Cloning of a glycosylphosphatidylinositol-anchored alpha-2-macroglobulin cDNA from the ascidian, Ciona intestinalis, and its possible role in immunity

Molecular approaches were used to study thiolester-containing genes in the ascidian, Ciona intestinalis. RT-PCR, RACE and genome mining revealed that this animal expresses not only conventional alpha-2-macroglobulin (α2m) and two forms of C3 but also a gene encoding a glycosylphosphatidylinositol (GPI)-anchored α2m. Previously, GPI-anchored α2ms have been reported only for humans and mice. We propose that GPI-anchored α2ms constitute a third subgroup of the α2m superfamily and may represent an important evolutionary stage in the phylogeny of the thiolester containing proteins. Its occurrence in an ascidian shows its origin pre-dates the evolution of the vertebrates. In C. intestinalis this GPI-anchored α2m, designated Ciona α2m-GPI, is expressed in the hepatopancreas, circulating coelomic blood cells and the gut of adults. It is also expressed in 3-5 days old larvae. Its tissue distribution coupled with its sequence characteristics and unusual domain structure indicate that the encoded protein probably assists in host defence by entrapping and inhibiting proteases from micro-organisms.