Identification and functional analysis of grouper (Epinephelus coioides) B-cell linker protein BLNK

B-cell linker protein (BLNK) is an adaptor protein that plays a crucial role in the B cell antigen receptor (BCR) signal pathway. To investigate the function of BLNK in teleost fish, we cloned a BLNK ortholog gene from the orange-spotted grouper (Epinephelus coioides). Homology analysis showed that the grouper BLNK (EcBLNK) had a 34%-77% amino acid identity in comparison to other vertebrates and shared the highest amino acid identity with BLNK from the Asian seabass Lates calcarifer. EcBLNK comprises an N-terminal SAM domain and a C-terminal B-cell linker SH2 domain. Ten tyrosine residues were well conserved between teleost fish and mammals. Tissue distribution analysis showed that EcBLNK was expressed mainly in immune organs and expression was at the highest level in head kidney. Co-localization of EcBLNK and EcCD79a was observed in transfected HEK293T cells. Overexpression of EcBLNK did not activate nuclear factor kappa-light-chain-enhancer of activated B cells. The protein level of EcBLNK in grouper head kidney leukocytes was increased by stimulation with lipopolysaccharide. In groupers infected with Cryptocaryon irritans, EcBLNK was regulated in the infected sites and the systemic organ which suggests that EcBLNK was activated in the immune response to parasite infection.