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Complement component 1q (C1q) with a characteristic C1q globular domain is an important pattern recognition molecule in the classical complement systems and plays a major role in the crosslinking between innate immunity and specific immunity in vertebrates. In this study, a homologous gene encoding typically C1q domains was obtained from the razor clam Sinonovacula constricta (designated ScC1qDC) by rapid amplification of the cDNA end. The full-length cDNA of ScC1qDC was 1225 bp in length with a 5′UTR of 258 bp, a 3′UTR of 223 bp, and an open reading frame of 744 bp encoding a polypeptide of 247 amino acids containing a typical C1q globular domain. The mRNA transcripts of ScC1qDC were constitutively transcribed in all examined tissues with higher expression in the hepatopancreas. Time-course expression analysis indicated that ScC1qDC was significantly up-regulated both in hepatopancreas and gills after Vibrio parahaemolyticus challenge. The recombinant ScC1qDC (rScC1qDC) displayed high binding activities to various pathogen-associated molecular patterns, including LPS, PGN, and MAN. Recombinant ScC1qDC showed no agglutinating activity to Gram-positive bacterium of Micrococcus luteus but showed obvious activities towards all the three examined Gram-negative bacteria. All our results indicated that ScC1qDC might be served as a pattern recognition receptor and promoted Gram-negative bacteria agglutination during the pathogen challenge.Full-length cDNA of C1q-domain-containing protein was identified from Sinonovacula constricta (ScC1qDC).ScC1qDC was ubiquitously transcribed in all examined tissues with higher expression in the hepatopancreas.ScC1qDC mRNA was rapidly induced in hepatopancreas and gill after Vibrio parahaemolyticus challenge.The recombinant ScC1qDC (rScC1qDC) displayed higher binding activities to LPS, PGN and MAN.rScC1qDC showed no agglutinating activity to gram-positive bacteria of Micrococcaceae luteus, but obvious activities to all examined gram-negative bacteria.