Genetic dissection of the multiple functions of alfalfa mosaic virus coat protein in viral RNA replication, encapsidation, and movement.

Coat protein (CP) of alfalfa mosaic virus (AMV) binds as a dimer to the 3' termini of the three genomic RNAs and is required for initiation of infection, asymmetric plus-strand RNA accumulation, virion formation, and spread of the virus in plants. A mutational analysis of the multiple functions of AMV CP was made. Mutations that interfered with CP dimer formation in the two-hybrid system had little effect on the initiation of infection or plus-strand RNA accumulation but interfered with virion formation and reduced or abolished cell-to-cell movement of the virus in plants. Six of the 7 basic amino acids in the N-terminal arm of CP (positions 5, 6, 10, 13, 16, and 25) could be deleted or mutated into alanine without affecting any step of the replication cycle except systemic movement in plants. Mutation of Arg-17 interfered with initiation of infection (as previously shown by others) and cell-to-cell movement of the virus but not with plus-strand RNA accumulation or virion formation. The results indicate that in addition to the RNA-binding domain, different domains of AMV CP are involved in initiation of infection, plus-strand RNA accumulation, virion formation, cell-to-cell movement, and systemic spread of the virus.