Cell-to-cell movement and assembly of a plant closterovirus: roles for the capsid proteins and Hsp70 homolog

Abstract

Diverse animal and plant viruses are able to translocate their virions between neighboring cells via intercellular connections. In this work, we analyze the virion assembly and cell-to-cell movement of a plant closterovirus and reveal a strong correlation between these two processes. The ®lamentous virions of a closterovirus possess a long body formed by the major capsid protein (CP) and a short tail formed by the minor capsid protein (CPm). Genetic and biochemical analyses show that the functions of these virion components are distinct. A virion body is required primarily for genome protection, whereas a tail represents a specialized device for cell-to-cell movement. Furthermore, tail assembly is mediated by the viral Hsp70 homolog (Hsp70h) that becomes an integral part of the virion. Inactivation of the ATPase domain of Hsp70h results in assembly of tailless virions that are incapable of translocation. A dual role for the viral molecular chaperone Hsp70h in virion assembly and transport, combined with the previous ®nding of this protein in intercellular channels, allowed us to propose a model of closteroviral movement from cell to cell.

Complex molecular architecture of beet yellows virus particles

“…It was demonstrated that, in the absence of functional CPm, CP is capable of encapsidating the entire BYV genome (23), suggesting that the tail has not evolved merely for RNA protection. Because the Hsp70h, p64, and CPm are each required for BYV cell-to-cell movement, it was proposed that the tail represents a viral transport device (17,23). This interpretation seems to be even more likely in relation to the tip segment.…”

“…Previous work demonstrated that the BYV Hsp70h and p64 proteins are required for assembly of the virion tails, but not the bodies (17,23), suggesting that each of these proteins is an integral tail component. It was also suggested that the tails are assembled in the proximity of the RNA 5Ј terminus, although the length of the RNA fragment encapsidated by the tails was not determined (29).…”

“…The fifth virion protein of BYV is a 20-kDa protein (p20) that interacts with Hsp70h (18). BYV CP, CPm, p64, and Hsp70h are each required for virion assembly and subsequent cell-to-cell movement (17,(22)(23)(24), indicating that the formation of the ''tailed'' virions is a prerequisite for intercellular trafficking. In contrast, p20 is dispensable for virion assembly and cell-to-cell movement, but is necessary for BYV transport through the plant vascular system (18).…”

Helical capsids of plant viruses: architecture with structural lability

“…Of these proteins, Hsp70h may be the key element of BYV transport machinery, since this protein, when expressed in the absence of other viral products, is able to localize to motile granules, which are transported to the cell periphery in an actin-dependent manner, and can associate with plasmodesmata (Prokhnevsky et al, 2005). Moreover, the ATPase activity of Hsp70h is required for cell-to-cell BYV transport (Alzhanova et al, 2001).…”

Whitefly-transmitted criniviruses of cucurbits: current status and future prospects

“…Hsp70h is a multi-functional protein which possesses several conserved motifs such as those involved in ATPase activity [39]. The crinivirus-encoded Hsp70h may be associated with virion tail assembly and disassembly similar to the closterovirus Beet yellows virus (BYV) [9]. Further, Hsp70h may be involved in plasmodesmatal attachment and virion translocation through ATPase activity [9].…”