Sequence and functional features of a novel scavenger receptor homolog, SCARA5 from Yellow drum (Nibea albiflora)

As an important member in SR-As, member 5 (SCARA5) can swallow apoptotic cells and foreign bodies, and participate multiple signaling pathways to inhibit tumor occurrence, development growth and metastasis. To explore its immune function, SCARA5 was identified from the yellow drum (Nibea albiflora) according to its transcriptome data, and its full-length cDNA was 6968 bp (named as NaSCARA5, GenBank accession no: MW070211) encoding 497 amino acids with a calculated molecular weight of 55.12 kDa, which had the typical motifs of SR family, such as transmembrane helix region, coil region, Pfam collagens region and SR region. BLASTp and the phylogenetic relationship analysis illustrated that the sequences shared high similarity with known SCARA5 of teleosts. Quantitative real time RT-PCR analysis showed that NaSCARA5 was expressed in intestine, stomach, liver, kidney, gill, heart and spleen, with the highest in the spleen (24.42-fold compared with that in heart). After being infected with Polyinosinic:polycytidylic acid (PolyI:C), Vibrio alginolyticus and Vibrio parahaemolyticus, NaSCARA5 mRNA were up-regulated with time dependent mode in spleen, which suggested that NaSCARA5 might play an important role in the immune process of fish. The extracellular domain of NaSCARA5 was successfully expressed in BL21 (DE3), and yielded the target protein of the expected size with many active sites for their conferring protein-protein interaction functions. After being purified by Ni-NAT Superflow resin and renatured, it was found to bind all the tested bacteria (V.parahaemolyticus,V.alginolyticus and Vibrio harveyi). The eukaryotic expression vector of the NaSCARA5-EGFP fusion protein was constructed and transferred into epithelioma papulosum cyprini (EPC) cells, and it was mainly expressed on the cell membrane indicating that NaSCARA5 was a typical transmembrane protein. The aforementioned results indicated that NaSCARA5 played a significant role in the defense against pathogenic bacteria infection as PRRs, which may provide some further understandings of the regulatory mechanisms in the fish innate immune system for SR family.