The Tobamovirus Turnip Vein Clearing Virus 30-Kilodalton Movement Protein Localizes to Novel Nuclear Filaments To Enhance Virus Infection

Plant viruses overcome the barrier of the plant cell wall by encoding cell-to-cell movement proteins (MPs), which direct newly replicated viral genomes to, and across, the wall. The paradigm for how a single MP regulates and coordinates these activities is the Tobacco mosaic virus (TMV) 30-kDa protein (MPTMV). Detailed studies demonstrate that TMV multiplies exclusively in the cytoplasm and have documented associations of MPTMV with endoplasmic reticulum (ER) membrane, microtubules, and plasmodesmata throughout the course of infection. As TMV poorly infects Arabidopsis thaliana, Turnip vein clearing virus (TVCV) is the tobamovirus of choice for studies in this model plant. A key problem, which has contributed to confusion in the field, is the unproven assumption that the TVCV and TMV life cycles are identical. We engineered an infectious TVCV replicon that expressed a functional fluorescence-tagged MPTVCV and report here the unexpected discovery that MPTVCV, beyond localizing to ER membrane and plasmodesmata, targeted to the nucleus in a nuclear localization signal (NLS)-dependent manner, where it localized to novel F-actin-containing filaments that associated with chromatin. The MPTVCV NLS appeared to be conserved in the subgroup 3 tobamoviruses, and our mutational analyses showed that nuclear localization of MPTVCV was necessary for efficient TVCV cell-to-cell movement and systemic infection in Nicotiana benthamiana and Arabidopsis thaliana. Our studies identify a novel nuclear stage in TVCV infection and suggest that nuclear MP encoded by TVCV and other subgroup 3 tobamoviruses interacts with F-actin and chromatin to modulate host defenses or cellular physiology to favor virus movement and infection.