= Cotranslational protein translocation
<Cotranslational protein translocation> is passage of a growing <polypeptide chain> across a membrane during <biological translation>. <SRP> recognizes an exposed <hydrophobic> <ER signal sequence>. The <SRP receptor> enables handoff to the <Sec61 complex>. <Biological translation> can slow during delivery; the <ribosome> remains on the <cytosol> side.<Membrane Protein Insertion at the Endoplasmic Reticulum by Sichen Shao and Ramanujan S. Hegde>[Shao and Hegde, Membrane Protein Insertion at the Endoplasmic Reticulum, 2011; “Cotranslational Targeting”, Figure 3]{ref}
The useful engineering feature is coordination: a targeting instruction can become accessible while the rest of its cargo is still being made. Recognition, docking and crossing remain distinct operations. A defect in any one can prevent successful import even when the <protein primary structure> contains an appropriate signal.
This route is not universal. Many <tail-anchored proteins> use delivery after <biological translation>, and their membrane-spanning segment lies near the <C-terminus>. Thus <protein targeting> signals need not be at the <N-terminus>.<Membrane Protein Insertion at the Endoplasmic Reticulum by Sichen Shao and Ramanujan S. Hegde>[Shao and Hegde, 2011; “Posttranslational Membrane Protein Topogenesis”, Figure 6]{ref}
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