Identification of C1q receptor associated surface molecules on the monocyte‐like cell line U937

The C1q receptor (C1qR) comprises of at least two unrelated surface proteins, which bind C1q via either its globular heads (gC1qR) or collagen tail (cC1qR). Although these non‐transmembrane proteins bind C1q and induce biological responses, the mechanism by which they do so is incompletely understood. The present studies were undertaken to identify potential new receptors or associated molecules that may have eluded identification due to limitations of previous techniques. Therefore, we used solubilized membrane proteins from U937 cells (total 2×10 8 cells) that were first surface biotinylated by a method that targeted either the amino groups or the carbohydrate moieties of proteins. Then, immunocapture ELISA was used to confirm that the known receptors—gC1qR and cC1qR—are surface labeled. Next, we used anti‐gC1qR mAb 74.5.2 to identify membrane proteins that are co‐captured with gC1qR. The results showed the presence of CD44 and β1 integrin. Furthermore, when membrane proteins were incubated with C1q coated wells, both CD44 and β1 integrin were bound to C1q in addition to the expected gC1qR and cC1qR. These results indicate that C1qR probably consists of transmembrane (CD44 and β1 integrin) and non‐transmembrane (gC1qR and cC1qR) surface proteins that together forge a docking/signaling partnership. [Supported by NIH‐NIAID Grant R01 AI‐060866]