The cell nucleus communicates with the cytoplasm through a nucleocytoplasmic linker that maintains the shape of the nucleus and mediates its migration. In contrast to animal nuclei, which are moved by motor proteins (kinesins and dyneins) along the microtubule cytoskeleton, plant nuclei move rapidly and farther along an actin filament cytoskeleton. This implies that plants use a distinct nucleocytoplasmic linker for nuclear dynamics, although its molecular identity is unknown. Here, we describe a new type of nucleocytoplasmic linker consisting of a myosin motor and nuclear membrane proteins. In the Arabidopsis thaliana mutant kaku1, nuclear movement was impaired and the nuclear envelope was abnormally invaginated. The responsible gene was identified as myosin XI-i, which encodes a plant-specific myosin. Myosin XI-i is specifically localized on the nuclear membrane, where it physically interacts with the outer-nuclear-membrane proteins WIT1 and WIT2. Both WIT proteins are required for anchoring myosin XI-i to the nuclear membrane and for nuclear movement. A striking feature of plant cells is dark-induced nuclear positioning in mesophyll cells. A deficiency of either myosin XI-i or WIT proteins diminished dark-induced nuclear positioning. The unique nucleocytoplasmic linkage in plants might enable rapid nuclear positioning in response to environmental stimuli.
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