Identification and characterization of a protein destruction signal in theencephalomyocarditis virus 3C protease

The amino acid sequence LLVRGRTLW, which is probably located in a strand-tu rn-strand structure, has been identified as a protein destruction signal in the rapidly degraded encephalomyocarditis virus 3C protease. Mutations wit hin this sequence reduced the susceptibility of the 3C protease toward ubiq uitination and degradation in rabbit reticulocyte lysate. This signal is tr ansferable, since poliovirus 3C protease, which is a poor ubiquitin-mediate d proteolytic system substrate, was found to be ubiquitinated and degraded when the signal sequence was either generated at an internal location in th e protein or fused to the N terminus. An evaluation of the behavior of 3C p rotease proteins containing mutations in the signal region indicates that c onsiderable variability in the primary structure is tolerated, although the conservation of certain features appears to be required for signal functio n. Two E3 ubiquitin-protein ligases that recognize the encephalomyocarditis virus 3C protease as a substrate were also partially purified. One of thes e was identified as the previously described E3 alpha, and this was shown t o require the destruction signal sequence to catalyze efficiently the ubiqu itination of the 3C protease. The other is a UbcB-dependent E3 that appears to recognize a different, unidentified feature of the 3C protease.