Profilin coordinates actin network assembly during cell-edge protrusion

Profilin, an actin-binding protein, acts as a regulator of branched actin networks that push the edge of a cell forward, according to a peer-reviewed experimental and modeling study led by scientists at the Helmholtz Centre for Infection Research in Braunschweig. The researchers found that profilin promotes the activity and localization of the Arp2/3 complex while counterbalancing proteins that accelerate actin-filament growth. These opposing actions help explain how lamellipodia—the broad, sheet-like protrusions associated with cell migration—are assembled and renewed as the cell advances.

Lamellipodia contain dense networks of actin filaments that convert the assembly of cytoskeletal polymers into pushing forces against the cell membrane. Several molecular components cooperate in this process. The Arp2/3 complex branches existing filaments to create the branched networks characteristic of lamellipodia. Ena/VASP family polymerases accelerate filament growth by adding actin monomers, whereas a heterodimeric capping protein blocks rapidly growing filament ends and shields them from further polymerization. Profilin binds actin monomers and prepares them for incorporation into filaments, placing it at a central position in the network. Its precise role, however, had been controversial, with earlier studies proposing that profilin either directed actin monomers toward Ena/VASP-dependent structures or, alternatively, favored Arp2/3-dependent networks.

To distinguish these possibilities, the team used CRISPR/Cas9 genome editing to disrupt individual components alone and in combination. The researchers then examined the effects on lamellipodium formation and function and measured the relative distribution of the remaining proteins. These experiments identified a step-wise set of functional relationships. Profilin counteracted Ena/VASP while promoting Arp2/3 complex activity, and Ena/VASP and capping protein acted antagonistically. Combination experiments separated additional pathways: profilin remained important for localizing the Arp2/3 complex even when Ena/VASP was absent, but this relationship was not observed with capping protein. Conversely, the absence of Ena/VASP eliminated the effect of profilin on capping protein. The results therefore support distinct molecular routes through which profilin influences the two antagonistic regulators.

Mathematical modeling by the team and Martin Falcke at the Max Delbrück Center Berlin reproduced the experimental findings and further indicated that Ena/VASP and capping protein must compete for a common recruitment mechanism at lamellipodial edges. The study does not identify the molecular nature of that recruitment structure, which remains an open question for future work. The findings establish a mechanistic explanation for how branched actin networks generate membrane protrusion, but they do not by themselves establish a treatment for cancer metastasis, infection-associated migration, or other disorders involving abnormal cell movement. Instead, the work provides a framework for investigating such processes at the molecular level.

Subject of Research: Molecular regulation of branched actin networks in lamellipodium protrusion

Article Title: How cells move – and how they regulate this

Article References: How cells move – and how they regulate this. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: profilin, actin networks, lamellipodia, cell migration, Arp2/3 complex, Ena/VASP, capping protein, CRISPR screening, mathematical modeling, cells, move, they