Characterization of class B scavenger receptor type 1 (SRB1) in turbot (Scophthalmus maximus L.)

Class B scavenger receptor type 1 (SRB1) serves as a high-density lipoprotein (HDL) receptor essential for HDL metabolism, and plays vital roles in innate immunity. In this study, the turbot (Scophthalmus maximus) SRB1 was cloned and characterized. The gene structure consists of a coding region of 1,527 bp nucleotides dividing into 13 exons and 12 introns. Such genome structure is highly conserved among teleost fishes. The deduced SRB1 encodes 508 amino acids that mainly has a CD36 transmembrane domain. Tissue distribution of SRB1 showed the lowest expression in liver, while the highest expression was found in intestine. Significantly down-regulation pattern of SmSRB1 expression in intestine was shared after infection with Vibrio anguillarum and Streptococcus iniae. Brach and site models in CODEML program showed that SmSRB1 underwent a conservative evolutionary and three potential positive selected sites 470K, 496E, and 501Y were detected, which requires further investigation and confirmation using base-editing technologies. Subcellular localization demonstrated that turbot SRB1 was distributed in the membrane and cytoplasm. rSmSRB1 showed binding ability in vitro to bacteria, LPS, PGN, LTA and virus. Protein-protein interaction network agrees the function of SRB1 as lipoprotein receptor. Our results indicated SmSRB1 might act as co-receptors to TLRs and NLRs to modulate the immune response to pathogens. Further studies should pay attention to evaluate the specific co-receptor for SRB1 in recognition of different pathogens and selective mechanisms involved.