Amino acid differences between helper component-proteinase (HC-Pro) and coat protein (CP) that are putatively associated with biological differences between 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 gene of PVA-B11 with the CP gene of PVA-U reduced virus accumulation in tobacco 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 terminus reduced virus accumulation 10-fold, but restored aphid-transmissibility of PVA-B11. Simultaneous mutation of HC-Pro and replacement of CP in B11 delayed systemic movement in tobacco and limited cell-to-cell movement in potato. These results imply coordinated functions of HC-Pro and CP in accumulation and movement of PVA, because the phenotypic effects caused by simultaneous mutation of the two genes were different from the expected 'sum' of phenotypic changes observed following mutation of only one gene at a time.