CDNA CLONING AND PRIMARY STRUCTURE-ANALYSIS OF C1QR(P), THE HUMAN C1QMBL/SPA RECEPTOR THAT MEDIATES ENHANCED PHAGOCYTOSIS IN-VITRO/

The complement protein C1q, mannose-binding lectin (MEL), and pulmonar y surfactant protein A (SPA) are structurally similar molecules that e nhance 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 clone d. C1qR(p) is a novel type I membrane protein with the following putat ive structural elements: a C-type carbohydrate recognition domain, fiv e EGF-like domains, a transmembrane domain, and a short cytoplasmic ta il. All peptides identified by amino acid sequencing are encoded by th e cDNA. Additionally, an anti-peptide antiserum was generated, which i s reactive with C1qR(p). The data indicate that the cloned cDNA encode s the receptor that plays a role in C1q/MBL/SPA-mediated removal or de struction of pathogens and immune complexes by phagocytosis.