The eriophyid mite transmitted Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) shares a common genome organization with aphid transmitted species of the genus Potyvirus. Although both tritimoviruses and potyviruses encode helper component-proteinase (HC-Pro) homologues (required for nonpersistent aphid transmission of potyviruses), sequence conservation is low (amino acid identity, ∼16%), and a role for HC-Pro in semipersistent transmission of WSMV by the wheat curl mite (Aceria tosichella [Keifer]) has not been investigated. Wheat curl mite transmissibility was abolished by replacement of WSMV HC-Pro with homologues of an aphid transmitted potyvirus (Turnip mosaic virus), a rymovirus (Agropyron mosaic virus) vectored by a different eriophyid mite, or a closely related tritimovirus (Oat necrotic mottle virus; ONMV) with no known vector. In contrast, both WSMV-Sidney 81 and a chimeric WSMV genome bearing HC-Pro of a divergent strain (WSMV-El Batán 3; 86% amino acid sequence identity) were efficiently transmitted by A. tosichella. Replacing portions of WSMV-Sidney 81 HC-Pro with the corresponding regions from ONMV showed that determinants of wheat curl mite transmission map to the 5′-proximal half of HC-Pro. WSMV genomes bearing HC-Pro of heterologous species retained the ability to form virions, indicating that loss of vector transmissibility was not a result of failure to encapsidate. Although titer in systemically infected leaves was reduced for all chimeric genomes relative to WSMV-Sidney 81, titer was not correlated with loss of vector transmissibility. Collectively, these results demonstrate for the first time that HC-Pro is required for virus transmission by a vector other than aphids.