The two actin-related subunits of the Arp2/3 complex, Arp2 and Arp3, are proposed to form a pseudo actin dimer that nucleates actin polymerization. However, in the crystal structure of the inactive complex, they are too far apart to form such a nucleus. Here, we show using EM that yeast and bovine Arp2/3 complexes exist in a distribution among open, intermediate and closed conformations. The crystal structure docks well into the open conformation. The activator WASp binds at the cleft between Arp2 and Arp3, and all WASp-bound complexes are closed. The inhibitor coronin binds near the p35 subunit, and all coronin-bound complexes are open. Activating and loss-of-function mutations in the p35 subunit skew conformational distribution in opposite directions, closed and open, respectively. We conclude that WASp stabilizes p35-dependent closure of the complex, holding Arp2 and Arp3 closer together to nucleate an actin filament.

Dual-color total internal reflection fluorescence microscopy revealed that the N-terminal half of Srv2 (N-Srv2) directly catalyzes severing of cofilin-decorated actin filaments. N-Srv2 formed novel six-bladed structures resembling ninja throwing stars (shurikens), and N-Srv2 activities were critical for actin organization in vivo and were lethal in combination with Aip1.

The two-domain architecture of the Shaker channel is consistent with the recently proposed "hanging gondola" model for the T1 domain, putting the T1 domain at a distance from the membrane domain but attached to it by thin connectors. The space between the two domains is sufficient to permit cytoplasmic access of ions and the N-terminal inactivation domain to the pore region. A hanging gondola architecture has also been observed in the nicotinic acetylcholine receptor and the KcsA structure, suggesting that it is a common element of ion channels.

Srv2/CAP is a conserved actin-binding protein with important roles in driving cellular actin dynamics in diverse animal, fungal, and plant species. However, there have been conflicting reports about whether the activities of Srv2/CAP are conserved, particularly between yeast and mammalian homologs. Yeast Srv2 has two distinct functions in actin turnover: its hexameric N-terminal-half enhances cofilin-mediated severing of filaments, while its C-terminal-half catalyzes dissociation of cofilin from ADP-actin monomers and stimulates nucleotide exchange. Here, we dissected the structure and function of mouse CAP1 to better understand its mechanistic relationship to yeast Srv2. Although CAP1 has a shorter N-terminal oligomerization sequence compared with Srv2, we find that the N-terminal-half of CAP1 (N-CAP1) forms hexameric structures with six protrusions, similar to N-Srv2. Further, N-CAP1 autonomously binds to F-actin and decorates the sides and ends of filaments, altering F-actin structure and enhancing cofilin-mediated severing. These activities depend on conserved surface residues on the helical-folded domain. Moreover, N-CAP1 enhances yeast cofilin-mediated severing, and conversely, yeast N-Srv2 enhances human cofilin-mediated severing, highlighting the mechanistic conservation between yeast and mammals. Further, we demonstrate that the C-terminal actin-binding β-sheet domain of CAP1 is sufficient to catalyze nucleotide-exchange of ADP-actin monomers, while in the presence of cofilin this activity additionally requires the WH2 domain. Thus, the structures, activities, and mechanisms of mouse and yeast Srv2/CAP homologs are remarkably well conserved, suggesting that the same activities and mechanisms underlie many of the diverse actin-based functions ascribed to Srv2/CAP homologs in different organisms.