Four C1q domain-containing proteins involved in the innate immune response in Hyriopsis cumingii

• Four C1q domain-containing proteins (HcC1qDC1-4) were identified from Hyriopsis cumingii.
• HcC1qDC1-4 were widely expressed in the several tissues.
• HcC1qDC1-4 showed different expression profiles under challenge with bacteria.
• HcC1qDC1-4 showed different binding activity to bacteria.

C1q is a key subcomponent of the complement C1 complex. This subcomponent contains a globular C1q (gC1q) domain with remarkable ligand binding properties. C1q domain-containing (C1qDC) proteins are composed of all proteins with a gC1q domain. C1qDC proteins exist in many invertebrates and recognize non-self-ligands. In our study, four C1qDC genes, namely, HcC1qDC1–HcC1qDC4, were identified from Hyriopsis cumingii. HcC1qDC1–HcC1qDC4 encode a protein of 224, 204, 305, and 332 amino acids, respectively. All C1qDC proteins consist of a gC1q domain at the C terminal. In addition to the gC1q domain, a coiled-coil region is found in HcC1qDC4. Multiple alignments and phylogenetic tree analysis revealed that the C1qDC proteins highly differ from one another. Tissue distribution analysis demonstrated that HcC1qDC1–HcC1qDC4 are widely distributed in hemocytes, hepatopancreas, gills, mantle, and foot. These C1qDC genes are regulated by bacteria to varying degrees. These recombinant HcC1qDC proteins exhibit a binding activity against different bacterial species. Our results may suggest the roles of HcC1qDC genes in anti-bacterial immune defense.