L-Type lectin from the kuruma shrimp Marsupenaeus japonicus promotes hemocyte phagocytosis.

L-Type lectins (LTLs) contain a luminal carbohydrate recognition domain, which exhibits homology to leguminous lectins. These type I membrane proteins are involved in the early secretory pathway of animals, and have functions in glycoprotein sorting, trafficking and targeting. Recent studies suggest that LTLs may be involved in immune responses in vertebrates, but no functional studies have been reported. This study reports an LTL, designated as MjLTL1, from the kuruma shrimp Marsupenaeus japonicus. MjLTL consists of a signal peptide, leguminous lectin domain, and transmembrane region. It was upregulated following challenge of shrimp with Vibrio anguillarum. MjLTL1 could agglutinate several bacteria with the presence of calcium, and bind to several Gram-positive and Gram-negative bacteria through lipopolysaccharide and peptidoglycan binding. MjLTL1 could enhance the clearance of V. anguillarum in vivo. MjLTL1 silencing by RNA interference could impair bacterial clearance ability. Further study suggested that MjLTL1 promoted hemocyte phagocytosis. To analyze the possible mechanism, a disintegrin and metalloprotease-like protein (MjADAM) mediating the proteolytic release of extracellular domains from the membrane-bound precursors was also studied in the shrimp. MjADAM exhibited similar tissue location and expression profiles to MjLTL1. After knockdown of MjADAM, the hemocyte phagocytosis rate also declined significantly. ADAM was reported to have an ectodomain shedding function to LTL and release the ectodomain of the lectin from cell membrane. Therefore, our results suggest that the extracellular domain of MjLTL1 might be released from the cell surface as a soluble protein by MjADAM, and function as an opsonin involved in the antibacterial immune responses in shrimp.