Identification, Molecular Characterisation, Expression and Functional Analysis of FADD in Response to Streptococcus agalactiae Infection in Nile Tilapia

Fas-associated death domain protein (FADD) was initially identified for its involvement in apoptosis signal transduction, while subsequent research has revealed its additional functions in proliferation, cell cycle regulation, inflammation, and innate immunity. However, current research on FADD predominantly focuses on mammals, with only limited studies conducted on fish. In this study, we cloned FADD from Nile tilapia (Oreochromis niloticus) and analysed its sequence characteristics. Subsequently, qPCR was used to detect the gene expression of FADD in healthy tissues and its changes in Streptococcus agalactiae infected tissues. After obtaining FADD through prokaryotic expression, its ability to bind to S. agalactiae was assessed using ELISA. Finally, the growth of S. agalactiae was assessed by adding FADD to the bacterial culture medium. The results indicate that the coding sequence of FADD spans 585 bp, encoding 194 amino acids. Predicted to be an intracellular protein, FADD exhibits high hydrophilicity and lacks transmembrane regions, with a structure predominantly composed of α-helices. FADD is widely expressed, with the highest levels found in the gill. Notably, the expression of FADD in peripheral blood, spleen, head kidney, and brain tissues changed significantly with the invasion of S. agalactiae. Furthermore, ELISA and absorbance assays confirmed that FADD binds to S. agalactiae and inhibits its growth, indicating the participation in tilapia resistance against S. agalactiae infection.