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A guide-directed DNA nuclease. Target recognition depends on a guide RNA and an appropriate protospacer adjacent motif; cleavage is followed by cellular repair, so cleavage alone does not specify the final allele.

Antibody-targeted nuclease cleavage and release of DNA near a chosen chromatin-associated protein or mark. Background, antibody specificity and cleavage conditions still need controls.

A nuclease that cuts internal bonds of a nucleic-acid chain. For an RNA substrate, internal cleavage can create new ends for further degradation.

Genome editing deliberately changes the DNA sequence at selected sites in a genome. Changes can include a small insertion or deletion, replacement of a sequence, or introduction of a specific base change. This differs from merely measuring gene expression or transiently changing it without altering the underlying DNA.

In a common CRISPR-Cas9 approach, a guide RNA directs a nuclease to a matching DNA region, and cellular DNA repair determines the resulting sequence change. Repair can disrupt a gene or use a supplied template to introduce a designed change. Le Cong and colleagues demonstrated programmable targeting of endogenous mammalian sites and simultaneous editing at several sites in 2013.^[ref]

A targeted cut is therefore an intermediate operation, not proof of the intended edit. DNA sequencing of the resulting alleles distinguishes the planned change from unintended repair products.

Micrococcal nuclease is a calcium-dependent nuclease capable of cleaving DNA and RNA. In chromatin analysis, accessible connecting DNA is often cut more readily than DNA protected by nucleosomes. The recovered fragment distribution can therefore report protection patterns, but sequence preference and digestion extent affect interpretation: a fragment boundary is not automatically an exact unperturbed cellular boundary. ^[ref]

A nuclease is an enzyme that cleaves nucleic acids. ribonucleases act on RNA; other activities act on DNA. Nuclease sensitivity can reveal a nucleic-acid-dependent assembly, but loss of association can also reflect secondary structural disruption.

An assay asking whether nucleic acid remains inaccessible to a nuclease under specified conditions. Protection can reflect protein binding or compartmental enclosure; appropriate disruption controls distinguish them.

Latent viral DNA has regularly spaced nucleosome protection, whereas much of the nuclear viral DNA during productive infection lacks that regular protection. In the latent sample, increasing digestion resolves a ladder of protected fragments, as expected when micrococcal nuclease cuts exposed linker DNA between nucleosomes. In the productive sample, viral signal is lost rapidly into heterogeneous small material rather than accumulating as a comparable ladder. That is consistent with a large pool of newly replicated or otherwise differently packaged viral DNA being more accessible.

The total-DNA ladders show that host chromatin remains nucleosomal and that the assay can recover protected fragments in each cellular condition. The virus-specific Southern blot distinguishes viral DNA from this abundant host background, and the uninfected lane controls probe background. The conclusion concerns the dominant detectable viral population: productive cultures can contain both chromatinized templates in viral latency and accessible replication products. Absence of a ladder does not establish absence of every histone molecule, and differences in DNA recovery or nuclease accessibility should be considered before assigning a precise fraction of chromatinized genomes.

The cytoplasmic viral signal stays predominantly at high molecular size despite longer nuclease treatment. The viral DNA is protected from digestion, plausibly inside viral capsids or assembled particles. This differs from a nucleosomal ladder: a capsid can shield a whole genome instead of producing repeated small protected units. Release into the cytoplasm during productive infection makes particle-associated DNA a reasonable interpretation.

The virus-negative lane argues against nonspecific hybridization. Protection alone nevertheless does not prove that all detected particles are mature or infectious; an inaccessible compartment or another protective assembly could also shield DNA. Comparing intact material with material whose protective envelope or capsid has been disrupted would distinguish enclosure from intrinsic nuclease resistance of the DNA. Fractionation controls would address nuclear contamination. The result is about physical accessibility of viral DNA, not a quantitative infectivity assay.

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.

Peter J. Skene coauthored research on nuclease.^[ref]

A nuclease is an enzyme that cleaves nucleic acids. ribonucleases act on RNA; other activities act on DNA. Nuclease sensitivity can reveal a nucleic-acid-dependent assembly, but loss of association can also reflect secondary structural disruption.

A short target-adjacent DNA motif recognized by a CRISPR-associated nuclease. The motif requirement depends on the nuclease; the commonly used Cas9 from Streptococcus pyogenes recognizes NGG.

The bacterial species supplying the widely used SpCas9 nuclease for CRISPR genome editing.

A protein shell enclosing a viral genome. Protection from nuclease is consistent with enclosure but does not alone demonstrate infectivity.

A nuclease participating in primary piRNA processing in Drosophila, distinct from the Dicer cleavage pathway.