Class-1 Myosins at the Interphase between Spindles and Membranes

Abstract

Class-1 myosins play key roles in the dynamic organization of the cytoskeleton and membrane shape as they act as divalent cross-linkers interconnecting and generating force between both actin and membrane systems. Previously we have shown that the long-tailed Dictyostelium myosin-1C not only is involved in membrane remodeling during endocytic processes, but also plays a role in spindle dynamics during mitosis through direct microtubule interactions. Here, we characterized myosin-1D as the second myosin-1 member with roles in mitosis. Our studies reveal that myosin-1D binds spindle pole microtubules and associates with the nuclear envelope of dividing cells. Myosin-1D targets microtubules and nuclear structures early during mitosis and decorates the membrane embedded chromatin masses until the end of cell division. This suggests a role of the motor in nuclear membrane fission by mediating microtubule-membrane interactions. Since the nuclear envelope in Dictyostelium remains intact throughout mitosis, except for regions of perforations that are found near the spindle poles, myosin-1D appears to act as a dynamic linker connecting spindle pole microtubules to the nuclear membrane important to hold the envelope together as it gets stretched upon spindle elongation. To investigate the mechanisms that govern myosin-1s to distinct mitotic structures, we have generated a set of motor and tail domain constructs and characterized binding affinities and kinetics of intermolecular interactions with lipid membranes, actin, and microtubules. Our results show that differences in strength and specificity of lipid and microtubule binding and regulatory roles of the tails account for the specialized functions of the motors in mitosis.