Potyvirus helper component-proteinase and coat protein (CP) have coordinated functions in virus-host interactions and the same CP motif affects virustransmission and accumulation

Amino acid differences between helper component-proteinase (HC-Pro) and coa t protein (CP) that are putatively associated with biological differences b etween isolates PVA-B11 and PVA-U of potato A potyvirus (PVA) were studied using an infectious cDNA clone of PVA-B11. Replacement of the entire CP gen e of PVA-B11 with the CP gene of PVA-U reduced virus accumulation in tobacc o 5-fold, to the level of PVA-U. In contrast, four simultaneous amino acid substitutions made in PVA-B11 HC-Pro (according to PVA-U HC-Pro) increased virus accumulation 2- to 4-fold. A single substitution (S7G) at the CP N te rminus reduced virus accumulation 10-fold, but restored aphid-transmissibil ity of PVA-B11. Simultaneous mutation of HC-Pro and replacement of CP in B1 1 delayed systemic movement in tobacco and limited cell-to-cell movement in potato. These results imply coordinated functions of HC-Pro and CP in accu mulation and movement of PVA, because the phenotypic effects caused by simu ltaneous mutation of the two genes were different from the expected 'sum' o f phenotypic changes observed following mutation of only one gene at a time .