Capsid protein self-assembles to form an icosahedral capsid with a T=3 symmetry, about 31-34 nm in diameter, and consisting of 180 capsid proteins. Plays an essential role in virion formation by interacting, via its N-terminal region, with the bipartite viral RNA genome and specifically with the 3' terminus of RNA-1 and the TA element on RNA-2. Participates also in symptom development, viral RNA accumulation and systemic movement within the host. Homomultimer. The N-terminal R domain is rich in basic residues and is essential for RNA-binding and virion assembly functions. Belongs to the icosahedral plant coat protein family.