Tobacco mosiac virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP),which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in theintercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory haveprovided evidence that the vRNA movement process is correlated with the ability of the MP to interact withmicrotubules, although the exact role of this interaction during infection is not known. Here, we have used avariety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associatedprotein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MPin whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against thehypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes withkinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driventransport processes during infection.