EVALUATION OF THE SUSCEPTIBILITY OF THE 3C PROTEASES OF HEPATITIS-A VIRUS AND POLIOVIRUS TO DEGRADATION BY THE UBIQUITIN-MEDIATED PROTEOLYTIC SYSTEM

The picornavirus 3C proteases are required for the processing of viral polyproteins during infections of host cells. Here we report that the 3C protease of the hepatitis A virus, like that of the encephalomyoca rditis virus, is a substrate for rapid, ubiquitin-mediated degradation in vitro. Ubiquitin was shown to stimulate the turnover of the hepati tis virus 3C protease, and labeled protease was found to become incorp orated into a mixture of high molecular weight species, which is chara cteristic of conjugation with polyubiquitin chains. In the presence of methylated ubiquitin, a new 33 kDa species formed, consistent with th e generation of a monoubiquitin-3C protease conjugate. The rate of deg radation of the 3C protease was reduced by inhibitors of the 26S prote asome. A similar evaluation of the 3C protease of poliovirus revealed that it is stable protein and is not conjugated with ubiquitin. It was also determined that the hepatitis A and encephalomyocarditis virus 3 C proteases compete with each other for conjugation with ubiquitin and for degradation. This suggests that the two 3C proteases are both rec ognized by the same ubiquitin system enzyme, or enzymes, responsible f or selecting them as targets for destruction. (C) 1997 Academic Press.