IDENTIFICATION OF FUNCTIONAL DOMAINS ON GC1Q-R, A CELL-SURFACE PROTEIN THAT BINDS TO THE GLOBULAR HEADS OF C1Q, USING MONOCLONAL-ANTIBODIESAND SYNTHETIC PEPTIDES

A membrane protein (33 kDa) that binds to the globular ''heads'' of Cl q (gC1q-R) has been recently described, The full length cDNA encoding gC1q-R has been cloned, expressed in E. coli and using the purified re combinant protein (rgC1q-R) as an immunogen, a panel of IgG monoclonal antibodies (MAb) has been produced by fusion of spleen cells from hyp erimmunized BALB/c mice with NSO mouse myeloma partners, From this fus ion, 60 anti-gC1q-R hybridomas were selected and evaluated for their a bility to (1) discriminate between the mature form (MF) of gC1q-R (res idues 74-282) and a truncated form (TF) lacking residues 74-95, which contains a major Clq binding site, (2) recognize two functionally defi ned synthetic peptides derived from the NH2-(XN(18)) and COOH-(XC(15)) terminus of gC1q-R, and (3) bind to microtiter well fixed intact Raji cells, Several clones were identified: MAbs 46.23 and 60.11 (IgG(1 ka ppa), reacted strongly with ELISA plate-fixed intact Raji and K562 cel ls, MF, and the XN(18) peptide, but had poor or no reactivity with TF; MAbs 74.5.2 > 25.15 (IgG(1 kappa)) recognized both MF and TF and are directed against epitopes in the XC(15) peptide that contains a bindin g site for high-molecular-weight kininogen and Factor XII.