Two Distinct Mechanisms for Actin Capping Protein Regulation—Steric and Allosteric Inhibition

Abstract

Author SummaryActin is a ubiquitous eukaryotic protein that polymerizes into bidirectional filaments and plays essential roles in a variety of biological processes, including cell division, muscle contraction, neuronal development, and cell motility. The actin capping protein (CP) tightly binds to the fast-growing end of the filament (the barbed end) to block monomer association and dissociation at this end, thus acting as an important regulator of actin filament dynamics in cells. Using X-ray crystallography, we present the atomic structures of CP in complex with fragments of two inhibitory proteins, V-1 and CARMIL, to compare the modes of action of these two regulators. The structures demonstrate that V-1 directly blocks the actin-binding site of CP, thereby preventing filament capping, whereas CARMIL functions in a very different manner. Detailed comparison of several CP structures revealed that CP has two stable domains that are continuously twisting relative to each other. CARMIL peptides were found to bind across the two domains of CP on a surface distinct from its actin binding sites. We propose that CARMIL peptides attenuate the binding of CP to actin filaments by suppressing the twisting movement required for tight barbed end capping. Our comparative structural studies therefore have revealed substantial insights in the variety of mechanisms by which different actin regulatory factors function.