Mutations in the potyvirus helper component protein: effects on interactions with virions and aphid stylets.

Mutations of K --> E in the highly conserved 'KITC' motif of the potyvirus helper component (HC) protein result in loss of HC function in aphid transmission, presumably because of inability to interact with virions, stylets or both. In this study we show that HC of potato virus C (PVC), a naturally occurring variant of potato virus Y (PVY) that has the K --> E mutation, lacks the ability to be retained in stylets, whereas PVY HC is retained. The K --> E mutation in either PVC or a site-directed mutant of tobacco etch virus (TEV) did not hinder binding to capsid protein, nor did deletion of the N-terminal 107 aa of TEV HC. An additional mutation, F -->, L at aa 10 of TEV HC, which renders HC non-functional but does not affect binding to capsid protein, is reported. Collectively, the results suggest that the N-terminal domain is required for interaction of HC with stylets rather than for binding to virions.

A specific interaction between coat protein and helper component correlates with aphid transmission of a potyvirus.

Specific binding between the coat protein (CP) and the helper component (HC) of the tobacco vein mottling potyvirus (TVMV) was characterized using a protein blotting-overlay protocol. In this in vitro assay, HC interacted with either virions or CP monomers originating from the aphid-transmissible TVMV-AT but not from the non-aphid-transmissible TVMV-NAT. There was a strong correlation between the aphid transmissibility of a series of TVMV variants having mutations in the DAG motif of the CP and their ability to bind HC. Expression of TVMV CP derivatives in bacteria allowed a precise determination of the minimum domain mediating HC binding. This domain is composed of seven amino acids, including the DAG motif (DTVDAGK), located in the N-terminus of the TVMV CP at amino acid positions 2 to 8.