A novel multi-domain C1qDC protein from Zhikong scallop Chlamys farreri provides new insights into the function of invertebrate C1qDC proteins

• A novel multi-domain C1qDC protein was identified from scallop Chlamys farreri.
• The mRNA expression of CfC1qDC-2 could be induced by PAMP stimulation and was increased in D-hinged, veliger and eye-spot larvae.
• CfC1qDC-2 could bind various PAMPs, microorganisms and scallop apoptotic cells.
• CfC1qDC-2 could interact with human heat-aggregated IgG and IgM.
• CfC1qDC-2 could inhibit the C1q-dependent hemolysis of rabbit serum.

The C1q domain containing (C1qDC) proteins are a family of proteins possessing globular C1q (gC1q) domains, and they rely on this domain to recognize various ligands such as PAMPs, immunoglobulins, ligands on apoptotic cell. In the present study, a novel multi-domain C1qDC protein (CfC1qDC-2) was identified from scallop Chlamys farreri, and its full length cDNA was composed of 1648 bp, encoding a signal peptide and three typical gC1q domains. BLAST analysis revealed significant sequence similarity between CfC1qDC-2 and C1qDC proteins from mollusks. Three gC1q domains were predicted in its tertiary structure to form a tightly packed bell-shaped trimer, and each one adopted a typical 10-stranded sandwich fold with a jelly-roll topology and contained six aromatic amino acids forming the hydrophobic core. The mRNA transcripts of CfC1qDC-2 were mainly detected in the tissues of hepatopancreas and gonad of adult scallops, and the expression level was up-regulated in hemocytes after stimulated by LPS, PGN and β-glucan. During the embryonic development of scallop, the mRNA transcripts of CfC1qDC-2 were presented in all the detected stages, and the expression level was up-regulated from D-hinged larvae and reached the highest at eye-spot larvae. The recombinant protein of MBP-CfC1qDC-2 (rCfC1qDC-2) could bind various PAMPs including LPS, PGN, LTA, β-glucan, mannan as well as polyI:C, and different microorganisms including three Gram-negative bacteria, three Gram-positive bacteria and two yeasts, as well as scallop apoptotic cells. Meanwhile, rCfC1qDC-2 could interact with human heat-aggregated IgG and IgM, and inhibit the C1q-dependent hemolysis of rabbit serum. All these results indicated that CfC1qDC-2 could recognize not only PAMPs as a PRR, but also the apoptotic cells. Moreover, the similar structures and functions shared by CfC1qDC-2 and complement C1q provided a new insight into the evolution of C1qDC proteins in complement system.