The Kex2p proregion is essential for the biosynthesis of an active enzyme and requires a C-terminal basic residue for its function

The Saccharomyces cerevisiae prohormonc-processing enzyme Kex2p is biosynth esized as an inactive precursor extended by its N-terminal proregion. Here we show that deletion of the proregion renders Kex2p inactive both in vivo and in vitro. Absence of the proregion impaired glycosylation and stability and resulted in the retention of the enzyme in the endoplasmic reticulum. These phenotypes were partially complemented by expression of the proregion in trans. Trans complementation was specific to Kex2p proregion because ex pression of any of the seven mammalian prohormone convertase propeptides ha d no effect. These data are consistent with a model whereby Kex2p proregion functions as an intramolecular chaperone and indicate that covalent linkag e to the protein is not an absolute requirement for proregion function. Fur thermore, extensive mutagenesis revealed that, in addition to their functio n as proteolytic recognition sites, C-terminal basic residues play an activ e role in proregion-dependent Kex2p activation.