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A detergent is a substance or formulation that assists cleaning, commonly through surfactants with water-compatible and oil-compatible molecular regions. These molecules collect at interfaces and can form micelles that help disperse oily material in water. Agitation, concentration and the composition of both the soil and washing solution affect the process.

A simple comparison is to shake an oil-water mixture with and without a suitable detergent and observe how readily droplets disperse and how long they persist. Lowering surface tension can improve wetting, while adsorption and aggregation change how oily material is carried away. Stable dispersion and removal are different steps: the surrounding fluid must still transport the dispersed material.

Detergents can also disrupt a lipid bilayer. In a protease protection assay, removing that membrane barrier tests whether it caused the apparent protection of an enclosed protein.

First establish that proteins cross the organelle boundary; then define its resident proteome and determine which residents arrive by import. A protein detected near an organelle may be a surface binder, contaminant, transient visitor or locally synthesized product. These possibilities require different controls.

Start with an operational definition of the new organelle: its location, boundary, ultrastructure and reproducible biochemical enrichment. Fluorescence microscopy with independent markers and electron microscopy can determine whether it is membrane-bounded and whether it contacts or engulfs other compartments. A condensate without a membrane can recruit proteins by partitioning; that should not be mistaken for translocation through a membrane. Purification by density-gradient centrifugation or affinity isolation should include markers for likely contaminants and measurements of organelle integrity. An enriched fraction is a starting material, not evidence of import by itself.

Choose candidate proteins from fractionation, imaging or sequence predictions and make minimally perturbing labeled versions. Follow newly synthesized protein using a pulse-chase experiment and independently verify its position relative to the organelle boundary. Loss of a cytosolic precursor followed by accumulation of an internal mature form is suggestive, but proteolytic processing alone does not demonstrate translocation. Tags can obscure targeting signals or change folding, so compare different tag positions and validate endogenous localization where possible.

The key biochemical test is an organelle import assay coupled to a protease protection assay. Incubate isolated intact organelles with a labeled precursor, then remove externally accessible protein with protease. Imported lumenal or matrix protein should be protected in intact organelles but become sensitive after detergent disruption. Surface-bound controls must remain accessible; an independent internal marker tests organelle sealing. Include organelle-free incubations to identify protease-resistant aggregates, and verify that detergent does not simply inactivate the protease. Carbonate extraction or membrane fractionation can distinguish an integral membrane product from a soluble internal one. For membrane proteins, map protected fragments and terminal accessibility rather than demanding protection of the entire chain.

Perturbation then tests the mechanism. Compare incubation time and temperature; deplete and restore relevant energy sources; dissipate a membrane potential only after confirming that this treatment preserves membrane integrity; and compete with an excess of unlabeled substrate. Energy requirements must be discovered rather than assumed. Mitochondrial matrix entry, ER translocation and peroxisomal import use different machinery. In particular, stable folding is not a universal barrier: primary peroxisome experiments demonstrated import of stably folded substrates.^[ref]

Mutating a candidate targeting signal should reduce import, while appending that signal to a suitable reporter can test sufficiency. Disrupting a candidate receptor or translocation component, followed by rescue with the wild-type protein, tests necessity. Because a blocked pathway may destroy the organelle indirectly, pair import measurements with morphology, membrane integrity and the import of unrelated controls. An import-defective allele that leaves the compartment otherwise functional is more informative than complete organelle loss. A negative isolated-organelle result does not exclude import if purification removed a required factor; compare with an intact-cell assay. Conversely, protection shows internalization but does not alone distinguish direct protein translocation from delivery inside a vesicle or engulfed compartment. Route-specific perturbations and topology measurements must resolve that distinction.

For the protein complement, combine quantitative spatial proteomics with complementary methods. Measure protein abundance across an entire fractionation gradient and identify proteins with reproducible localization profiles, rather than accepting every protein in a single pellet. Compare intact-organelle protease treatment with disrupted controls, and compare wild type with an import-defective condition. Analyze both soluble and membrane fractions. Liquid chromatography-mass spectrometry identifies peptides, but poorly soluble, scarce or unusual proteins may escape detection, so fractionation and acquisition strategies should be varied.

