EVIDENCE THAT THE 2 C1Q BINDING MEMBRANE-PROTEINS, GC1Q-R AND CC1Q-R,ASSOCIATE TO FORM A COMPLEX

Two types of widely coexpressed, highly acidic, cell membrane binding proteins that display preferential domain specificity for Clq have bee n described: a 60-kDa calreticulin homologue, designated cC1q-R, that binds to the collagen-like ''stalk'' and a 33-kDa glycoprotein with af finity for the globular ''heads'' (gC1q-R), Although the two molecules are known to be coexpressed on all cell types examined to date and of ten coelute during purification, there is no direct evidence showing t hat they associate with each other either on the membrane or when exam ined in a purified system, In this report we present the first evidenc e that 1) biotinylated cC1q-R binds to recombinant as well as native g C1q-R, as assessed by solid phase ELISA; 2) binding sites for cC1q-R a re located within N-terminal residues 76 through 93 of the mature form of gC1q-R and within residues 204 through 218; 3) this interaction is inhibited by two mAbs, 60.11 and 46.23, that recognize primarily epit opes within the N terminus of gC1q-R corresponding to residues 74 thro ugh 96 and by mAb 74.5.2 that recognizes epitopes within residues 204 through 218; and 4) biotinylated cC1q-R binds to microtiter-fixed Raji and K562 cells, and this interaction is inhibited by mAb 60.11, Furth ermore, coimmunoprecipitation analysis of Raji cell membranes with ant i-gC1q-R mAbs showed the presence of cC1q-R in addition to gC1q-R. Tak en together, the evidence suggests that cC1q-R is able to form a compl ex with gC1q-R and may associate with gC1q-R on the cell surface.