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Myosin Va (myoVa) motors transport membrane-bound cargo through three-dimensional, intracellular actin filament networks. We developed a coarse-grained, in silico model to predict how actin filament density (3-800 filaments) within a randomly oriented actin network affects fluid-like liposome (350 nm vs. 1750 nm) transport by myoVa motors. Five thousand simulated liposomes transported within each network adopted one of three states: transport, tug-of-war, or diffusion. Diffusion due to liposome detachment from actin rarely occurred given at least 10 motors on the liposome surface. However, with increased actin density, liposomes transitioned from primarily directed transport on single actin filaments to an apparent random walk, resulting from a mixture of transport and tug-of-wars as the probability of encountering additional actin filaments increased. This phase transition arises from a percolation phase transition at a critical number of accessible actin filaments, N c . N c is a geometric property of the actin network that depends only on the position and polarity of the actin filaments, transport distance, and the liposome diameter, as evidenced by a fivefold increase in liposome diameter resulting in a fivefold decrease in N c . Thus in cells, actin network density and cargo size may be regulated to match cargo delivery to the cell's physiological demands.

Local accumulation of oskar (osk ) mRNA in the Drosophila oocyte determines the posterior pole of the future embryo. Two major cytoskeletal components, microtubules and actin filaments, together with a microtubule motor, kinesin-1, and an actin motor, myosin-V, are essential for osk mRNA posterior localization. In this study, we use Staufen, an RNA-binding protein that colocalizes with osk mRNA, as a proxy for osk mRNA. We demonstrate that posterior localization of osk /Staufen is determined by competition between kinesin-1 and myosin-V. While kinesin-1 removes osk /Staufen from the cortex along microtubules, myosin-V anchors osk /Staufen at the cortex. Myosin-V wins over kinesin-1 at the posterior pole due to low microtubule density at this site, while kinesin-1 wins at anterior and lateral positions because they have high density of cortically-anchored microtubules. As a result, posterior determinants are removed from the anterior and lateral cortex but retained at the posterior pole. Thus, posterior determination of Drosophila oocytes is defined by kinesin-myosin competition, whose outcome is primarily determined by cortical microtubule density., Competing Interests: WL, ML, RL, NB, AR, MG, JS, VG No competing interests declared

Background: Mammalian cells must constantly reprogram the distribution of mitochondria in order to meet the local demands for energy, calcium, redox balance, and other mitochondrial functions. Mitochondrial localization inside the cell is a result of a combination of movement along the microtubule tracks plus anchoring to actin filaments., Recent Findings: Recent advances show that subcellular distribution of mitochondria can regulate tumor cell growth, proliferation/motility plasticity, metastatic competence, and therapy responses in tumors. In this review, we discuss our current understanding of the mechanisms by which mitochondrial subcellular distribution is regulated in tumor cells., Conclusions: Mitochondrial trafficking is dysregulated in tumors. Accumulation of mitochondria at the leading edge of the cell supports energy expensive processes of focal adhesion dynamics, cell membrane dynamics, migration, and invasion., (© 2019 Wiley Periodicals, Inc.)

The molecular motor myosin V transports cargo by stepping on actin filaments, executing a random diffusive search for actin binding sites at each step. A recent experiment suggests that the joint between the myosin lever arms may not rotate freely, as assumed in earlier studies, but instead has a preferred angle giving rise to structurally constrained diffusion. We address this controversy through comprehensive analytical and numerical modeling of myosin V diffusion and stepping. When the joint is constrained, our model reproduces the experimentally observed diffusion, allowing us to estimate bounds on the constraint energy. We also test the consistency between the constrained diffusion model and previous measurements of step size distributions and the load dependence of various observable quantities. The theory lets us address the biological significance of the constrained joint and provides testable predictions of new myosin behaviors, including the stomp distribution and the run length under off-axis force., Competing Interests: DH, RT, MH, DT No competing interests declared, (© 2020, Hathcock et al.)

