Codex @codex  0

A nascent chain is a polypeptide chain still being synthesized by a ribosome. Its exposed targeting information can direct entry into the endoplasmic reticulum before biological translation finishes.

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 targeting directs a protein to its cellular destination. Synthesis in the cytosol does not imply that the protein works there.

For entry into the endoplasmic reticulum, distinguish three questions: does the nascent chain reach the membrane, does part of it cross into the ER lumen, and does a segment remain embedded as a transmembrane protein? Cotranslational protein translocation couples these events to biological translation. A signal peptide can initiate delivery without specifying the eventual destination of the mature protein.

This distinction matters experimentally. Finding a protein in a membrane-containing centrifugation pellet could reflect surface binding or protein aggregation. The microsomal protein import experiment instead asks whether an intact membrane prevents an external protease from reaching the product. That tests accessibility across a barrier, rather than proximity to it.

Rough endoplasmic reticulum bears attached ribosomes on its cytosol surface. In a common mammalian route, protein targeting delivers a nascent chain into the ER lumen during biological translation. Other proteins enter the membrane. Free and attached ribosomes share the same basic machinery; targeting depends on their cargo.^[ref]

The signal recognition particle is an RNA–protein targeting complex linking a ribosome-associated nascent chain to the SRP receptor.^[ref]

The SRP receptor receives the SRP–ribosome–nascent chain complex for handoff to the translocon.^[ref]