Sequence upstream of the 24K protease enhances cleavage of the cowpea mosaic virus B RNA-encoded polyprotein at the junction between the 24K and 87K proteins

Abstract

To investigate cleavage at the junction between the cowpea mosaic virus (CPMV) 24K and 87K proteins, plasmids were constructed containing the sequence of bottom-component (B) RNA encoding the 110K protein plus a variable length of upstream coding sequence. Transcripts derived from these clones were translated in rabbit reticulocyte lysate and the appearance of the 87K protein was used to assess the efficiency of cleavage at the 24K-87K junction. The results show that the 110K protein, containing the contiguous sequence of the 24K and 87K proteins, is stable and that efficient cleavage at 24K–87K junction requires the presence of amino acids upstream of the 24K protease. These observations show that the 170K protein rather than the 110K protein is the precursor of the 87K protein and suggest a mechanism whereby both the B RNA-encoded 110K and 87K proteins can accumulate during infection.