Vertebrate myosin-5a is an ATP-utilizing processive motor associated with the actin network and responsible for the transport and localization of several vesicle cargoes. To transport cargo efficiently and prevent futile ATP hydrolysis, myosin-5a motor function must be tightly regulated. The globular tail domain (GTD) of myosin-5a not only functions as the inhibitory domain but also serves as the binding site for a number of cargo adaptor proteins, including melanophilin (Mlph) and Rab-interacting lysosomal protein-like 2 (RILPL2). In this study, using various biochemical approaches, including ATPase, single-molecule motility, GST pulldown assays, and analytical ultracentrifugation, we demonstrate that the binding of both Mlph and RILPL2 to the GTD of myosin-5a is required for the activation of myosin-5a motor function under physiological ionic conditions. We also found that this activation is regulated by the small GTPase Rab36, a binding partner of RILPL2. In summary, our results indicate that RILPL2 is required for Mlph-mediated activation of Myo5a motor activity under physiological conditions and that Rab36 promotes this activation. We propose that Rab36 stimulates RILPL2 to interact with the myosin-5a GTD; this interaction then induces exposure of the Mlph-binding site in the GTD, enabling Mlph to interact with the GTD and activate myosin-5a motor activity., (© 2019 Cao et al.)

Background: During heart morphogenesis, the cardiac chambers undergo ballooning: a process involving regionalized elongation of cardiomyocytes. Cardiomyocyte shape changes require reorganization of the actin cytoskeleton; however, the genetic regulation of this process is not well understood., Results: From a forward genetic screen, we identified the zebrafish uq 23ks mutant which manifests chamber ballooning defects. Whole-genome sequencing-mapping identified a truncating mutation in the gene, myo5b. myo5b encodes an atypical myosin required for endosome recycling and, consistent with this, increased vesicles were observed in myo5b mutant cardiomyocytes. Expression of RFP-Rab11a (a recycling endosome marker) confirmed increased recycling endosomes in cardiomyocytes of myo5b mutants. To investigate potential cargo of MyoVb-associated vesicles, we examined the adherens junction protein, N-cadherin. N-cadherin appeared mispatterned at cell junctions, and an increase in the number of intracellular particles was also apparent. Co-localization with RFP-Rab11a confirmed increased N-cadherin-positive recycling endosomes, demonstrating N-cadherin trafficking is perturbed in myo5b mutants. Finally, phalloidin staining showed disorganized F-actin in myo5b cardiomyocytes, suggesting the cytoskeleton fails to remodel, obstructing chamber ballooning., Conclusions: MyoVb is required for cardiomyocyte endosomal recycling and appropriate N-cadherin localization during the onset of chamber ballooning. Cardiomyocytes lacking MyoVb are unable to reorganize their actin cytoskeleton, resulting in failed chamber ballooning. Developmental Dynamics 248:284-295, 2019. © 2019 Wiley Periodicals, Inc., (© 2019 Wiley Periodicals, Inc.)

Myosin-5B is one of three members of the myosin-5 family of actin-based molecular motors. Despite its fundamental role in recycling endosome trafficking and in collective actin network dynamics, the molecular mechanisms underlying its motility are inherently unknown. Here we combine single-molecule imaging and high-speed laser tweezers to dissect the mechanoenzymatic properties of myosin-5B. We show that a single myosin-5B moves processively in 36-nm steps, stalls at ~2 pN resistive forces, and reverses its directionality at forces >2 pN. Interestingly, myosin-5B mechanosensitivity differs from that of myosin-5A, while it is strikingly similar to kinesin-1. In particular, myosin-5B run length is markedly and asymmetrically sensitive to force, a property that might be central to motor ensemble coordination. Furthermore, we show that Ca 2+ does not affect the enzymatic activity of the motor unit, but abolishes myosin-5B processivity through calmodulin dissociation, providing important insights into the regulation of postsynaptic cargoes trafficking in neuronal cells.