Evaluation of the roles of specific regions of the Cucumber necrosis virus coat protein arm in particle accumulation and fungus transmission.

The Cucumber necrosis virus (CNV) particle is a T=3 icosahedron composed of 180 identical coat protein (CP) subunits. Each CP subunit includes a 34-amino-acid (aa) arm which connects the RNA binding and shell domains. The arm is comprised of an 18-aa "beta" region and a 16-aa "epsilon" region, with the former contributing to a beta-annular structure involved in particle stability and the latter contributing to quasiequivalence and virion RNA binding. Previous work has shown that specific regions of the CNV capsid play important roles in transmission by zoospores of the fungal vector Olpidium bornovanus and that particle expansion is essential for this process. To assess the importance of the two arm regions in particle accumulation, stability, and virus transmission, five CP arm deletion mutants were constructed. Our findings indicate that beta(-) mutants are capable of producing particles in plants; however, the arm(-) and epsilon(-) mutants are not. In addition, beta(-) particles bind zoospores less efficiently than wild-type CNV and are not fungally transmissible. Beta(-) particles are also less thermally stable and disassemble under swelling conditions. Our finding that beta(-) mutants can accumulate in plants suggests that other features of the virion, such as RNA/CP interactions, may also be important for particle stability.