MUTATIONS IN THE HC-PRO GENE OF ZUCCHINI YELLOW MOSAIC POTYVIRUS - EFFECTS ON APHID TRANSMISSION AND BINDING TO PURIFIED VIRIONS

Transmission of zucchini yellow mosaic virus (ZYMV) by aphids was exam ined by introducing mutations within the highly conserved proline-thre onine-lysine (PTK) motif of the helper component proteinase (HC-Pro) u sing a cDNA full-length clone. Replacement of proline by alanine (ATK) in the PTK motif abolished transmission almost completely both from p lants and from membranes, Substitution of the basic lysine by glutamic acid (PTE) did not reduce the rate of transmission compared with the wildtype, Replacement of threonine by valine (PVK) or serine (PSK) res ulted in a rate of transmission that was lower than that of the wild-t ype, The rate was lower for PSK than for PVK, Western blot comparison did not permit attribution of HC-Pro functionality in transmission to its level in the host. The HC-Pro of strains that effected transmissio n (with the wildtype PTK motif, and with the mutated PTE and PVK motif s) could also bind in vitro to virions of ZYMV. HC-Pro with a PSK moti f, which was less effective in assisting transmission, could bind only weakly to virions, while HC-Pro of the almost non-transmissible strai ns (with PAK and ATK motifs) did not bind at all. Interestingly, posit ive binding was recorded for transmission-defective ZYMV-Ct, which has a PTK motif but has glutamic acid instead of lysine in the lysine-leu cine-serine-cysteine (KLSC) motif. These findings support the 'bridge hypothesis', and confirm the binding of the HC-Pro to the virion, The possible role of the PTK and KLSC motifs in binding to the virus and t o the mouthparts of the aphid is discussed.