Functional Study On A New Subfamily Of Penaeidin In The Innate Immunity Of Kuruma Shrimp

Pathogens like bacteria and virus are great threaten for normal physiological function of animals. Invertebrates have not developed a complex self-defence system as vertebrates have. They defend against harmful bacteria and viruses through the innate immune system. Antimicrobial peptides (AMPs) play an important role in pathogen elimination. As a kind of AMP specifically exists in penaeid shrimp, penaeidin can not only kill bacteria directly or inhibit their growth and proliferation, but also work as a regulatory factor in innate immunity. Penaeidins are an important family of antimicrobial peptides (AMPs) in penaeid shrimp. To date, five groups of penaeidins have been identified in penaeid shrimp. All are composed of a proline-rich N-terminus and a C-terminus containing six cysteine residues engaged in three disulfide bridges. In this study, a new type of penaeidin from Marsupenaeus japonicus was identified. The full-length penaeidin contains a unique serine-rich region and a penaeidin domain, which consists of a proline-rich region and a cysteine-rich region. Here, we classify all penaeidins into two subfamilies. All reported penaeidins are in subfamily Ⅰ (Pen-Ⅰ), and the new penaeidin identified in M. japonicus is designated as Penaeidin subfamily Ⅱ (Pen-Ⅱ). Different from Pen-Ⅰ which mainly distributed in hemocytes, MjPen-Ⅱ was expressed in all tested tissues, including hemocytes, heart, hepatopancreas, gills, stomach and intestine, and was upregulated after bacterial challenge. MjPen-Ⅱ had antibacterial activity toward both Gram-positive and Gram-negative bacteria. And the serine-rich region itself could inhibit the growth of the tested bacteria. MjPen-Ⅱ could bind to bacteria by binding to peptidoglycan, lipopolysaccharide and lipoteichoic acid on the surface of bacteria, thus promoting bacterial agglutination. The serine-rich region bound bacteria mainly by binding to peptidoglycan and lipoteichoic acid. Although the serine-rich region itself could not promote the agglutination of bacteria, it significantly enhanced the agglutination activity of MjPen-Ⅱ. The proline-rich domain had a stronger bacterial-binding activity and polysaccharide-binding activity than the cysteine-rich domain. MjPen-Ⅱ was also found to be involved in the phagocytosis of bacteria and efficiently improved the phagocytosis rate. Therefore, MjPen-Ⅱ eliminates bacteria through direct bacterial inhibition as well as by promoting phagocytosis in shrimp.