A MUTANT KEX2 ENZYME WITH A C-TERMINAL HDEL SEQUENCE RELEASES CORRECTLY FOLDED HUMAN INSULIN-LIKE GROWTH FACTOR-I FROM A PRECURSOR ACCUMULATED IN THE YEAST ENDOPLASMIC-RETICULUM

Mutations in the pro region of the yeast DNA hybrid of prepro-alpha-factor and human insulin-like growth factor-I (IGF-1) cause the accumulation, in the yeast Saccharomyces cerevisiae, of an unglycosylated precursor protein where the pre sequence is missing. The prepro sequence of the prepro-alpha-factor consists of a pre or signal sequence and a proregion which possesses three sites for N-glycosylation. Isolation of a precursor, where the pro region is still linked to IGF-1 through a pair of dibasic amino acid residues, implies that the polypeptide may have translocated into the endoplasmic reticulum (ER) but has not been processed by the Golgi membrane-bound Kex2 endoprotease. However, the lack of any N-glycosylation in the translocated polypeptide is surprising. The mutated pro region, can be processed, in vitro, by treatment with a soluble form of the Kex2 enzyme. It is also possible to release the pro region, in vivo, by coexpressing a mutant Kex2 protease which is partially retained in the ER with the help of the C-terminal tetrapeptide sequence, HDEL. The mature IGF-1, which is secreted from the intracellular pool of precursor proteins, is predominantly an active, monomeric molecule, corroborating observations that early removal of the pro region before folding in the ER helps to prevent aberrant intermolecular disulfide-bond formation in IGF-1. These results have revealed the utility of the ER-retained Kex2 enzyme as a novel in vivo biochemical tool.