Asignal sequence (gray) at the n-terminal end of the insulin precursor protein targets the protein to the lumen of the er. in the er, the signal sequence is cleaved off, and three disulfide bonds are formed between pairs of cys residues. proteases then cleave the precursor protein at two sites, each adjacent to a pair of basic amino acids (called dibasic sites, green), releasing the c peptide (blue). finally, the dibasic sites are removed. this generates the mature insulin protein, consisting of the a (yellow) and b (purple) chains joined by a pair of disulfide bonds. it is this mature form of the insulin protein that is secreted into the blood.