We have purified a serine proteinase from the membrane of U-937 cells that was inhibited in a tight-binding manner by recombinant gp120 and by peptides mimicking the V3 loop of gp120 [(1993) FEBS Lett. 317, 167-1721. This proteinase has now been characterized, both structurally and functionally. It has a dual trypsin-and chymotrypsin-like specificity, and N-terminal sequence analysis of the iirst 32 residues indicates complete identity with leukocyte cathepsin G. Cathepsin G-like material was located at the surface of U-937 cells using a monoclonal antibody directed against leukocyte cathepsin G, and polyclonal anti-cathepsin G antibodies precipitated the purified proteinase. However, the U-937 enzyme differs slightly from commercial leukocyte cathepsin G in its apparent M, because of different glycosylation. No other protein structurally related to cathepsin G was found upon screening a U-937 cDNA library using several oligonucleotide probes constructed from the membrane proteinase N-terminal amino acid sequence. The possible interaction of a cathepsin G-like proteinase at the surface of U-937 cells with the V3 loop of HIV-l gp120 is discussed.

A trypsin-like proteinase which is inhibited by recombinant gp120 and by synthetic peptides of various lengths spanning the conserved sequence of the V3 loop has been purified and partially characterized from a U-937 cell membrane extract. V3 loop peptides behave as competitive inhibitors of the enzyme, while gp120 exerts a tight-binding inhibition, reacting in stoichiometric amounts with the proteinase to provide significant inhibition. Though the properties of the U-937 membrane proteinase towards gp120 and synthetic peptides of the V3 loop resemble those of the Molt-4 T-cell tryptase TL2, these two proteinases differ by their physicochemical properties and their susceptibility to other inhibitors of serine proteinases. These results give support to the concept of a membrane-associated proteinase as a complementary or alternative receptor to the CD4, for allowing virus to enter host cells and thus spreading HIV infection.

A cell surface-associated cathepsin G has been reported to be a possible complementary factor for H1V-1 infection of U-937 cells. The effect of recombinant gpl20 (IIIB) and a series of V3 loop peptides derived from the sequence of different strains of HIV on the activity of U-937 cathepsin G was assayed. The sequence on the N-terminal side of the highly conserved GPGRAF V3 loop segment was required for interaction with cathepsin G. The inhibition was stable for several hours and there was no cleavage of the peptides derived from the HIV-I(IIIB) strain. Recombinant gpl20 (IIIB) also remained uncleared after incubation with cathepsin G for 3 h, but some cleavage occurred, generating 2 fragments (50 kDa and 70 kDa), after 16 h. Linear peptides derived from HIV-1 Mal, ELI, MN, CDC4 and SF162 strains, and consensus V3 peptides all had inhibitory properties towards cathepsin G, although they were significantly cleaved after one hour. The cleavage site was at the carboxy-terminus of Tyr 323 which is conserved in all these H1V-I strains but not in HIV-I(IIIB). There was no cleavage at the Arg residue of the GPGRAF sequence, whatever the V3 peptide sequence, the amount of proteinase, or the incubation time. We conclude that the inhibition of membrane-associated cathepsin G of U-937 cells by the gpl20 V3 loop of HIV-1 does not occur via a Kunitz-type mechanism, and that the proteinase-V3 loop interaction does not result in a significant cleavage of the V3 loop, though it has been suggested that this event is required for the entry phase of the virus.