Molecular mechanism of the interactions between white spot syndrome virus anti-apoptosis protein AAP-1 (WSSV449) and shrimp effector caspase

AAP-1 (WSSV449), an anti-apoptosis protein encoded by white spot syndrome virus (WSSV), blocked apoptosis in insect cells (SF9) induced by Penaeus monodon effector caspase (Pm caspase). Here, to characterize in detail the anti-Pm caspase activity of AAP-1, both proteins were expressed and purified from Escherichia coli and their interactions were assayed in vitro. We found that although AAP-1 could inhibit Pm caspase activity, the inhibition was not as efficient as that of baculovirus anti-apoptosis protein P35. We further confirmed the binding and cleavage of AAP-1 by Pm caspase, and detected three AAP-1 cleavage products. Mutational analysis and protein N-terminal sequencing revealed that whereas both Asp233 and Asp272 residues of AAP-1 are involved in binding and cleavage by Pm caspase, only the Asp272 is involved in Pm caspase inhibition. Asp233, on the other hand, negatively regulates AAP-1's anti-Pm caspase activity. Lastly, AAP-1 homotypically interacts with each other both in vitro and in insect cells.