SPECIFICITY OF PROHORMONE CONVERTASE ENDOPROTEOLYSIS OF PROGASTRIN INATT-20 CELLS

Biologically active peptide hormones are synthesized from larger precu rsor proteins by a variety of posttranslational processing reactions, Endoproteolytic cleavage at the Lys(74) Lys(75) dibasic processing sit e of progastrin is the major determinant for the relative distribution of gastrin heptadecapeptide and tetratriacontapeptide in tissues. Thu s, we explored the ability of two prohormone convertases, PC1/PC3 and PC2, to cleave this important site within progastrin. We expressed wil d-type human gastrin cDNA and mutant cDNAs in which the Lys(74)Lys(75) Site was changed to Lys(74)Arg(75), Arg(74)Arg(75), and Arg(74)Lys(75 ) residues in AtT-20 cells, Because AtT-20 cells express PC1/PC3 but n ot PC2, we also coexpressed a cDNA encoding PC2 in both wildtype and m utant gastrin-producing AtT-20 cells, Wild-type Lys(74)Lys(75) and mut ant Arg(74)Arg(75) progastrin processing sites were efficiently cleave d in AtT-20 cells only after coexpression of PC2, Mutant Lys(74)Arg(75 ) progastrin was readily processed in cells in the presence or absence of PC2 coexpression, but, in contrast, mutant Arg(74)Lys(75) progastr in was inefficiently cleaved regardless of PC2 coexpression, Northern analysis revealed the presence of PC2 but not PC1/PC3 in canine antral gastrin-producing G cells, These data suggest that PC2 but not PC1/PC 3 is responsible for the cleavage of the Lys(74)Lys(75) Site in wild-t ype progastrin.