A novel C1q family member of amphioxus was revealed to have a partial function of vertebrate C1q molecule.

C1q is the target recognition protein of the classical complement pathway and a major connecting link between innate and adaptive immunities. Its globular signature domain is also found in a variety of noncomplement protein that can be grouped together as a C1q family. In this study, we have cloned and identified a novel C1q family member in cephalochordate amphioxus and named it as AmphiC1q1. The high transcriptional levels of this gene were detected during all stages of embryonic development, and the section in situ hybridization demonstrated that AmphiC1q1 was mainly expressed in the ovary, intestine, and nerve system of mature individuals. The transcript of AmphiC1q1 was up-regulated by LPS and gram-negative bacteria, but hardly by lipoteichoic acid and Staphylococcus aureus. The recombinant AmphiC1q1 protein could not bind with N-acetyl-glucosamine and did not possess hemagglutinating activity, indicating that AmphiC1q1 could not act as its lamprey homologue. But both the full-length protein and its truncated globular domain of C1q protein could interact with LPS. Moreover, recombinant AmphiC1q1 protein could inhibit collagen-induced platelet aggregation, but the truncated globular C1q domain protein would not, indicating that the blocking activity of AmphiC1q1 protein was via the collagen region of the protein. Our study on the primitive form of C1q family in protochordate will shed a light on understanding the gradual functional evolution of C1q family and eventual formation of mammalian homologues.