Molecular and cellular biology of proprotein convertases : analysis of carboxy-terminal domain function

The convertases are a recently discovered family of mammalian serine proteases related in structure to the bacterial subtilisins and to the yeast enzyme kexin. These proteases share with the subtilisins the presence within their protein structure of a signal peptide, a prosegment, and a conserved catalytic region. However, they also possess additional P and COOH-terminal domains, which are not observed in subtilisins, but are present in their eucaryotic yeast counterpart, kexin. While the function of the P domain has been investigated, that of the COOH-terminal domain, excepting transmembrane and cytosolic tail structures of some convertases, remains unresolved.In the present manuscript, the convertases' COOH-terminal domain function was investigated through the studies of two models of multiple convertase isoforms. These isoforms, which arise through differential splicing, are structurally identical, except within their COOH-terminal region. Studies by protein overexpression in cultured mammalian cells of multiple Drosophila furin isoforms and analysis of the cleavage products generated by these enzymes in coexpression with different substrates permitted to demonstrate that structural modifications of the COOH-terminal domain do not affect the cleavage specificity of the adjoined protease. However, particular COOH-terminal assemblages may affect the processing efficiency displayed by these enzymes. Further cellular localization studies by immunofluorescence and electron microscopy of two PC5 isoforms demonstrated that specific structural motives present in the COOH-terminal domain of these proteins are indeed implicated in the subcellular localization of these endoproteases, a fact which may ultimately affect their cleavage efficiency and substrate selectivity.In conclusion, in some convertases' COOH-terminal domains, distinctive structural motives are involved in the sorting of these endoproteases to the appropriate subcellular compartment(s), and likely influence the trafficking of these proteins through the secretory pathway