Epitope analysis of gctq-r using monoclonal antibodies

There are two types of ubiquitously coexpressed, highly acidic cell membrane binding proteins which display preferential domain specificity for Clq: a 60 kDa, calreticulin homologue, which binds to the 'collagen-like' domain (cClq-R) and a 33 kDa glycoprotein with affinity for the globular 'heads'.The mature form (MF) of gClq-R corresponding to residues 74-282 as well as a truncated form (TF) lacking residues 74-97 present in the mature form have been cloned and expressed in E.coli and several IgGl monoclonal antibodies (MAbs) have been raised against MF and characterized. Two of these, 60.11 and 46.23 were very strongly reactive with MF as well as with a synthetic peptide corresponding to the NH₂-terminus (residues 76-93) but only weakly rective with TF. A second set of antibodies 25.1 and 74.5.3 recognized both MF and TF indicating that they are directed against an epitope(s) which is common to both. Further analyses showed that 25.1 recognizes epitopes contained within residues 204-218 of gClq-R. However, although all of these MAbs reacted strongly with microtiter plate fixed Raji and K562 cells only 60.11 reacted weakly at best with the same cells in suspension as assessed by FACS analysis and confocal microscopy. The data suggest that that these epitopes are cryptic in circulating cells and not accessible to the MAbs. Furthermore, since the NHz-terminus of gClq-R contains a major Clq binding site, MAbs 60.11 and 46.23 are likely to be useful tools in elucidating some aspects of gClq-R structure and function.