Studies on Hybrid Comoviruses Reveal the Importance of Three-Dimensional Structure for Processing of the Viral Coat Proteins and Show That the Specificity of Cleavage Is Greater in trans Than in cis

Abstract

A series of cowpea mosaic virus (CPMV)-based hybrid comoviral RNA-2 molecules have been constructed. In these, the region encoding both the large (L) and small (S) viral coat proteins was replaced by the equivalent region from bean pod mottle virus (BPMV). The hybrid RNA-2 molecules were able to replicate in cowpea protoplasts in the presence of CPMV RNA-1. Though processing of the hybrid polyproteins by the CPMV-specific 24K proteinase at the site between the 58/48K and L proteins could readily be achieved, no processing at the site between the L and S coat proteins could be obtained even when the sequence of amino acids between the two coat proteins was made CPMV-like. As a result, none of the hybrids was able to form functional virus particles, and they could not infect cowpea plants. Comparison with the processing of the L–S site in cis in reticulocyte lysates demonstrated that the requirements for processing are more stringent in trans than in cis. The results suggest that the L–S cleavage site is defined by more than just a linear sequence of amino acids and probably involves interactions between the L–S loop and the β barrels of the viral coat proteins.