Titus, M. A. | bioRxiv Science

bioRxiv2021

VASP mediated actin dynamics activate and recruit a filopodia myosin

Filopodia are thin, actin-based structures that cells use to interact with their environments. Filopodia initiation requires a suite of conserved proteins but the mechanism remains poorly understood. The actin polymerase VASP and a MyTH-FERM (MF) myosin, DdMyo7 in amoeba, are essential for filopodia initiation. DdMyo7 is localized to dynamic regions of the actin-rich cortex. Analysis of VASP mutants and treatment of cells with anti-actin drugs shows that myosin recruitment and activation in Dictyostelium requires localized VASP-dependent actin polymerization. Targeting of DdMyo7 to the cortex alone is not sufficient for filopodia initiation; VASP activity is also required. The actin regulator locally produces a cortical actin network that activates myosin and together they shape the actin network to promote extension of parallel bundles of actin during filopodia formation. This work reveals how filopodia initiation requires close collaboration between an actin binding protein, the state of the actin cytoskeleton and MF myosin activity.

cell biology↗

bioRxiv2020

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

Cells from across the eukaryotic tree use actin polymers and a number of conserved regulators for a wide variety of functions including endocytosis, cytokinesis, and cell migration. Despite this conservation, the actin cytoskeleton has undergone significant evolution and diversification, highlighted by the differences in the actin cytoskeletal networks of mammalian cells and yeast. Chytrid fungi diverged before the emergence of the Dikarya (multicellular fungi and yeast), and therefore provide a unique opportunity to study the evolution of the actin cytoskeleton. Chytrids have two life stages: zoospore cells that can swim with a flagellum, and sessile sporangial cells that, like multicellular fungi, are encased in a chitinous cell wall. Here we show that zoospores of the amphibian-killing chytrid Batrachochytrium dendrobatidis (Bd) build dynamic actin structures that resemble those of animal cells, including pseudopods, an actin cortex, and filopodia-like actin spikes. In contrast, Bd sporangia assemble actin patches similar to those of yeast, as well as perinuclear actin shells. Our identification of actin cytoskeletal elements in the genomes of five species of chytrid fungi indicate that these actin structures are controlled by both fungal-specific components as well as actin regulators and myosin motors found in animals but not other fungal lineages. The use of specific small molecule inhibitors indicate that nearly all of Bds actin structures are dynamic and use distinct nucleators: while pseudopods and actin patches are Arp2/3-dependent, the actin cortex appears formin-dependent, and actin spikes require both nucleators. The presence of animal- and yeast-like actin cytoskeletal components in the genome combined with the intermediate actin phenotypes in Bd suggests that the simplicity of the yeast cytoskeleton may be due to evolutionary loss.

cell biology↗