Ena/VASP proteins capture actin filament barbed ends

Actin filaments are central to cell motility and shape changes. Prior research has shown that Ena/VASP proteins influence actin dynamics in various contexts, including cell migration and pathogen invasion. However, the exact mechanisms by which these proteins regulate filament assembly remain unclear. Existing studies suggest roles in filament bundling and protection from capping proteins. No prior work had resolved how Ena/VASP proteins interact with growing filament ends. This uncertainty drove the need for direct observation of actin-VASP interactions. Researchers have proposed that Ena/VASP proteins may stabilize or guide filament elongation. Yet, the precise molecular interactions at barbed ends had not been visualized. This gap motivated the current investigation into Ena/VASP function at the single-filament level.

This study aimed to clarify how Ena/VASP proteins regulate actin filament assembly at barbed ends. The specific problem was to determine whether these proteins influence filament elongation or capping. The motivation stemmed from conflicting in vitro observations of Ena/VASP activity. Researchers sought to observe interactions in real time using single-molecule techniques. The goal was to test whether VASP promotes filament growth by recruiting profilin-actin. The study focused on barbed end dynamics in the presence of VASP. The research team aimed to distinguish between filament bundling and end-specific effects. Understanding these mechanisms could inform broader actin regulation models.

The study used total internal reflection fluorescence microscopy to track individual actin filaments. Filaments were observed growing in the presence of VASP and profilin. Surfaces were coated with VASP to capture filaments at barbed ends. Filament elongation rates were measured under varying protein conditions. Researchers monitored filament attachment and detachment events. Profilin-actin mixtures were compared with profilin-actin plus VASP. Filament growth was analyzed for pauses and resumption patterns. The experimental setup allowed direct observation of barbed end interactions.

Ena/VASP proteins did not change filament growth rates on their own. Filament elongation increased when profilin-actin was combined with VASP. VASP-coated surfaces captured growing barbed ends directly. Filaments attached to surfaces via barbed end interactions. Some filaments paused at the end before resuming growth. Paused filaments often resumed after side attachments formed. VASP promoted filament assembly by recruiting profilin-actin. The findings suggest that VASP blocks capping at barbed ends.

The authors propose that Ena/VASP proteins regulate actin assembly by interacting with barbed ends. These proteins recruit profilin-actin to growing filaments. VASP also prevents filament capping, allowing continued elongation. Filaments captured at barbed ends may remain attached via side interactions. The study suggests that VASP enhances filament growth through multiple mechanisms. Filament bundling and protection from capping are key functions. The findings align with prior observations of Ena/VASP activity in cells. The results support a model where VASP stabilizes barbed ends during assembly.