Question

In some ER proteins the signal sequence acts as a topogenic sequence. How do these sequences have a dual role regarding insertion into the lipid bilayer?

Answer 1

A topogenic sequence is a combined term used for a sequence of peptides located near the growing proteins, which are necessary for their insertion and orientation in the cell membranes. These sequences also help in the translocation of the proteins across the various intracellular membranes and also make sure that they are transported to the right organelle after their synthesis. The site of the sequence may be in mid parts of the nascent protein, for example, in-between the signal-anchor sequences and stop-transfer anchor sequences, or at the end, like at the sequence of N-terminal. If the sequence is at the end or N terminal of the polypeptide, it is incised off after entering into the lumen of ER (endoplasmic reticulum) by a signal peptidase through translocon, and subsequently, it will be degraded.