Vasodilator-stimulated phosphoprotein (VASP) is an actin polymerase that binds to the ‘barbed’ ends of actin filaments to drive polymerisation and prevent binding of capping proteins, which terminate polymerisation. Thus, VASP is a known regulator of the growth and stability of actin-rich filopodial protrusions. Previously, Stephanie Gupton and colleagues found that VASP itself is regulated by and colocalises with the E3 ubiquitin ligase TRIM9 at the tips of filopodia. There, TRIM9 monoubiquitylates VASP; however, the mechanistic impact of this post-translational modification was not known. Now, in this study (McCormick et al., 2024), they use biochemical assays to show that VASP is monoubiquitylated at two sites, and that these modifications attenuate the ability of VASP to bundle and elongate actin filaments. Interestingly, however, ubiquitylated VASP maintains its ability to bind to the barbed ends of filaments and protect them from capping proteins. The authors also electroporate recombinant ubiquitylated VASP protein into murine embryonic fibroblasts, where the protein shows reduced localisation to filopodia and lamellipodia compared with that by ubiquitin-free VASP, resulting in altered cell spreading morphology, including greater numbers of ruffled cells and fewer filopodia-rich cells. Together, these findings suggest that multi-monoubiquitylation of VASP by TRIM9 negatively regulates VASP activity, thus impacting actin filament and filopodia dynamics.
