IDENTIFICATION ON MELANOMA-CELLS OF P39, A CYSTEINE PROTEINASE THAT CLEAVES C3, THE 3RD COMPONENT OF COMPLEMENT - AMINO-ACID-SEQUENCE IDENTITIES WITH PROCATHEPSIN-L

We previously identified, on normal or tumour cells, two membrane prot einases, p57 and p65, that cleave human C3, the third component of com plement, thus regulating C3's biological properties. Whereas p57 was p urified from human erythrocytes, p65 was identified using polyclonal a nti-p57 antibodies on a human melanoma cell line resistant to compleme nt lysis. Analysis of cell distribution of C3-cleaving proteinases est ablished that DSm, a murine melanoma cell line, expressed a C3-cleavin g proteinase distinct from p57 and p65 proteinases. Thus we purified t he C3-cleaving proteinase solubilized from membranes of DSm cells. The purified proteinase, termed 'p39' on the basis of its molecular mass of 39 kDa, was identified, using specific proteinase inhibitors, as a cysteine proteinase. Anti-p39 antibodies, prepared against highly puri fied p39, localized the p39 C3-cleaving proteinase mainly at the cell surface and demonstrated that p39 is also secreted. Anti-p39 antibodie s inhibited solubilized C3-cleaving activity. Preincubation of DSm cel ls with anti-p39 F(ab')(2) fragments increased up to 60% complement ce ll susceptibility. Amino acid analysis of N-terminal and three other r egions of p39 demonstrated that this C3-cleaving proteinase carries 10 0% identity within four regions of procathepsin L. This is the first d emonstration that a melanoma cell line expresses on its surface and se cretes a p39 C3-cleaving cysteine proteinase that shares sequence iden tities with procathepsin L. Thus the p39 cysteine proteinase represent s a new member of the C3-cleaving proteinase family associated with, a nd/or expressed on, the cell surface.