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K. Veluthambi is a plant researcher whose work with B. W. Poovaiah investigated hormonal control of polypeptide patterns during strawberry development. Their 1984 study combined manipulation of auxin supply with SDS-PAGE measurements, connecting whole-fruit growth to changes in molecular composition.^[ref]

For example, an approximately $81\mathrm{kDa}$ polypeptide appeared between five and ten days after pollination in the controls. Removing the achenes prevented its appearance; treatment with the synthetic auxin naphthaleneacetic acid restored it. Other bands changed at later developmental stages, showing why the timing of collection matters.^[ref]

An illuminating comparison is between two samples collected on the same day, one deprived of auxin and one supplied with it. Comparing samples of different ages instead would mix treatment effects with ordinary development. This makes the study an example of pairing a physiological intervention with a molecular measurement, while leaving the identities and causal roles of individual proteins as further questions.

An Escherichia coli transcription unit encoding lactose-use proteins. Expression integrates LacI-dependent repression, allolactose induction, catabolite activator protein activation and inducer exclusion.

The lac repressor, also called LacI, is the regulatory protein that reduces transcription of the lac operon by binding a lac operator in Escherichia coli. Binding of a small-molecule inducer changes the repressor's regulatory state through allostery. Allolactose connects this switch with lactose metabolism, while IPTG supplies an experimentally useful inducer. The lac repressor is distinct from the operator DNA and from the RNA polymerase whose transcriptional output is controlled.

Walter Gilbert and Benno Müller-Hill's 1966 isolation experiment gave the proposed regulator a biochemical measurement. Their equilibrium dialysis assay used radioactive IPTG: small free inducer could cross the membrane, while the macromolecular binding component was retained. Extra total labelled inducer on the protein side reported binding. The assay deliberately used inducer recognition rather than assuming that the exact mechanism of transcriptional repression was already established.^[ref]

An original one-site binding model explains that readout. If the free inducer concentration is $I$, the concentration of independent binding sites is $B_T$ and the dissociation constant is $K_I$, the bound concentration isAt equilibrium dialysis with matched solvent conditions, the protein side contains free plus bound inducer, $I+B$, while the opposite side contains free inducer $I$. Choosing $B_T=2\mu \mathrm{M}$ and $I=K_I=1\mu \mathrm{M}$ gives $B=1\mu \mathrm{M}$ and total concentrations $2$ versus $1\mu \mathrm{M}$. Equality of the free concentration does not require equality of the total concentration. The numbers are illustrative; the native lac repressor has a more detailed oligomeric and allosteric structure.

A separate one-operator model connects binding to expression. If a concentration $R$ of binding-competent repressor occupies the lac operator with probability $\theta =R/(K_O+R)$, and occupied DNA produces no transcripts in this model, thenReducing $R/K_O$ from $9$ to $1$ raises the model's transcription rate from $0.1r_0$ to $0.5r_0$. It does not follow that every measured cellular response obeys this one-site expression: operator arrangement, additional regulators and the later stages of gene expression affect the output. Measuring inducer binding and measuring repression therefore test related but different parts of the mechanism.

The repressor controlling the lac operon. Operator binding represses transcription, while inducer binding shifts the conformational equilibrium towards lower operator occupancy.

A lamin is a type of intermediate filament protein forming part of the nuclear lamina, a network associated with the inner surface of the nuclear envelope. Lamins contribute to nuclear shape and mechanical organization and interact with components connecting the envelope to chromatin. They differ from laminin, an extracellular-matrix protein, despite the similar names. ^[ref]

Regulated assembly is useful during cell division. Phosphorylation of lamins helps promote lamina disassembly as the nuclear envelope breaks down in open mitosis; subsequent reassembly supports daughter nuclei. Thus a structural protein can form a load-bearing network while retaining a chemically regulated route for dismantling it.

The large ribosomal subunit is the ribosomal subunit containing the peptidyl transferase center, where peptide bonds are formed during biological translation. It contains ribosomal RNA and ribosomal proteins, and provides a ribosomal exit tunnel through which the growing polypeptide passes. The small ribosomal subunit organizes decoding of messenger RNA; synthesis requires coordination between both parts of the ribosome.^[ref]

For a reaction example, let one peptidyl-tRNA carry a chain of four amino acid residues and an incoming aminoacyl-tRNA carry one more. Peptidyl transfer produces a five-residue chain attached to the incoming transfer RNA. The existing chain is transferred and extended; the reaction does not join five free amino acids all at once. Subsequent movement of the ribosome prepares the complex for another elongation cycle.

A 2000 structural experiment examined an archaeal large ribosomal subunit and complexes containing two substrate analogues. The analogues contacted conserved 23S ribosomal RNA in the catalytic region, with no protein side-chain atoms within about 18 angstroms of the bond-forming position. This supported the identification of the ribosome as a ribozyme. The same study found that both RNA and proteins contribute to the ribosomal exit tunnel.^[ref] The structural evidence identifies the reaction environment; it does not make every chemical step or proton-transfer mechanism directly visible.

A maturing degradative compartment distinguished from early sorting endosomes by changes in proteins, lipids, lumenal conditions and fusion competence.

