salt and ligands. Molecular chaperones help avoid aggregation or kinetic traps; a stable fold is not a perfectly rigid conformation, and some proteins have disordered regions.

For a soluble globular protein, a typical pattern is a nonpolar core and a polar, hydrated surface. A transmembrane protein instead presents nonpolar side chains to the hydrocarbon interior of the lipid bilayer. Membrane-spanning alpha helices satisfy their polar backbone groups through internal hydrogen bonds; a roughly membrane-thickness helix commonly contains about twenty residues, depending on tilt and bilayer thickness. Multiple helices can enclose polar channels or binding sites. Membrane-spanning beta barrels likewise expose mainly nonpolar groups outward towards lipid while arranging backbone hydrogen bonds between strands and often lining an aqueous pore with polar groups. Exposed loops and domains contacting water can resemble soluble proteins. Not every membrane-associated protein spans the membrane: peripheral attachment and lipid anchoring are distinct possibilities.

The organizing distinction is the environment of each surface: water favors accessible polar groups, while the membrane core favors outward-facing nonpolar groups. Buried polar groups can still be essential to a transport pathway or catalytic site.

Affinity purification and proximity labeling answer related but different questions. The former enriches material that remains associated during extraction; the latter marks a neighbourhood before extraction. Neither alone proves a direct protein-protein interaction.

In affinity purification-mass spectrometry, isolate a bait protein with an antibody or an endogenous epitope tag. Lyse under conditions chosen to preserve the relevant protein complex, capture the bait on an affinity matrix, wash away background and identify co-purifying proteins by liquid chromatography-mass spectrometry after digestion into peptides. Co-immunoprecipitation is the antibody-based version of this association test. Gentle conditions preserve more weak interactions but also increase background; stringent washing improves specificity at the cost of labile partners. Extraction can disrupt membrane assemblies or create associations that were not present in intact cells.

Controls should include matched tag-only or untagged cells, an appropriate antibody control, input abundance and, where feasible, a binding-defective bait that retains its normal localization. Check that tagging has not changed the bait's abundance, localization or function. Reciprocal capture of a candidate partner is useful validation. If an association might be bridged by RNA or DNA, compare nuclease-treated and untreated samples while recognizing that nuclease treatment may dismantle a biologically genuine assembly.

For proximity labeling, fuse the bait to TurboID, which activates biotin and covalently labels accessible nearby proteins in living cells. After a timed labeling pulse, harsh lysis can be used because recovery no longer requires the original assembly to survive. Capture biotinylated material using streptavidin and identify enriched proteins by mass spectrometry. This can recover transient or insoluble neighbourhoods missed by ordinary extraction. A localization-matched control enzyme, no-added-biotin condition and matched expression help separate the bait's neighbourhood from diffuse labeling and endogenous biotinylated proteins.^[ref]

The label records proximity, accessibility and time spent nearby. A neighbour need not bind the bait; a real binder might label poorly if reactive groups are inaccessible. Pulse duration therefore trades temporal selectivity against labeling yield, rather than defining an exact universal distance cutoff.

Quantification distinguishes enrichment from a list of contaminants. Use label-free quantification of peptide signals across independent bait and control purifications, or SILAC in compatible cell cultures to compare light and heavy samples. In SILAC, isotope-containing amino acids enter newly synthesized proteins; matched peptides form distinguishable mass pairs. A label swap helps identify isotope-condition confounding. Tandem mass tags offer multiplexed comparison using reporter signals, but interfering co-isolated peptides can compress ratios.

Estimate enrichment and its reproducibility relative to matched controls, accounting for bait recovery and input abundance. Common abundant contaminants should not be promoted solely because many spectra identify them. Statistical scoring such as SAINT models the contrast between specific and background recovery across controls and replicates, allowing candidate associations to be ranked with estimated error rates.^[ref] A reproducibly enriched candidate is evidence for the assay-defined association or neighbourhood, not automatically a direct binding partner or fixed stoichiometric subunit.

A polyprotein is a polypeptide chain that supplies several protein products through proteolysis. One translation product can therefore contain the sequences of multiple eventual products. This differs from a protein complex assembled by association of independently made chains.

The relationship can be tested by mapping product sequences back onto a precursor. In 1981, investigators determined the poliovirus RNA sequence and identified a long open reading frame. Amino acid sequence analysis mapped twelve viral polypeptides to cleavage products of the encoded polyprotein. Agreement between the sequence and the isolated products supported a common precursor, rather than merely grouping similar-sized bands together.^[ref]

For an original accounting example, suppose a hypothetical precursor consists of consecutive regions of 200, 150 and 250 amino acid residues. Two cuts at the boundaries can yield three chains containing 600 residues altogether. Cleavage increases the number of molecules without synthesizing additional amino acid residues. Partial processing would also produce intermediates containing more than one of the regions.

The precursor and the final products need not have identical functions or lifetimes. An assay recognizing a sequence retained in several intermediates can detect more than one molecular species. Identifying a polyprotein thus requires following both its protein primary structure and its processing state.

A protein complex is an assembly of associated proteins. Its composition specifies component identities and their stoichiometry; variable accessory occupancy and transient partners can make the assembly a molecular population of related forms rather than one fixed object.

A protein subunit is an individual polypeptide component of a protein complex. The number of protein subunits counts chain copies, whereas the number of subunit types counts distinct kinds of chain. An assembly $A_2{B}_2$ has four protein subunits of two types. A protein domain is a region within a chain and need not be a separate protein subunit; the arrangement of several chains belongs to protein quaternary structure.

The alpha/beta tubulin heterodimer illustrates the distinction: it contains two related but different polypeptides, not two domains cut out of one chain. The 1998 electron-crystallographic model resolved the alpha/beta architecture at $3.7$ angstrom resolution, using zinc-induced tubulin sheets.^[ref]

Subunit counting can also become an optical experiment. In total internal reflection fluorescence microscopy, tagged membrane proteins can be observed as isolated fluorescent spots; successive photobleaching steps reveal individual fluorescent tags disappearing. Subunit counting in membrane-bound proteins by Maximilian H. Ulbrich and Ehud Y. Isacoff used this approach on membrane proteins expressed in frog oocytes. For NR1/NR3B receptor assemblies, tagging one component type mainly gave two steps, while tagging both gave four, supporting a two-plus-two arrangement.^[ref]

A count of visible steps need not equal the true number of protein subunits: some tags fail to fluoresce, and two tags may bleach too close together to resolve. The experiment therefore combines step counts with a model of labeling and detection, rather than interpreting every two-step trace as proof of a two-chain protein complex.^[ref]

RNA immunoprecipitation followed by sequencing of recovered RNA. Enrichment suggests association with the selected protein complex under the assay conditions, not necessarily direct binding to every recovered transcript.

Suhas S. P. Rao coauthored research on cohesin.^[ref]

Cohesin is a multisubunit chromosome-associated protein complex involved in sister-chromatid cohesion and chromosome organization. Cleavage of a cohesin component helps permit sister separation at anaphase. Cohesin also contributes to chromosomal loop organization: acute removal can eliminate loop domains without implying that every aspect of gene expression changes immediately. Its segregation and interphase-organization roles are related but experimentally separable.