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Cauliflower mosaic virus (CaMV) open reading frame III (ORF III) codes for a virion-associated protein (Vap), which is one of two viral proteins essential for aphid transmission. However, unlike the aphid transmission factor encoded by CaMV ORF II, Vap is also essential for systemic infection, suggesting that it is a multifunctional protein. To elucidate the additional function or functions of Vap, we tested the replication of noninfectious ORF III-defective mutants in transfected turnip protoplasts. PCR and Western blot analyses revealed that CaMV replication had occurred with an efficiency similar to that of wild-type virus and without leading to reversions. Electron microscopic examination revealed that an ORF III frameshift mutant formed normally structured virions. These results demonstrate that Vap is dispensable for replication in single cells and is not essential for virion morphogenesis. Analysis of inoculated turnip leaves showed that the ORF III frameshift mutant does not cause any detectable local infection. These results are strongly indicative of a role for Vap in virus movement.

All plant pararetroviruses belong to theCaulimoviridaefamily. This family contains six genera of viruses with different biological, serological, and molecular characteristics. Although some important mechanisms of viral replication and host infection are understood, much remains to be discovered about the many functions of the viral proteins. The focus of this study, the virion-associated protein (VAP), is conserved among all members of the group and contains a coiled-coil structure that has been shown to assemble as a tetramer in the case of cauliflower mosaic virus. We have used the yeast two-hybrid system to characterize self-association of the VAPs of four distinct plant pararetroviruses, each belonging to a different genus ofCaulimoviridae. Chemical cross-linking confirmed that VAPs assemble into tetramers. Tetramerization is thus a common property of these proteins in plant pararetroviruses. The possible implications of this conserved feature for VAP function are discussed.