MyTH4-FERM myosins have an ancient and conserved role in filopod formation

Abstract

The formation of filopodia in Metazoa and Amoebozoa requires the activity of myosin 10 (Myo10) in mammalian cells and of Dictyostelium unconventional myosin 7 (DdMyo7) in the social amoeba Dictyostelium. However, the exact roles of these MyTH4-FERM myosins (myosin tail homology 4-band 4.1, ezrin, radixin, moesin; MF) in the initiation and elongation of filopodia are not well defined and may reflect conserved functions among phylogenetically diverse MF myosins. Phylogenetic analysis of MF myosin domains suggests that a single ancestral MF myosin existed with a structure similar to DdMyo7, which has two MF domains, and that subsequent duplications in the metazoan lineage produced its functional homolog Myo10. The essential functional features of the DdMyo7 myosin were identified using quantitative live-cell imaging to characterize the ability of various mutants to rescue filopod formation in myo7-null cells. The two MF domains were found to function redundantly in filopod formation with the C-terminal FERM domain regulating both the number of filopodia and their elongation velocity. DdMyo7 mutants consisting solely of the motor plus a single MyTH4 domain were found to be capable of rescuing the formation of filopodia, establishing the minimal elements necessary for the function of this myosin. Interestingly, a chimeric myosin with the Myo10 MF domain fused to the DdMyo7 motor also was capable of rescuing filopod formation in the myo7-null mutant, supporting fundamental functional conservation between these two distant myosins. Together, these findings reveal that MF myosins have an ancient and conserved role in filopod formation.C ells interact with their environment through protrusions such as filopodia that form in response to extracellular cues and mediate initial contact with the substrate. Filopodia are slender, actin-filled membrane projections that are highly dynamic, growing and shrinking from peripheral regions of cells, such as lamellipodia and the dorsal surface (1). A wide variety of cell types including amoebae such as Dictyostelium discoideum (2) and Acanthamoeba (3), as well as mammalian vascular endothelial cells (4) and developing neurons (5), extend filopodia. Filopodia are typically 1-10 μm long and 0.1-0.3 μm in diameter and have a core of 10-30 parallel actin filaments with a protein-rich complex at their tip (1, 6, 7). Modified forms of filopodia such as dendritic spines, cytonemes, and tunneling nanotubes promote intercellular communication during multicellular development (8-10). Defects in filopod formation alter cell spreading and adhesion (2, 11, 12), whereas overproduction of filopodia or filopodia-like protrusions is associated with increased invasiveness of metastatic cancer cells (13)(14)(15).Filopod elongation is triggered by small GTPase activity (1, 16) and is driven by the activity of actin elongation factors including vasodilator-stimulated phosphoprotein (VASP) and formins, and the actin core is stabilized by actin cross-linking proteins (1). A MyTH4-FERM (myosin tail homology ...

The actin networks of chytrid fungi reveal evolutionary loss of cytoskeletal complexity in the fungal kingdom

“…Although all chytrid species analyzed have at least one copy of Myo1, 2, 5, and 17, only some chytrid species appear to have retained Myo22. These myosins have one or two MyTH-FERM domains in the C-terminal tail region (Kollmar and Mühlhausen 2017) and are largely associated with the assembly and function of cellular protrusions composed of parallel bundles of actin, such as the filopodia of Dictyostelium and animal cells (Tuxworth et al 2001;Petersen et al 2016;Weck et al 2017). Although a subset of chytrid fungi including Rg and Sp have a single Myo22, neither Am, Bd nor Bs have a Myo22.…”

Myosin-X knockout is semi-lethal and demonstrates that myosin-X functions in neural tube closure, pigmentation, hyaloid vasculature regression, and filopodia formation

“…Myo10 is a member of a phylogenetically ancient group of myosins characterized by one or more MyTH4-FERM domains (Myosin Tail Homology 4 and band 4.1, Ezrin, Radixin, Merlin). The MyTH4-FERM myosins are strongly associated with protrusions based on actin bundles, such as filopodia, microvilli, and inner ear stereocilia 10 , 11 . Of the four MyTH4-FERM myosins expressed in humans, Myo7a is expressed in several tissues and localizes to inner ear stereocilia 12 , Myo7b is expressed in transporting epithelia and localizes to the tips of microvilli 13 , Myo15a is expressed in the inner ear and localizes to the tips of stereocilia 14 , and Myo10 is expressed in most cells and localizes to the tips of filopodia 3 .…”

Structure of Myo7b/USH1C complex suggests a general PDZ domain binding mode by MyTH4-FERM myosins

“…A common property of MyTH4-FERM myosins is their involvement in the formation or elongation/stabilization of actin-bundle supported structures, including filopodia, microvilli, and stereocilia (15). This property first appeared in ancient MyTH4-FERM myosins and is conserved over 1 billion y of evolution (14,16,17). Mutations of MyTH4-FERM myosins cause a broad spectrum of human diseases, including hearing loss, vision defects, digestive disorders, and cancers (18-21), often because of defects in actin bundle-supported protrusions (i.e., filopodia, microvilli, or stereocilia).…”