Structure and homology of human C1q receptor (collectin receptor)

In this paper we report partial amino acid sequence for C1q receptor (C1qR). The N-terminal amino acid sequence of isolated C1qR and the sequences of peptides obtained by V8/trypsin digestion show a high degree of similarity to the cDNA-derived amino acid sequence of a human protein which was initially reported as a component of RoSSA and subsequently as calreticulin. This sequence in turn shows homology with Onchocerca volvulus antigen (RAL-1) and B50 murine melanoma antigen. A component of approximately 53,000 MW, isolated from human spleen, was found to have identical mobility on SDS-PAGE to C1qR and identical N-terminal sequence, but a different overall charge. Human antibodies from Sjögren's syndrome patients did not recognize C1qR, but showed positive reaction with the purified 53,000 MW component from spleen. Rabbit antibodies against denatured C1qR, in contrast, recognized both C1qR and the purified 53,000 MW component. The 53,000 MW spleen component thus has an identical N-terminal sequence to calreticulin, and to the reported RoSSA component, and is recognized by antibodies in Sjögren's syndrome sera. The data obtained indicate that C1qR and the reported calreticulin/RoSSA component are similar but not identical molecules, which belong to the same protein superfamily.