The branching of actin filaments mediates various cellular processes related to movement, such as cellular motility and endocytosis. The Arp2/3 protein complex plays a major role in nucleating, or creating, new branches of actin filaments. This complex, in turn, is carefully regulated by a family of proteins called WASP through a mechanism not yet fully understood.
New findings now reveal how WASP binds to Arp2/3 through a combination of biophysical, biochemical and computational methods.This work, published recently in the Proceedings of the National Academy of Sciences, comes from groups in University of Washington in Seattle and University of Oregon.
Recent data have shown that WASP binds the complex at two sites, leading to development of structural binding models. One WASP segment engages “barbed ends” of the Arp3 and Arp2 subunits while the another segment binds the back side of the complex on Arp3. Electron microscopy reconstructions, however, have challenged this model.
In their paper, the team then used “chemical cross-linking and mass spectrometry (XL-MS) along with computational docking and structure-based mutational analysis” to experimentally map the two WASP binding regions on the Arp2/3 complex. What they found was data that “corroborate the barbed end and back side binding models,” with one WASP binding site on the back side on Arp3, and a second site that spans two Arp subunits.
"What we discovered was exciting because knowing precisely how the activator protein binds to Arp2/3 complex is the first step in understanding how it turns on its branching activity," says senior co-author Brad Nolen.
Understanding how this branching activity is turned on is important in some disease states, such as HIV and cancer, in which cells can lose control of their actin cytoskeleton. The process is thus relevant in developing new cancer drugs.
For example, Nolen said, a drug that blocks the site on Arp2/3 where the activator protein touches would prevent actin branching. That could stop tumor cells from crawling, or metastasizing. This is similar to the approach used by the successful chemotherapy drug paclitaxel, which targets tubulin, another filament-forming protein.
Image: Small molecule inhibitors that target Arp2/3 to control actin cytoskeletal dynamics may have clinical potential. Image courtesy of the Nolen Lab of University of Oregon.