The Papaya ringspot virus (PRSV) is one of the limiting factors affecting papaya and cucurbits production worldwide. PRSV belongs to the potyvirus genus which consists of 30% of known plant viruses. Two serological closely related strains, namely type-P and -W, have been reported. PRSV type-P infects both papaya and cucurbits, while type-W infects only cucurbits. The genome of PRSV Thailand isolate consists of a (+) RNA molecule of 10323 nucleotides, which is first translated into a single polypeptide and further cleaved by three viral encoded proteases into ten gene products. Helper-component proteinase (HC-Pro), which is encoded by the 2nd cistron of the potyviral genome, has been implicated in aphid transmission, viral movement, viral replication and suppression of host viral defense system. Studies of the Tobacco etch virus (TEV), Lettuce mosaic virus (LMV), Onion yellow dwarf virus (OYDV) and Wheat streak mosaic virus (WSMV) indicate that the N-terminal of HC-Pro is dispensable for systemic infection in their respective hosts. However, deletion analysis of the Tobacco vein mottling virus (TVMV) indicates otherwise. In this study, we examined whether HC-Pro is essential for PRSV systemic infection in cucurbits and the role of its N-terminal in systemic infection. Our results indicated that HC-Pro is indispensable for PRSV infection in zucchini. Deletion analysis of PRSV HC-Pro showed that deletion of as few as 54 amino acids at the N-terminal of HC-Pro completely abolished the infectivity of the corresponding cDNA clone. Therefore, it is proposed that the N-terminal of HC-Pro is involved in systemic infection of PRSV, in addition to its conserved function in aphid transmission.