Evidence for two nonoverlapping functional domains in the potato virus X 25K movement protein

To study subdomain organization of the potato virus X (PVX) movement protei n (MP) encoded by the first gene in the triple gene block (TGB), we mutated the 25-kDa TGBp1 protein. The N-terminal deletion of the helicase motifs I , IA, and II resulted in toss of the ATPase activity and RNA binding. A fra meshift mutation truncating the C-terminal motifs V and VI gave rise to inc rease of the TGBp1 ATPase activity and had little effect on RNA binding in vitro. Fusions of the green fluorescent protein with 25-kDa MP and its deri vative lacking motifs V-VI exhibited similar fluorescence patterns in epide rmal cells of Nicotiana benthamiana leaves. Cell-to-cell movement of the 25 K-deficient PVX genome was not complemented by the TGBp1 of Plantago asiati ca mosaic potexvirus (PIAMV) but was efficiently complemented by a chimeric TGBp1 consisting of the N-terminal part of PIAMV protein (motifs I-IV) and the PVX-specific C-terminal part (motifs V-VI). These results suggest that NTP hydrolysis, RNA binding, and targeting to the specific cellular compar tment(s) are associated with the N-terminal domain of the TGBp1 including t he helicase motifs I-IV and that the C-terminal domain is involved in speci fic interactions with other virus proteins. (C) 1999 Academic Press.