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.Key words: Actin cortex, Dictyostelium discoideum, guanine nucleotide exchange factor, myosin, Rac GTPase
Received 12 December 1999, revised and accepted for publication 31 January 2000The actin cytoskeleton is essential for the maintenance of cell shape and locomotion, but also provides tracks for active intracellular transport. Many actin-based processes, such as motility, cytokinesis, phagocytosis, endocytosis, polarized secretion and exocytosis, organelle movement and mRNA transport (1) have been shown to involve myosins. These motor proteins form a superfamily currently encompassing 15 phylogenetic and structural classes (1-3). Despite adaptation to diverse tasks in the cell, all myosins share a tripartite modular structure: the head or motor domain, site of the actin-activated ATPase activity; the neck acting as lever arm and binding site for light chains; and a class-specific tail reflecting functionality by its domain composition.Recent data indicate that myosins also play an active role in signal transduction (4,5). The tail domain of various myosins contains elements including Src homology 3 (SH3) and pleckstrin homology (PH) domains thought to mediate proteinprotein or protein -lipid interactions within signaling cascades. Particularly, the tail of class IX myosins harbors a GTPase activating protein (GAP) domain specific for Rho (6), a member of the family of small GTPases that play key roles in growth factor-regulated cytoskeletal reorganization and cell cycle progression (7 -9). Mammalian cells overexpressing myr5, a rat myosin IX, lose stress fibers and focal contacts and consequently round up (6). T...
