FIGURE 4.

Cleavage of polyubiquitin chains by PLpro from IBV. A, picture of the gel from SDS-PAGE analysis of reaction products of Lys⁴⁸-linked tetraubiquitin chains treated with IBV PLpro at different time intervals. B, analysis similar to that in A except the substrate used was Lys⁶³-linked tetraubiquitin. C, SDS-PAGE gel picture of reaction products of linear ubiquitin chains treated with PLpro from IBV. The protease could not cleave linear ubiquitin under reaction conditions similar to those used for cleaving Lys⁴⁸- and Lys⁶³-linked tetraubiquitin. M, molecular mass markers; ′, minutes.