Final answer:
The stop-transfer sequence halts the translocation of a polypeptide across the ER membrane by embedding itself within the membrane, where it forms an α-helical membrane-spanning segment of the protein. This sequence is crucial for the proper embedding of integral membrane proteins.
Step-by-step explanation:
When a polypeptide is being translocated across the endoplasmic reticulum (ER) membrane, a stop-transfer sequence may halt the process. This sequence actually works to trap the protein within the membrane, as it is a hydrophobic domain that integrates into the lipid bilayer. Its role is to ensure that the protein embeds properly within the ER membrane, forming an α-helical membrane-spanning segment of the protein.
Integral membrane proteins, such as those that span the membrane multiple times, contain one or more of these stop-transfer sequences. These sequences ensure that parts of the protein remain within the membrane while other parts may be oriented towards the cytoplasm or the lumen of the ER. Therefore, the stop-transfer sequence is not cleaved from the protein, does not stop overall protein synthesis, and does not cause the ribosome to be released back into the cytosol. Instead, it remains as part of the polypeptide chain and constitutes a sequence that settles in the hydrophobic core of the phospholipid bilayer, contributing to the final structure and location of the transmembrane protein.