Studies on the interaction between "Tobacco Mosaic Virus" movement protein and microtubules

During the invasion of a susceptible host, Tobacco mosaic virus (TMV) transports its RNA genome from sites of viral synthesis into neighbouring cells, thus potentiating the spread of infection. In many systems, RNA trafficking is known to be a highly coordinated process, often involving the complex interplay of numerous factors. In plants, however, the mechanisms that govern RNA transport are not well understood. Since viruses have a propensity to exploit pre-existing host machinery for their own purposes, TMV has become a popular model system for the study of RNA trafficking in plants. TMV viral RNA (vRNA) is thought to be transported as a ribonucleoprotein (RNP) with the virally encoded movement protein (MP). Localization studies have demonstrated a temporal redistribution of MP from endoplasmic reticulum (ER)derived replication bodies onto microtubules (MTs) at mid to late stage of infection. A functional link between the association of MP with microtubules (MT) and RNA trafficking has been established, and furthermore, this association does not require other viral components or plant-specific factors, thereby indicating a direct interaction between MP and MTs. In vitro, purified MP interacts directly with preformed, dynamically suppressed MTs and has no requirement for MT polymerization or polymer-specific structural forms. Immunohistochemical studies indicate that MP may interact with the extreme C-termini of α- and β-tubulin, possibly forming a stabilizing sheath. Consistent with the proposal that MP functions as a structural microtubuleassociated protein (MAP), MP:MT complexes assembled in vitro are highly stable. A possible functional overlap between the putative RNA and MT binding domains of MP has been identified, and although MP:MT complexes are unlikely to support RNA transport directly, such complexes have been implicated to modulate MT-dependent molecular motor activity. Collectively, in vitro data are in support of the observation that MP associates with MTs to high amounts, leading to the hypothesis that MPmediated RNA transport and the MP:MT interaction are related, yet functionally distinguishable processes. Furthermore, based on the similarity of MP to endogenous MAPs, a new model is proposed in which MP-mediated trafficking may occur in the form of a translationally competent ER-vesicle, concurrent with the deployment of MP to the MT surface.