Molecular mechanism of Ena/VASP-mediated actin-filament elongation.

Ena/VASP proteins are implicated in a variety of fundamental cellular processes including axon guidance and cell migration. In vitro, they enhance elongation of actin filaments, but at rates differing in nearly an order of magnitude according to species, raising questions about the molecular determinants of rate control. Chimeras from fast and slow elongating VASP proteins were generated and their ability to promote actin polymerization and to bind G-actin was assessed. By in vitro TIRF microscopy as well as thermodynamic and kinetic analyses, we show that the velocity of VASP-mediated filament elongation depends on G-actin recruitment by the WASP homology 2 motif. Comparison of the experimentally observed elongation rates with a quantitative mathematical model moreover revealed that Ena/VASP-mediated filament elongation displays a saturation dependence on the actin monomer concentration, implying that Ena/VASP proteins, independent of species, are fully saturated with actin in vivo and generally act as potent filament elongators. Moreover, our data showed that spontaneous addition of monomers does not occur during processive VASP-mediated filament elongation on surfaces, suggesting that most filament formation in cells is actively controlled.

Clustering of VASP actively drives processive, WH2 domain-mediated actin filament elongation.

Vasodilator-stimulated phosphoprotein (VASP) is a key regulator of dynamic actin structures like filopodia and lamellipodia, but its precise function in their formation is controversial. Using in vitro TIRF microscopy, we show for the first time that both human and Dictyostelium VASP are directly involved in accelerating filament elongation by delivering monomeric actin to the growing barbed end. In solution, DdVASP markedly accelerated actin filament elongation in a concentration-dependent manner but was inhibited by low concentrations of capping protein (CP). In striking contrast, VASP clustered on functionalized beads switched to processive filament elongation that became insensitive even to very high concentrations of CP. Supplemented with the in vivo analysis of VASP mutants and an EM structure of the protein, we propose a mechanism by which membrane-associated VASP oligomers use their WH2 domains to effect both the tethering of actin filaments and their processive elongation in sites of active actin assembly.

Analysis of actin assembly by in vitro TIRF microscopy.

Since directed movement toward an extracellular chemoattractant requires rapid and continuous reorganization of the actin cytoskeleton to form complex structures such as a protruding lamellipodium, it is of great interest to analyze and understand the individual contribution of proteins specifically involved in this process. Over the last decade, enormous progress has been made toward understanding the versatile molecular mechanisms underlying actin-based cell motility and the regulation of site-specific F-actin assembly and disassembly. In spite of this wealth of knowledge and due to the constant discovery of novel regulatory factors, many questions remain to be answered. In this chapter, we describe a powerful method that allows to study the effects of actin-binding proteins on the assembly of single filaments by in vitro total internal reflection fluorescence (TIRF) microscopy using purified proteins and fluorescently labeled actin.