N-terminal basic amino acids of alfalfa mosaic virus coat protein involved in the initiation of infection.

Alfalfa mosaic virus coat protein or its messenger RNA is required in the inoculum for virus infection. The N-terminus of the coat protein is required for activity; thus, changes were made in the amino acid sequence of this region. Six coat protein mutants were tested for activity in virus infection assays in protoplasts. A coat protein mutant in which N-terminal residues 3-19 were absent was inactive; whereas, a mutant in which residues 3-11 were absent (CP deltaN9) still had 73% of wild-type activity. Substitution of alanine for the basic residues at positions 14, 17, and 18 in full-length coat protein and in CP deltaN9 resulted in mutant proteins that were inactive in infection. Thus, one, two, or three of these basic residues in CP are required for activity.