Helicase ATPase activity of the Tobacco mosaic virus 126-kDa protein modulates replicase complex assembly

Mutations disrupting helicase domain motifs of the Tobacco mosaic virus 126/183-kDa proteins were investigated for their effect on replicase function and assembly. These mutations inhibited virus replication but did not affect 126-kDa induced N gene resistance or RNAi suppression. However, in vivo expressed 126-kDa motif mutants yielded two distinct cytoplasmic phenotypes that correlated with ATPase activity. Specifically, ATPase active 126-kDa proteins produced small cytoplasmic bodies that resembled the ovoid granular-like bodies found early in virus infection while 126-kDa proteins defective in ATPase activity produced large tubule containing cytoplasmic bodies similar to those observed late in infection. Additional studies indicate that the helicase ATPase activity resides predominantly within monomer and dimer helicase forms and that motifs affecting ATPase activity induce alterations in helicase assembly. Combined these findings indicate that helicase ATPase activity modulates the progression of replicase complex assembly and maturation.