The sea urchin genome

Here’s the relevant chunk of the paper. Does that help?

The complement system. The complement system of vertebrates is a complex array of soluble serum proteins and cellular receptors arranged into three activation pathways (classical, lectin, and alternative) that converge and activate the terminal or lytic pathway. This system opsonizes pathogenic cells for phagocytosis and sometimes activates the terminal pathway, which leads to pathogen destruction. An invertebrate complement system was first identified in the sea urchin [for reviews, see (39, 40)], and the analysis of the genome sequence presented a more complete picture of this important immune effector system. In chordates, collectins initiate the lectin cascade through members of the mannose-binding protein (MBP)—associated protease (MASP)/C1r/C1s family. Several genes encoding collectins, C1q and MBP, have been predicted (39) and were present in the genome; however, members of the MASP/C1r/C1s family were not identified. There was no evidence for the classical pathway, which links the complement cascade with immunoglobulin recognition in jawed vertebrates. The alternative pathway is initiated by members of the thioester protein family, which, in the sea urchin, was somewhat expanded with four genes. Two of the thioester proteins, SpC3 and SpC3-2, are known to be expressed, respectively, in coelomocytes and in embryos and larvae. Furthermore, there were three homologs of factor B, the second member of the alternative pathway (41). The terminal complement pathway in vertebrates acts to destroy pathogens or pathogen-infected cells with large pores called membrane attack complexes (MACs). Twenty-eight gene models were identified that encode MAC- perforin domains, but none of these had the additional domains expected for terminal complement factors (C6 through C9). Instead, these are members of a novel and very interesting gene family with perforin-like structure. In vertebrates, perforins carry out cell-killing functions by cytotoxic lymphocytes through the formation of small pores in the cell membranes. If the complement system in the sea urchin functions through multiple lectin and alternative pathways in the absence of the lytic functions of the terminal pathway, the major activity of this system is expected to be opsonization.