What method do proteins use to be sorted to the ER?

Proteins are sorted to the endoplasmic reticulum (ER) primarily through a mechanism known as transmembrane transport. This process involves the use of signal sequences, typically located at the amino-terminus of the nascent polypeptide chain. These signal sequences are recognized by the signal recognition particle (SRP) when the protein is still being synthesized by ribosomes.

Once the SRP binds to the signal sequence, it halts translation temporarily and directs the ribosome-protein complex to the SRP receptor on the surface of the ER membrane. After binding to the receptor, the ribosome is positioned correctly to resume translation, allowing the growing polypeptide chain to be threaded into the ER lumen or integrated into the ER membrane through a protein translocation channel.

This transmembrane transport is crucial for proteins that are destined for the secretory pathway or for inclusion in the ER membranes, as it allows for proper folding and modifications to take place within the ER. In contrast, exocytosis and endocytosis are involved in the movement of materials to and from the cell, but they do not specifically describe the mechanism by which proteins are sorted to the ER. Simple diffusion does not pertain to protein sorting, as it describes passive transport