Identification of a protein binding to the phagocytosis activating protein (PAP) in immunized black tiger shrimp

The phagocytosis activating protein (PAP) gene was isolated from Penaeus monodon infected with the white spot syndrome virus (WSSV). The endocytosis pathway of GST–PAP (glutathione-S-transferase–PAP) that caused activation of phagocytosis of shrimp haemocytes was investigated. In this study, a yeast two-hybrid screening assay of a WSSV-infected P. monodon cDNA library, was performed using PAP as bait to test for the interaction between PAP and other proteins in WSSV-infected shrimp. An alpha-2-macroglobulin (α2M) was one of the proteins that interacted with PAP. A GST pull-down assay showed that GST–PAP was capable of co-precipitating α2M whereas GST itself was not. An intramuscular injection with inactivated WSSV caused a significant increase in the expression of both the PAP and α2M gene in the haemolymph. The highest expression of both genes was detected at 24 h post-injection and remained constant for 1 week. Moreover, the uptake of GST–PAP by shrimp haemocytes was higher in the presence of α2M than in its absence. The results indicated that α2M may facilitate the entry of GST–PAP into phagocytic cells and increase the survival rate of the shrimp after being infected with WSSV.