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 encephalomyocarditis virus, is a substrate for rapid,
ubiquitin-mediated degradation in vitro.
Ubiquitin was shown to stimulate the turnover of the hepatitis virus 3C
protease, and labeled
protease was found to become incorporated into a mixture of high molecular weight species, which is characteristic of conjugation with
polyubiquitin chains. In the presence of methylated
ubiquitin, a new 33 kDa species formed, consistent with the generation of a monoubiquitin-3C
protease conjugate. The rate of degradation of the 3C
protease was reduced by inhibitors of the
26S proteasome. 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
3C proteases compete with each other for conjugation with
ubiquitin and for degradation. This suggests that the two
3C proteases are both recognized by the same
ubiquitin system
enzyme, or
enzymes, responsible for selecting them as targets for destruction.