Lateral mobility of membrane proteins is movement along a membrane surface. It differs from membrane transport through the boundary and from reversing membrane topology. Attachment, crowding and barriers can restrict diffusion; the fluid mosaic model does not require every membrane protein to wander freely.^[ref]

An original local model treats a small, approximately flat patch as an unbounded plane with uniform diffusion coefficient $D$, no drift and no trapping. Represent the two displacement coordinates by independent Brownian motions, each with zero expected value and variance $2Dt$. Since $r^2=x^2+y^2$, addition of the two coordinate expectations givesThis is a root-mean-square displacement, not distance traveled along a jagged trajectory or the displacement of every protein. For a stipulated $D=0.08\mu {\mathrm{m}}^2{\mathrm{s}}^{-1}$ and $t=5\mathrm{s}$, $⟨r^2⟩=1.6\mu {\mathrm{m}}^2$ and $r_{\mathrm{rms}}\approx 1.265\mu \mathrm{m}$. Reaching a stipulated root-mean-square scale $L=2\mu \mathrm{m}$ gives $t=L^2/(4D)=12.5\mathrm{s}$. These model values are not measured diffusion coefficients or mixing times from the historical experiment.

Multiplying the target displacement scale by two multiplies the model time by four. This quadratic scaling is a useful way to see why local encounters and redistribution over an entire biological cell need not happen on the same timescale. A curved, bounded cell membrane, restricted paths and different mobile populations require more than this flat-patch model; disappearance of an initial pattern alone does not determine $D$.

A protein that binds particular carbohydrate features. A lectin stain reports accessible binding sites rather than every molecule in a cell wall.

Lei Jin coauthored structural research on how heparin activates the inhibitory protein antithrombin. A 1997 paper compared crystallographic structures involving active and latent antithrombin and its high-affinity five-sugar binding sequence at 2.9-ångström resolution.^[ref]

The study linked binding to changes in the main beta sheet and exposure of the reactive-center loop, providing a structural account of how a carbohydrate ligand changes the behaviour of a serpin.^[ref] The point is more informative than simply observing that heparin and antithrombin bind: the locations of structural changes suggest how binding can alter access to the inhibitory machinery.

A structure is a constrained molecular snapshot. Establishing a complete time sequence or the rate of inhibition requires additional kinetic evidence; it cannot be read directly from a static arrangement of atoms.

Coauthor of research on antibody unfolding and binding in force spectroscopy. The study connected molecular mechanical states with the interpretation of recognition measurements, rather than treating a measured binding event as independent of protein conformation.^[ref]

Lethal giant larvae denotes a conserved polarity-protein family first characterized in Drosophila. Its members help organize cortical domains and epithelial polarity, with activity and localization regulated partly through phosphorylation. Mammalian LLGL proteins are related components rather than literal larval structures. Their contribution depends on cellular context, so polarity effects and effects on proliferation should be measured separately. ^[ref]

Leucine is an amino acid with a branched hydrophobic side chain. In proteins, that side chain often participates in packing away from water, while the amino-acid backbone forms peptide bonds.

Leucine is also a metabolic signal. Direct leucine sensing by Sestrin2 connects amino-acid availability with the mTORC1 pathway; the original study used binding and signalling measurements to distinguish sensing from mere correlation.^[ref]

Ligand binding is association of a ligand with a binding partner, such as a receptor or another protein. For a simple equilibrium $P+LightleftharpoonsPL$, the dissociation constant is $K_d=[P][L]/[PL]$. If free ligand is controlled and sites are independent, fractional occupancy is $[L]/(K_d+[L])$. Half-occupancy occurs at free $[L]=K_d$. This statement concerns binding; activation can require additional conformational or assembly steps. ^[ref]

A biochemical ligand is an ion or molecule that binds a protein. For a receptor, a ligand may activate signaling, prevent activation, or have another effect. This usage concerns protein binding; it is distinct from the coordination-chemistry use of the word. See the protein-binding reference under receptor.

A motivating experiment used isolated membranes from A-431 biological cells, which supplied abundant EGFR. Separate membrane aliquots were incubated at $0{}^{\circ }\mathrm{C}$ with ATP carrying radioactive phosphorus in its terminal phosphate group, plus magnesium or manganese ions. Radioactivity in material recovered by chemical acid-induced precipitation supplied the readout. Adding EGF beforehand enhanced incorporation relative to the unstimulated preparation; this placed a ligand-dependent biochemical response in a cell-free membrane system.^[ref]

The follow-up comparisons separated increased addition from suppressed removal: phosphate removal was rapid but unaffected by EGF, and anti-EGF antibody reversed activation by removing the ligand. These observations support reversible regulation of membrane-associated protein kinase activity.^[ref] Total incorporation alone cannot identify which protein or amino acid residue received the label, prove intrinsic receptor catalysis, or predict an intact biological cell's response.

The same account records an instructive identification error: receptor phosphorylation was initially assigned to the wrong amino acid residue because released modified amino acids migrated together in the electrophoretic electric field. Reanalysis identified phosphotyrosine residues. Retained radioactivity is evidence about the assay's separation and detection, not unique chemical identity.^[ref]

A pigment–protein assembly collecting light and transferring excitation towards a photosynthetic reaction centre. Excitation transfer between pigments differs from electron transfer after charge separation.

Limited proteolysis cleaves a protein under controlled conditions to probe exposed regions or recover stable domains. Resistance can reflect accessibility and dynamics as well as sequence specificity of the chosen protease.

Coauthor of research on antibody unfolding and binding in force spectroscopy. The study connected molecular mechanical states with the interpretation of recognition measurements, rather than treating a measured binding event as independent of protein conformation.^[ref]

A lipid bilayer nanodisc is a small bilayer patch stabilized at its edge, often by a surrounding scaffold. It permits controlled lipid composition and study of membrane-associated proteins, while lacking the two separately sealed aqueous compartments of a closed lipid vesicle.

A protein that binds and transports a lipid between membranes or promotes its exchange at a contact site.