STOICHIOMETRY AND HETEROGENEITY OF THE PRO-REGION CHAIN IN TETRAMERICHUMAN CATHEPSIN-C

The subunit structure and composition of mature human cathepsin C, an oligomeric cysteine proteinase, has been characterised in detail. The heavy chain, light chain and pro-region peptides are shown to be held together solely by non-covalent interactions, and to be present in equ imolar ratio, suggesting an important structural role for the residual pro-region chain which is strongly bound to the enzyme. The mass of t he light chain, as determined by mass spectrometry, combined with its N-terminal sequence, determines the position of cleavage from the heav y chain. Amino-acid sequencing has led to definition of the 13.5 kDa N -terminal part of the pro-region which remains in the mature enzyme, t he C-terminal moiety of 10 kDa being cleaved out and lost from the pro -peptide on activation. The residual pro-region is heterogeneous, a pr oportion being intact and the remainder being cleaved tit alternative positions 58 or 61, yielding two smaller peptides joined by a disulphi de bond. The proportion of cleaved form was found to vary with tissue and enzyme preparation but did not affect enzyme activity. The molecul ar masses cf the constituent chains after deglycosylation lead to a pr otein mass of 158 kDa. All four potential glycosylation sites are glyc osylated. (C) 1998 Elsevier Science B.V.