Final answer:
In the synthesis of membrane-spanning (integral) proteins, the hydrophobic regions within the polypeptide chain are typically 18 to 24 amino acids long. These lengths allow for efficient ribosomal translocation, prevent translocon stalling, and facilitate signal sequence cleavage.
Step-by-step explanation:
In the synthesis of membrane-spanning (integral) proteins, stop-transfer sequences, which are hydrophobic regions within the polypeptide chain, trap the protein in the fatty acid interior of the membrane. These hydrophobic regions are typically 18 to 24 amino acids long. The optimal length for membrane integration is why the hydrophobic regions in signal anchors are 18 to 24 amino acids long. This length allows for efficient ribosomal translocation, as well as prevents translocon stalling. Moreover, it also facilitates signal sequence cleavage.