What is the mechanism for transport of insulin in mammalian cells?
The process is described here. This is my summary, plus a few extra details:
The primary gene product of the INS gene is preproinsulin. As the N-terminal signal sequence (pre-) emerges from the ribosome during polypeptide elongation the signal sequence binds to a signal recognition particle which arrests elongation until the nascent chain has engaged with the translocation apparatus in the membrane of the endoplasmic reticulum (ER). The preproinsulin polypeptide is translocated across the ER membrane in a cotranslational process, and the signal sequence is cleaved off by signal peptidase.
The proinsulin molecule folds in the ER lumen, assisted by various chaperones and, in particular, by protein disulphide isomerase, which ensures that the correct disulphide bonds form. The correctly-folded proinsulin polypeptide is exported from the ER in COPII-coated vesicles which deliver it to the Golgi. At this stage the proinsulin is cleaved by furin-type proteases (pro-hormone convertases; cleave at pairs of basic residues), removing the internal C-peptide (pro-) and after a little C-terminal trimming, leaving the insulin A and B peptides covalently attached via disulphides. This is mature insulin.
The mature insulin leaves the Golgi in secretory vesicles. Since insulin secretion is regulated, these vesicles do not fuse with the cell membrane constitutively, they are stimulated to fuse by a Ca2+-dependent signalling pathway. Vesicle-membrane fusion is mediated by SNARE proteins. At this stage the insulin has been released from the pancreatic β cell. Notice, however, that the insulin has been extracytoplasmic since it was translocated into the ER: all other steps in the transport process are achieved by rounds of vesicle formation and fusion.