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.