The connecting peptide, or C-peptide, is a short 31-amino-acid protein that connects insulin's A-chain to its B-chain in the proinsulin molecule. In the insulin synthesis pathway, first preproinsulin is secreted from the beta cells of the pancreas with an A-chain, a C-peptide, a B-chain, and a signal sequence. The signal sequence is cleaved from the N-terminus of the peptide by a signal peptidase, leaving proinsulin. Then the C-peptide is removed, leaving the A-chain and B-chain that constitute the insulin molecule.

to translocate polypeptides across the ER membrane

This is called a peptide bond, or peptide linkage.

The first signal required to activate a T cell is MHC(Major Histocompatibility Complex) presenting an antigen(foreign peptide) to the T cell receptor.

peptide

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