Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

The regulation of actin polymerization and cross-linking in Dictyostelium

It is clear that the polymerization and organization of actin filament networks plays a critical role in numerous cellular processes. Inhibition of actin polymerization by pharmacological agents will completely prevent chemotactic motility, macropinocytosis, endocytosis, and phagocytosis. Recently there has been great progress in understanding the mechanisms that control the assembly and structure of the actin cytoskeleton. Members of the Rho family of GTPases have been identified as major players in the signal transduction pathway leading from a cell surface signal to actin polymerization. The Arp2/3 complex has been added to the list of means by which new actin filaments can be nucleated. However, it is clear that actin polymerization by Arp2/3 complex is not the whole story. In principle, the final structures formed by actin filaments will depend on factors such as: the length of actin filaments, the degree of branching, how they are cross-linked and the tensions imparted on them. In addition, the means by which actin polymerization generates protrusion of membranes is still controversial. A phagosome, filopodium and a lamellipodium all require polymerization of new actin filaments, but each has a characteristic morphology and cytoskeletal structure. In the following chapter, we will discuss actin polymerization and filament organization, especially as it relates to the machinery of phagocytosis in Dictyostelium.

A role for myosin VII in dynamic cell adhesion

BACKGROUND

The initial stages of phagocytosis and cell motility resemble each other. The extension of a pseudopod at the leading edge of a migratory cell and the formation of a phagocytic cup are actin dependent, and each rely on the plasma membrane adhering to a surface during dynamic extension.

RESULTS

A myosin VII null mutant exhibited a drastic loss of adhesion to particles, consistent with the extent of an observed decrease in particle uptake. Additionally, cell-cell adhesion and the adhesion of the leading edge to the substratum during cell migration were defective in the myosin VII null cells. GFP-myosin VII rescued the phagocytosis defect of the null mutant and was distributed in the cytosol and recruited to the cortical cytoskeleton, where it appeared to be enriched at the tips of filopods. It was also localized to phagocytic cups, but only during the initial stages of particle engulfment. During migration, GFP-myosin VII is found at the leading edge of the cell.

CONCLUSIONS

Myosin VII plays an important role in mediating the initial binding of cells to substrata, a novel role for an unconventional myosin.

Myosin I phosphorylation is increased by chemotactic stimulation

Directed cell migration occurs in response to extracellular cues. Following stimulation of a cell with chemoattractant, a significant rearrangement of the actin cytoskeleton is mediated by intracellular signaling pathways and results in polarization of the cell and movement via pseudopod extension. Amoeboid myosin Is play a critical role in regulating pseudopod formation in Dictyostelium, and their activity is activated by heavy chain phosphorylation. The effect of chemotactic stimulation on the in vivo phosphorylation level of a Dictyostelium myosin I, myoB, was tested. The myoB heavy chain is phosphorylated in vivo on serine 322 (the myosin TEDS rule phosphorylation site) in chemotactically competent cells. The level of myoB phosphorylation increases following stimulation of starving cells with the chemoattractant cAMP. A 3-fold peak increase in the level of phosphorylation is observed at 60 s following stimulation, a time at which the Dictyostelium cell actively extends pseudopodia. These findings suggest that chemotactic stimulation results in increased myoB activity via heavy chain phosphorylation and contributes to the global extension of pseudopodia that occurs prior to polarization and directed motility.

A myosin I is involved in membrane recycling from early endosomes

Geometry-based mechanisms have been proposed to account for the sorting of membranes and fluid phase in the endocytic pathway, yet little is known about the involvement of the actin-myosin cytoskeleton. Here, we demonstrate that Dictyostelium discoideum myosin IB functions in the recycling of plasma membrane components from endosomes back to the cell surface. Cells lacking MyoB (myoA(-)/B(-), and myoB(-) cells) and wild-type cells treated with the myosin inhibitor butanedione monoxime accumulated a plasma membrane marker and biotinylated surface proteins on intracellular endocytic vacuoles. An assay based on reversible biotinylation of plasma membrane proteins demonstrated that recycling of membrane components is severely impaired in myoA/B null cells. In addition, MyoB was specifically found on magnetically purified early pinosomes. Using a rapid-freezing cryoelectron microscopy method, we observed an increased number of small vesicles tethered to relatively early endocytic vacuoles in myoA(-)/B(-) cells, but not to later endosomes and lysosomes. This accumulation of vesicles suggests that the defects in membrane recycling result from a disordered morphology of the sorting compartment.

Novel Dictyostelium unconventional myosin, MyoM, has a putative RhoGEF domain

We have cloned a novel unconventional myosin gene myoM in Dictyostelium. Phylogenetic analysis of the motor domain indicated that MyoM does not belong to any known subclass of the myosin superfamily. Following the motor domain, two calmodulin-binding IQ motifs, a putative coiled-coil region, and a Pro, Ser and Thr-rich domain, lies a combination of dbl homology and pleckstrin homology domains. These are conserved in Rho GDP/GTP exchange factors (RhoGEFs). We have identified for the first time the RhoGEF domain in the myosin sequences. The growth and terminal developmental phenotype of Dictyostelium cells were not affected by the myoM(-) mutation. Green fluorescent protein-tagged MyoM, however, accumulated at crown-shaped projections and membranes of phase lucent vesicles in growing cells, suggesting its possible roles in macropinocytosis.

The role of unconventional myosins in Dictyostelium endocytosis

Dictyostelium discoideum is a simple eukaryote amenable to detailed molecular studies of the endocytic processes phagocytosis and macropinocytosis. Both the actin cytoskeleton and associated myosin motors are well-described and a range of mutants are now available that enable characterization of the role of the cytoskeleton in a range of cellular functions. Molecular genetic studies have uncovered roles for two different classes of Dictyostelium unconventional myosins in endocytosis. The class I myosins contribute to both macropinocytosis and phagocytosis by playing a general role in controlling actin-dependent manipulations of the actin-rich cortex. A class VII myosin has been shown to be important for phagocytosis. This brief review summarizes what is known about the role of these different myosins in both fluid and particle uptake in this system.

The tail domain of myosin M catalyses nucleotide exchange on Rac1 GTPases and can induce actin-driven surface protrusions

Members of the myosin superfamily play crucial roles in cellular processes including management of the cortical cytoskeleton, organelle transport and signal transduction. GTPases of the Rho family act as key control elements in the reorganization of the actin cytoskeleton in response to growth factors, and other functions such as membrane trafficking, transcriptional regulation, growth control and development. Here, we describe a novel unconventional myosin from Dictyostelium discoideum, MyoM. Primary sequence analysis revealed that it has the appearance of a natural chimera between a myosin motor domain and a guanine nucleotide exchange factor (GEF) domain for Rho GTPases. The functionality of both domains was established. Binding of the motor domain to F-actin was ATP-dependent and potentially regulated by phosphorylation. The GEF domain displayed selective activity on Rac1-related GTPases. Overexpression, rather than absence of MyoM, affected the cell morphology and viability. Particularly in response to hypo-osmotic stress, cells overexpressing the MyoM tail domain extended massive actin-driven protrusions. The GEF was enriched at the tip of growing protuberances, probably through its pleckstrin homology domain. MyoM is the first unconventional myosin containing an active Rac-GEF domain, suggesting a role at the interface of Rac-mediated signal transduction and remodeling of the actin cytoskeleton.