Characterization of two new Scygonadin homologous SCY4 and SCY5 in Scylla paramamosain

Abstract

Two new peptides homologous to the reported antimicrobial peptide Scygonadin were characterized in Scylla paramamosain with 48.51% and 49.23% identity in the amino acid sequences, named as SCY4 and SCY5, respectively. The mRNA transcripts and proteins of the two peptides were abundant in the ejaculatory duct of male crabs, and they were transferred to the spermatheca of female crabs along with semen during mating. Immunofluorescence analysis showed that both peptides were localized in sperm cells at different developmental stages and in the acrosomal tubes of acrosomal reactions, suggesting their potential roles in protecting sperm and in the acrosomal reactions. The analysis of progesterone stimulation in vivo showed that the expression patterns of SCY4 and SCY5 were different, with SCY5 up-regulated and SCY4 down-regulated. The recombinant proteins (rSCY4 and rSCY5) were more active against Gram-positive bacteria than the Gram-negative bacteria we tested. It is noteworthy that although rSCY4 and rSCY5 had no bactericidal effect on the pathogen Vibrio alginolyticus in vitro, both proteins could obviously enhance the survival rate of crabs challenged with V. alginolyticus. In summary, SCY4 and SCY5 may play a role similar to Scygonadin in the reproductive process, thereby maintaining internal sterility during sperm transfer or in the spermatheca of female S. paramamosain.