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

Significance

The most prominent, protrusive structures of migrating cells on flat substrates include the sheet-like lamellipodia and actin-filament bundles termed filopodia or microspikes, the latter largely embedded in lamellipodia. Microspike formation requires clustering of Ena/VASP proteins at filament-barbed ends to enable sustained processive actin polymerization in the presence of heterodimeric capping protein. However, the factors and mechanisms mediating Ena/VASP clustering have remained elusive. Here, we systematically analyzed these processes in genetic knockout mutants derived from B16-F1 cells. Subsequent analysis revealed an unanticipated relationship of proteins implicated in control of dynamic actin protrusions showing, that contrary to previous assumptions, inverse BAR-domain proteins are not involved. Instead, we show that Ena/VASP clustering at microspike tips involves lamellipodin and strictly requires unconventional myosin-X.

Abstract

Sheet-like membrane protrusions at the leading edge, termed lamellipodia, drive 2D-cell migration using active actin polymerization. Microspikes comprise actin-filament bundles embedded within lamellipodia, but the molecular mechanisms driving their formation and their potential functional relevance have remained elusive. Microspike formation requires the specific activity of clustered Ena/VASP proteins at their tips to enable processive actin assembly in the presence of capping protein, but the factors and mechanisms mediating Ena/VASP clustering are poorly understood. Systematic analyses of B16-F1 melanoma mutants lacking potential candidate proteins revealed that neither inverse BAR-domain proteins, nor lamellipodin or Abi is essential for clustering, although they differentially contribute to lamellipodial VASP accumulation. In contrast, unconventional myosin-X (MyoX) identified here as proximal to VASP was obligatory for Ena/VASP clustering and microspike formation. Interestingly, and despite the invariable distribution of other relevant marker proteins, the width of lamellipodia in MyoX-KO mutants was significantly reduced as compared with B16-F1 control, suggesting that microspikes contribute to lamellipodium stability. Consistently, MyoX removal caused marked defects in protrusion and random 2D-cell migration. Strikingly, Ena/VASP-deficiency also uncoupled MyoX cluster dynamics from actin assembly in lamellipodia, establishing their tight functional association in microspike formation.