Ena/VASP: towards resolving a pointed controversy at the barbed end

Abstract

Ena/VASP proteins are conserved regulators of actin dynamics that have important roles in several physiological processes such as morphogenesis, axon guidance, endothelial barrier function, and cancer cell invasion and metastasis. Although considerable evidence points towards an anti-capping mechanism for Ena/VASP function, some controversy remains. Here, we evaluate the evidence for and against the anti-capping hypothesis, including results from some recent structural and biochemical studies that shed new light on this issue. In addition, we describe several alternate mechanisms that Ena/VASP proteins may utilize to regulate actin dynamics in vivo, including inhibition of branching, bundling and profilin-actin recruitment.

Fig. 1.

Intracellular distribution of Ena/VASP proteins. Immunofluorescence staining for VASP (green) and F-actin (red) in a fibroblast.

Fig. 2.

Domain structure of Ena/VASP proteins. The EVH1 and EVH2 domains and proline-rich region are indicated. EVH1 mediates protein:protein interactions; most, but not all, EVH1 ligands bind to a motif that has the consensus sequence (D/E)FPPPPX(D/E)(D/E). The proline-rich region harbors binding sites for profilin, including a high-affinity site adjacent to the EVH2 domain. The EVH2 domain contains a G-actin-binding site (GAB), an F-actin-binding site (FAB) and a coiled-coil at the very C-terminus that mediates tetramerization.

Fig. 3.

Mechanism of anti-capping by Ena/VASP proteins. (A) Tetramer of VASP in association with a bundle of actin filaments. Interactions between the EVH1 domain of Ena/VASP and FPPPP-motif-containing proteins help to position the Ena/VASP tetramer near the plasma membrane. (B) The profilin–G-actin loading mechanism by which Ena/VASP is thought to supply actin monomers to the barbed ends of filaments while antagonizing the ability of CP to terminate filament elongation.