Final answer:
The start-transfer sequence initiates protein integration into the ER membrane and interacts with the translocon, while the N-terminal ER signal sequence directs proteins into the ER lumen and interacts with SRP. Both sequences play crucial roles in protein synthesis and accurate cellular sorting.
Step-by-step explanation:
Understanding the protein synthesis and membrane integration process requires recognizing the roles of various signal sequences. The start-transfer sequence is a hydrophobic region within transmembrane proteins that initiates integration into the endoplasmic reticulum (ER) membrane. This sequence interacts with the translocon to facilitate membrane integration. In contrast, the N-terminal ER signal sequence directs co-translational entry of proteins into the ER lumen and is located at the N-terminus of the nascent polypeptide chain. The N-terminal signal sequence binds to a signal recognition particle (SRP), which then targets the ribosome-nascent chain complex to the ER membrane for translocation.
These mechanisms are critical for the accurate sorting and directing of proteins within the cell. For example, integral membrane proteins may contain multiple stop-transfer sequences allowing them to span the membrane several times. Additionally, proteins destined for mitochondria or chloroplasts carry specific signal sequences that are recognized post-translationally, different from the co-translational process of ER-targeted proteins.