An internally localized TurboID or other proximity labeling enzyme can reveal proteins accessible in the relevant compartment in living cells. A cytosolic control, a catalytically inactive labeling construct and matched expression levels help identify background. Proximity labeling establishes accessibility and neighborhood, not translocation or stable residency by itself; labeling radius, reagent access and perturbation by the enzyme must be considered.^[ref]

Finally distinguish resident, imported and all imported. A resident may have been synthesized by an organelle genome; an imported protein may only transit through the compartment. Time-resolved labeling, precursor processing, localization and dependence on a defined import pathway can classify these cases. Report coverage across growth conditions, detection limits and a confidence tier for each candidate. No single proteomic experiment proves absolute completeness, but convergent topology, kinetic and genetic evidence can produce a well-supported import catalogue.

Lane 4 combines detergent with the absence of the t-SNARE preparation. The lipid bilayer no longer protects inward-facing v-SNARE, and there is no t-SNARE partner to provide complex-dependent protection. Consequently little full-length labelled protein remains. This distinguishes membrane-dependent protection from partner-dependent protection in a protease protection assay. It also helps interpret lane 3: its strong signal requires the partner rather than lipid bilayer enclosure alone.

Use a membrane topology measurement on v-SNARE vesicles alone, so that partner binding cannot mask an outward-facing domain. A membrane-impermeant probe directed against the binding-domain region would label only the exposed population while vesicles remain intact. Compare that signal with total accessible protein after lipid bilayer disruption, using a trapped-lumen marker to check that the intact vesicles exclude the probe.

With comparable detection efficiencies and background subtraction,An analogous protease protection assay compares external accessibility with total accessibility. In either approach, lipid bilayer leakage and steric masking must be distinguished from genuinely inward-facing protein.

A protease protection assay asks whether a barrier prevents an added protease from reaching a protein. A membrane-disruption comparison tests whether protection depends on that barrier.

The membrane-disruption test was performed by David D. Sabatini and Günter Blobel in 1970. Rat-liver microsomes purified by sucrose centrifugation incorporated radioactive leucine during cell-free translation. Samples received cold trypsin and chymotrypsin, with or without membrane-solubilizing detergents. Electron microscopy checked vesicle preservation; size-exclusion chromatography separated protected fragment sizes. Large membrane-associated fragments lost protection after disruption, whereas small ribosome-protected fragments persisted. An exposed ribosome preparation remained digestible when mixed with microsomes, checking that membranes did not simply inhibit the proteases.^[ref]

To read the qualitative diagram, first compare untreated aliquots: they establish that detectable cargo exists before digestion. Next compare external protease with intact versus disrupted membranes. Persistence only while the barrier remains intact supports enclosure, provided the exposed-cargo control confirms active protease in both chemical conditions. The detergent-only aliquot checks that disruption itself does not destroy recovery or detection. Adding membranes after synthesis tests a different alternative: capture of a completed product rather than entry during biological translation.

This is an argument from controlled accessibility. Binding predicts membrane association but generally leaves an exposed cargo digestible. Enclosure predicts protection that depends on membrane integrity. However, protein aggregation, intrinsic resistance to proteolysis, inactive protease or undetected small fragments can obscure this distinction. For a transmembrane protein, expect protection of the enclosed portion rather than necessarily the full chain. Combine fragment size, compartment controls and membrane topology; a protected signal alone does not uniquely specify the whole protein's location.

Protein cell biology studies how proteins function within biological cells: where they are delivered, which partners they encounter, and how these interactions affect cellular processes. A protein's protein primary structure and abundance do not by themselves specify its location or available partners. RNA-binding proteins, G proteins and protein targeting illustrate different aspects of that problem.

The microsomal protein import experiment makes location experimentally testable. In 1975, Günter Blobel and Bernhard Dobberstein combined cell-free translation components with dog-pancreas microsomes. An immunoglobulin light chain product made while suitable membranes were present became processed and resistant to external proteolysis, whereas globin did not enter the same protected compartment. Adding membranes only after synthesis did not reproduce the effect in this preparation.^[ref]

The important observation is controlled accessibility, not just recovery near a membrane. A protein can bind a vesicle's outside without crossing it; enclosure can protect it from an external protease. Protease protection assays therefore complement measurements of total product and membrane topology.

For an original two-compartment example, suppose a fixed total of100 competent cargo molecules is divided between20 enclosed and80 exposed molecules. Redistributing the same total to70 enclosed and30 exposed changes accessibility even though total abundance stays100. If every enclosed molecule is protected and every exposed molecule is digested in the stipulated model, the protected fraction changes from0.20 to0.70. Real measurements also need checks for membrane integrity, protease activity and product recovery; the fractions alone do not identify every transport mechanism. This is why cellular organization belongs alongside the isolated molecule's chemistry.