White spot syndrome virus proteinsWSSV134 andWSSV322 have beenshown to bind with the p20domain
(residues 55e214) of Penaeus monodon caspase (PmCasp) protein through yeast two-hybrid screening.
Binding was confirmed for the p20 domain and the full-length caspase by co-immunoprecipitation.
WSSV134 is also known as the WSSV structural protein VP36A, but no function or conserved domains
have been ascribed toWSSV322. Discovery of the caspase binding activity of these two proteins led to an
investigation of their possible anti-apoptotic roles. Full-length PmCasp was confirmed to be an effector
caspase by inducing apoptosis in transfected Sf-9 cells as assessed by DAPI staining. Using the same cell
model, comparison of cells co-transfected with PmCasp and either WSSV134 or WSSV322 revealed that
both of the binding proteins had anti-apoptotic activity. However, using the same Sf-9 protocol with antiapoptosis
protein-1 (AAP-1; also called WSSV449) previously shown to bind and inactivate a different
effector caspase from P. monodon (Pm caspase) did not block apoptosis induced by PmCasp. The results
revealed diversity in effector caspases and their viral protein inhibitors in P. monodon.
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