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The bacterial SRP receptor GTPase that partners with Ffh in membrane targeting.

Reciprocal regulation of the receptor and SRP GTPases couples recognition to transfer of the nascent chain toward a membrane translocation site. This illustrates a targeting cycle in which simple binding is only one stage of productive delivery. ^[ref]

Hitoshi Nakatogawa is a researcher in molecular biology and biochemistry. His work includes ribosome stalling and the membrane changes involved in autophagy. A named institutional laboratory profile identifies both his autophagy research and his earlier joint SecM papers with Koreaki Ito.^[ref]

Nakatogawa and Koreaki Ito investigated how the sequence of an emerging nascent chain affects translation. Their 2002 paper identified an arrest sequence in SecM and suppressor mutations at the constriction of the ribosomal exit tunnel. The finding helped establish that a growing protein can influence the machinery synthesizing it.^[ref]

To see a quantitative consequence of pausing, consider an invented steady-input example. If $2$ ribosomes per second enter a segment and each normally spends $0.5$ seconds there, its mean occupancy is $2\times 0.5=1$ ribosome. A pause increasing the mean residence time to $5$ seconds would predict an occupancy of $10$, provided that the arrival rate remains unchanged and crowding can be neglected. A strong local accumulation can therefore reflect longer residence time, rather than faster production of completed proteins. At high occupancy, queues and collisions invalidate the independent-flow approximation; that failure is itself experimentally meaningful.

The connection with autophagy is methodological rather than a claim that these are one pathway: both questions ask how local molecular interactions produce a larger cellular behavior. His laboratory's stated autophagy questions concern where the membranes forming an autophagosome originate and how their assembly proceeds.^[ref]

Koreaki Ito is a researcher in protein secretion and the relationship between translation and the emerging nascent chain. With Hitoshi Nakatogawa, he showed that the ribosomal exit tunnel can participate in recognizing the sequence of a growing protein, rather than simply providing a passive passage out of the ribosome.^[ref]

Their SecM study connected two kinds of evidence. An arrest-causing sequence in the nascent chain produced ribosome stalling, while suppressor mutations in 23S ribosomal RNA or ribosomal protein L22 allowed elongation to continue. The altered residues faced the tunnel constriction. This localization supported an interaction between the tunnel and the emerging chain.^[ref]

A simplified experiment illustrates why suppressing a phenotype is informative but does not, by itself, prove a direct physical contact. Compare four combinations: ordinary versus altered SecM, each translated by ordinary versus altered ribosomes. If an altered ribosome restores elongation specifically for the arresting sequence, the result links the two components functionally. A general reduction in translation could instead change the apparent abundance of every product; comparing only one final band would miss that alternative. Spatial information about the changed ribosomal RNA and protein residues adds another constraint. Even then, a suppressor mutation might act by changing a nearby conformation rather than removing a direct contact. The experiment is powerful because genetic and structural evidence constrain one another, while leaving room to test the detailed physical mechanism.

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.

A reagent that can release nascent chains by accepting the growing peptide, useful as a control for translation-site signals. Global treatment also perturbs cellular physiology.

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]

A repeated peptide-epitope system detectable by fluorescent binders. Placing tags on nascent chains can make a biological translation site visible separately from completed proteins.