The Viral Nucleocapsid Protein of Transmissible Gastroenteritis Coronavirus (TGEV) Is Cleaved by Caspase-6 and -7 during TGEV-Induced Apoptosis

The transmissible gastroenteritis coronavirus (TGEV), like many other viruses, exerts much of its cytopathic effect through the induction of apoptosis of its host cell. Apoptosis is coordinated by a family of cysteine proteases, called caspases, that are activated during apoptosis and participate in dismantling the cell by cleaving key structural and regulatory proteins. We have explored the caspase activation events that are initiated upon infection of the human rectal tumor cell line HRT18 with TGEV. We show that TGEV infection results in the activation of caspase-3, -6, -7, -8, and -9 and cleavage of the caspase substrates eIF4GI, gelsolin, and α-fodrin. Surprisingly, the TGEV nucleoprotein (N) underwent proteolysis in parallel with the activation of caspases within the host cell. Cleavage of the N protein was inhibited by cell-permeative caspase inhibitors, suggesting that this viral structural protein is a target for host cell caspases. We show that the TGEV nucleoprotein is a substrate for both caspase-6 and -7, and using site-directed mutagenesis, we have mapped the cleavage site to VVPD(359)↓. These data demonstrate that viral proteins can be targeted for destruction by the host cell death machinery.