VASP mediated actin dynamics activate and recruit a filopodia myosin

Abstract

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 the MF myosin and together they shape the actin network to promote extension of parallel bundles 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.

The many roles of myosins in filopodia, microvilli and stereocilia

“…Filopodia emerge from MYO10 foci close to nascent focal adhesions rich in actin, integrin and vinculin at the leading edge of the cell 33,34 actin filaments to initiate filopodia. MYO10 and DdMyo7 are largely cytosolic and exist in an autoinhibited conformation mediated by interactions between the motor and tail regions, similar to other myosins 35,36 . Recruitment to the membrane or cortex relieves this autoinhibition, disrupting the head-tail interaction and inducing dimer formation 29,36 .…”

“…Recruitment to the membrane or cortex relieves this autoinhibition, disrupting the head-tail interaction and inducing dimer formation 29,36 . The exact mechanism by which the head interacts with the tail and how membrane/ cortical recruitment of the motor switches the monomer from a compact, closed state to an elongated dimer is not yet understood, although a role for PIP 3 binding and the cortical actin network has been identified for MYO10 and DdMyo7, respectively 35,37 . Activation in local clusters favors dimerization of the motor at initiation sites where it can pull on nearby actin filaments, promoting the bundling of actin filaments.…”

Activated I-BAR IRSp53 clustering controls the formation of VASP-actin-based membrane protrusions

“…VASP is a member of the Ena/VASP protein family that has been indicated to contribute to filopodia formation in cells ( 6-9, 14, 37, 38 ). VASP forms homo-tetramers that are weakly processive as actin polymerases.…”

Ena/VASP clustering at microspike tips involves Lamellipodin but not I-BAR proteins, and absolutely requires unconventional Myosin-X

“…Nevertheless, there are some marked differences, for instance regarding recruitment mechanisms. In Dictyostelium , DdMyo7 fails to be recruited to the leading edge in the absence of DdVASP, implying that DdVASP-mediated actin assembly and bundling is needed for DdMyo7 activation (63). In contrast, MyoX acts upstream of Ena/VASP and Lpd in microspike formation in B16-F1 cells, since even after Ena/VASP removal or lack of Lpd recruitment in MyoX-KO cells expressing MyoXΔMF, MyoX clusters at the leading edge are still being formed.…”