The family of subtilisin/kexin like pro-protein and pro-hormone convertases: Divergent or shared functions

The envelope glycoprotein of human immunodeficiency virus (HIV) initiates infection by mediating fusion of the viral envelope with the cell membrane. Fusion activity requires proteolytic cleavage of the gp160 protein into gp120 and gp41 at a site containing several arginine and lysine residues. Activation at basic cleavage sites is observed with many membrane proteins of cellular and viral origin. We have recently found that the enzyme activating the haemagglutinin of fowl plague virus (FPV), an avian influenza virus, is furin. Furin, a subtilisin-like eukaryotic endoprotease, has a substrate specificity for the consensus amino-acid sequence Arg-X-Lys/Arg-Arg at the cleavage site. We show here that the glycoprotein of HIV-1, which has the same protease recognition motif as the FPV haemagglutinin, is also activated by furin.

Subtilases are members of the family of subtilisin-like serine proteases. Presently, greater than 50 subtilases are known, greater than 40 of which with their complete amino acid sequences. We have compared these sequences and the available three-dimensional structures (subtilisin BPN', subtilisin Carlsberg, thermitase and proteinase K). The mature enzymes contain up to 1775 residues, with N-terminal catalytic domains ranging from 268 to 511 residues, and signal and/or activation-peptides ranging from 27 to 280 residues. Several members contain C-terminal extensions, relative to the subtilisins, which display additional properties such as sequence repeats, processing sites and membrane anchor segments. Multiple sequence alignment of the N-terminal catalytic domains allows the definition of two main classes of subtilases. A structurally conserved framework of 191 core residues has been defined from a comparison of the four known three-dimensional structures. Eighteen of these core residues are highly conserved, nine of which are glycines. While the alpha-helix and beta-sheet secondary structure elements show considerable sequence homology, this is less so for peptide loops that connect the core secondary structure elements. These loops can vary in length by greater than 150 residues. While the core three-dimensional structure is conserved, insertions and deletions are preferentially confined to surface loops. From the known three-dimensional structures various predictions are made for the other subtilases concerning essential conserved residues, allowable amino acid substitutions, disulphide bonds, Ca(2+)-binding sites, substrate-binding site residues, ionic and aromatic interactions, proteolytically susceptible surface loops, etc. These predictions form a basis for protein engineering of members of the subtilase family, for which no three-dimensional structure is known.