cDNA cloning and primary structure analysis of C1qR(P), the human C1q/MBL/SPA receptor that mediates enhanced phagocytosis in vitro.

The complement protein C1q, mannose-binding lectin (MBL), and pulmonary surfactant protein A (SPA) are structurally similar molecules that enhance phagocytic function in vitro. Monoclonal antibodies R3 and R139, which inhibit the enhancement triggered by these three ligands, were used to purify a 126,000 M(r) cell surface protein designated C1qR(P). Amino acid sequence was obtained and the corresponding cDNA was cloned. C1qR(P) is a novel type I membrane protein with the following putative structural elements: a C-type carbohydrate recognition domain, five EGF-like domains, a transmembrane domain, and a short cytoplasmic tail. All peptides identified by amino acid sequencing are encoded by the cDNA. Additionally, an anti-peptide antiserum was generated, which is reactive with C1qR(P). The data indicate that the cloned cDNA encodes the receptor that plays a role in C1q/MBL/SPA-mediated removal or destruction of pathogens and immune complexes by phagocytosis.