Molecular characterization and expression analysis of three membrane-bound complement regulatory protein isoforms in the ginbuna crucian carp Carassius auratus langsdorfii

Regulators of complement activation (RCA) play a role in protecting cells from excessive complement activation in humans. cDNA corresponding to three isoforms of teleost membrane-bound RCA protein (gTecrem) have been identified in the ginbuna crucian carp. gTecrem-1 consists of seven short consensus repeats (SCRs), whereas gTecrem-2 and gTecrem-3 have four SCRs. While gTecrem-1 possesses a tyrosine phosphorylation site in its cytoplasmic region, gTecrem-2 and gTecrem-3 lack the site. Tissue distribution analysis showed that gTecrem-1 and gTecrem-2 mRNAs were expressed in almost all tissues examined, whereas gTecrem-2 expression was not significantly detected in gill, liver, or intestine. Furthermore, analysis showed that gTecrem-1 was expressed in both peripheral blood leukocytes (PBLs) and erythrocytes and was also expressed in T cell subsets such as CD4+, CD8+ T cells, and IgM+ B cells. gTecrem-2 expression was not detected in either PBLs or erythrocytes, whereas gTecrem-3 was expressed only in erythrocytes. These results suggested that gTecrem isoforms may serve different functional roles; gTecrem-1, which is expressed in T cells and possesses a tyrosine phosphorylation site, may act as a complement regulator and a cellular receptor in adaptive immunity.