Which process directly facilitates the transfer of polypeptides across the ER membrane?

The process that directly facilitates the transfer of polypeptides across the endoplasmic reticulum (ER) membrane is translocation. Translocation is a mechanism that allows newly synthesized polypeptides, particularly those destined for secretion or for use within the cell's membrane, to be moved into the lumen of the ER or across the ER membrane itself. This occurs while the polypeptide is still being synthesized by ribosomes, a process known as co-translational translocation.

During translocation, the ribosome synthesizing the polypeptide associates with the ER membrane. The growing polypeptide chain is threaded through a channel in the membrane known as the translocon. This channel facilitates the movement of the polypeptide from the cytoplasm, where it is synthesized, into the ER, where it can undergo further modifications, including folding and processing. Overall, translocation is a crucial step that ensures polypeptides are properly directed to their functional sites within or outside the cell.