This review examines the multifaceted role of coat protein (CP) in the replication cycle of alfalfa mosaic virus (AMV) and ilarviruses, members of the Bromoviridae plant virus family. AMV and ilarviruses have tripartite RNA genomes packaged separately into particles. A key early discovery was that infection requires CP in the inoculum—a process termed "genome activation." The article details CP-RNA interactions through structural and mutational studies, identifying specific binding sites at the 3' ends of viral RNAs involving characteristic AUGC sequence motifs and stem-loop structures. These studies reveal that CP N-terminal regions, particularly an arginine at position 17 in AMV CP, are critical for RNA binding and genome activation. The review contrasts AMV and ilarviruses with other Bromoviridae members (bromoviruses and cucumoviruses), which lack this CP requirement, instead possessing tRNA-like structures at their 3' termini. Recent data demonstrate CP's role in translation initiation and replication complex formation, though CP itself does not participate in minus-strand RNA synthesis. The conserved interaction surfaces and consensus RNA-binding sequences suggest functional conservation between different plant viruses.

Key findings

• Coat protein binding to conserved AUGC motifs and stem-loop structures in the 3' untranslated regions of AMV and ilarvirus RNAs is essential for genome activation and infection initiation
• The arginine residue at position 17 of AMV CP N-terminus is crucial for RNA binding, with equivalent residues conserved in ilarvirus CPs
• CP is not required for minus-strand RNA synthesis but plays essential roles in translation, genome activation, and possibly replication complex formation
• A consensus RNA-binding sequence in plant virus CPs shares homology with RNA-binding motifs found in replicase proteins of other viruses, suggesting broader functional conservation