Early and late functions of alfalfa mosaic virus coat protein can be mutated separately

To investigate the role of alfalfa mosaic virus coat protein (CP) in genome activation, asymmetric plus-strand RNA accumulation, and cell-to-cell spread of the virus, mutations were made in the CP gene and putative CP binding sites in the 3'-untranslated region (UTR) of RNA 3. Mutants that produced no CP-related peptide or CP with an N-terminal deletion of 20 amino acids were defective in all three functions. Insertion of several nonviral amino acids at position 85 of CP had little effect on genome activation and plus-strand RNA accumulation but abolished cell-to-cell spread. A mutant encoding CP with a C-terminal deletion of 21 amino acids was defective in plus-strand RNA accumulation but showed substantial levels of genome activation and cell-to-cell spread. Mutations in the 3'-UTR that interfered with CP binding affected plus-strand RNA accumulation and cell-to-cell spread. Neither CP nor CP binding sites at the 3'-end of RNA 3 were required for minus-strand RNA accumulation. The results demonstrate that early and late functions of CP can be mutated separately, indicating that different domains of CP are involved in the three functions investigated.