gClq-R/p33, a member of a new class of multifunctional and multicompartmental cellular proteins, is involved in inflammation and infection

Human gC1q-R (p33, p32, C1qBP, TAP) is a ubiquitously expressed, multiligan d-binding, multicompartmental cellular protein involved in various ligand-m ediated cellular responses. Although expressed on the surface of cells, an intriguing feature of the membrane-associated form of gC1q-R is that its tr anslated amino acid sequence does not predict the presence of either a sequ ence motif compatible with a transmembrane segment or a consensus site for a glycosylphosphatidylinositol anchor. Moreover, the N-terminal sequence of the pre-pro-protein of gC1q-R contains a motif that targets the molecule t o the mitochondria and as such was deemed unlikely to be expressed on the s urface. However, several lines of experimental evidence clearly show that g C1q-R is present in all compartments of the cell, including the extracellul ar cell surface. First, surface labeling of B lymphocytes with the membrane -impermeable reagent sulfosuccinimidyl 6-(biotinamido)hexanoate shows speci fic biotin incorporation into the surface-expressed but not the intracellul ar form of gC1q-R. Second, FAGS and confocal laser scanning microscopic ana lyses using anti-gC1q-R IgG mAb 60.11 or 74.5.2, and the fluorophore Alexa 488-conjugated F(ab ')(2) goat anti-mouse IgG as a probe, demonstrated spec ific staining of Raji cells (> 95% viable). Three-dimensional analyses of t he same cells by confocal microscopy showed staining distribution that was consistent with surface expression. Third, endothelial gC1q-R, which is ass ociated with the urokinase plasminogen activator receptor, and cytokeratin 1 bind I-125-high molecular weight kininogen in a specific manner, and the binding is inhibited dose-dependently by mAb 74.5.2 recognizing gC1q-R resi dues 204-218. Fourth, native gC1q-R purified from Raji cell membranes but n ot intracellular gC1q-R is glycosylated, as evidenced by a positive periodi c acid Schiff stain as well as sensitivity to digestion with endoglycosidas e H and E Finally, cross-linking experiments using Clq as a ligand indicate that both cC1q-R and gC1q-R are co-immunoprecipitated with and-Clq. Taken together, the evidence accumulated to date supports the concept that in add ition to its intracellular localization, gC1q-R is expressed on the cell su rface and can serve as a binding site for plasma and microbial proteins, bu t also challenges the existing paradigm that mitochondrial proteins never l eave their designated compartment. It is therefore proposed that gC1q-R bel ongs to a growing list of a class of proteins initially targeted to the mit ochondria but then exported to different compartments of the cell through s pecific mechanisms which have yet to be identified. The designation 'multif unctional and multicompartmental cellular proteins' is proposed for this cl ass of proteins.