Identification of a human insulinoma cDNA encoding a novel mammalian protein structurally related to the yeast dibasic processing protease Kex2.

We have identified a human insulinoma cDNA (PC2) that encodes a protein homologous to the precursor processing Kex2 endoprotease of yeast by using a polymerase chain reaction to detect and amplify conserved sequences within the catalytic site. The 638-residue amino acid sequence of PC2 begins with a cleavable signal peptide, indicating that it enters the secretory pathway, and contains a 282-residue domain that is homologous to the catalytic modules of both Kex2 and the related bacterial subtilisins. Within this region 49 and 27% of the amino acids are identical to those in the aligned Kex2 and subtilisin BPN' sequences, respectively, and the catalytically essential Asp, His, and Ser residues are all conserved. Northern blot analysis revealed the presence of 2.8- and 5.0-kilobase hybridizing bands in mRNA from the insulinoma. The PC2 protein also shows great similarity to the incomplete NH2-terminal sequence of the human furin gene product, a putative membrane-inserted receptor-like molecule. We propose that PC2 is a member of a family of mammalian Kex2/subtilisin-like proteases that includes members involved in a number of specific proteolytic events within cells, including the processing of prohormones.