Cellular mechanisms governing non-muscle myosin 2 (NM2) filament assembly are largely unknown. Using EGFP-NM2A knock-in fibroblasts and multiple super-resolution imaging modalities, we characterized and quantified the sequential amplification of NM2 filaments within lamella, wherein filaments emanating from single nucleation events continuously partition, forming filament clusters that populate large-scale actomyosin structures deeper in the cell. Individual partitioning events coincide spatially and temporally with the movements of diverging actin fibers, suppression of which inhibits partitioning. These and other data indicate that NM2A filaments are partitioned by the dynamic movements of actin fibers to which they are bound. Finally, we showed that partition frequency and filament growth rate in the lamella depend on MLCK, and that MLCK is competing with centrally-active ROCK for a limiting pool of monomer with which to drive lamellar filament assembly. Together, our results provide new insights into the mechanism and spatio-temporal regulation of NM2 filament assembly in cells. The peri-centrosomal localization and morphology of the Golgi apparatus depends largely on the microtubule cytoskeleton and the microtubule motor protein dynein. Recent studies proposed that myosin 18A (M18A) also contributes to Golgi morphology by binding the Golgi protein GOLPH3 and walking along adjacent actin filaments to stretch the Golgi into its classic ribbon structure. Biochemical analyses have shown, however, that M18A is not an actin-activated ATPase and lacks motor activity. Our goal, therefore, was to define the precise molecular mechanism by which M18A determines Golgi morphology. We show that purified M18A remains inactive in the presence of GOLPH3, arguing against the Golgi-specific activation of the myosin. Using M18A-specific antibodies and expression of GFP-tagged M18A, we find no evidence that it localizes to the Golgi. Moreover, several cell lines with reduced or eliminated M18A expression exhibited normal Golgi morphology. Interestingly, actin filament disassembly resulted in a marked reduction in lateral stretching of the Golgi in both control and M18A-deficient cells. Importantly, this reduction was accompanied by an expansion of the Golgi in the vertical direction, vertical movement of the centrosome, and increases in the height of both the nucleus and the cell. Finally, stochastic variations in Golgi height correlated with variations in nuclear height in both untreated and M18A-deficient cells. Collectively, our data indicate that M18A does not localize to the Golgi or play a significant role in determining its morphology, and suggest that large-scale actin disassembly alters Golgi morphology indirectly by altering cell shape. Dendritic spines are signaling micro-compartments that serve as the primary site of synapse formation in neurons, and that house the machinery underlying memory formation. Actin plays a vital role in the generation and maintenance of spines, and changes in spine actin organization underlie memory formation. Not surprisingly, therefore, non-muscle myosin 2 (NM2) also plays a critical role in spine structure and function. Myosin 18A (M18A) is a NM2-like myosin expressed from flies to man that co-assembles with NM2 to make mixed filaments. Importantly, M18A is alternatively spliced to create many versions that contain unique N- and C-terminal extensions harboring both recognizable (e.g. PDZ domains, SH3 domain binding sites) and uncharacterized protein: protein interaction domains. For example, M18A-alpha possesses a 300-residue N-terminal extension that contains a PDZ domain, a binding site for F-actin, and a binding site for the Rac GEF Beta-Pix, which has been shown to play a key role in controlling spine actin assembly. Current thinking is that M18As primary biological function is to present these protein: protein interaction domains on the surface of mixed NM2 bipolar filaments to dramatically increase their functional diversity. Here we show that both endogenous and exogenous, GFP-tagged M18A-alpha are concentrated along with NM2 in the dendritic spines of cerebellar Purkinje neurons, and that M18A-alpha's N-terminal extension drives its spine localization. miRNA-mediated knockdown of M18A-alpha results in significant defects in dendrite number, spine number and spine length, all of which are rescued with an RNAi-immune version of M18A-alpha. Current efforts are directed at determining if M18A-alpha's interaction with Beta-Pix underlies its RNAi phenotype, consistent with previous work on this GEF in neurons. Myosin 10 (Myo10) is an unconventional myosin best known for its striking localization to the tips of filopodia. Despite the broad expression of Myo10 in vertebrate tissues, its functions at the organismal level remain largely unknown. We report here the generation of KO-first (Myo10tm1a/tm1a), floxed (Myo10tm1c/tm1c), and KO mice (Myo10tm1d/tm1d). Complete knockout of Myo10 was semi-lethal, with over half of homozygous KO embryos exhibiting exencephaly, a severe defect in neural tube closure. All Myo10 KO mice that survived birth exhibited a white belly spot, all had persistent fetal vasculature in the eye, most had a small kink near the tip of the tail, and 50% had webbed digits. Myo10 KO mice that survived birth could breed with one another to produce litters of KO embryos, demonstrating that Myo10 is not absolutely essential for mitosis, meiosis, adult survival, or fertility. Homozygous KO-first mice, as well as an independent spontaneous deletion (Myo10m1J/m1J), exhibited the same core phenotype. During retinal angiogenesis, the vascular fronts in KO mice exhibited 60% decrease in filopodia, demonstrating that Myo10 is required to form normal numbers of filopodia in vivo. The Myo10 mice generated here provide key tools for the field and demonstrate that Myo10 has important functions in mammalian development.
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