Proteins that cap the ends of the actin filament are essential regulators of cytoskeleton dynamics. While several proteins cap the rapidly-growing barbed end, tropomodulin (Tmod) is the only protein known to cap the slowly-growing pointed end. The lack of structural information severely limits our understanding of Tmod’s capping mechanism. We describe crystal structures of actin complexes with the unstructured N-terminal and the leucine-rich repeat C-terminal domains of Tmod. The structures and biochemical analysis of structure-inspired mutants showed that one Tmod molecule interacts with three actin subunits at the pointed end, while also contacting two tropomyosin molecules on each side of the filament. We found Tmod achieves high affinity binding through several discrete low-affinity interactions, which suggests a mechanism for controlled subunit exchange at the pointed end.

The Rho family GTPase effector IRSp53 has essential roles in filopodia formation and neuronal development, but its regulatory mechanism is poorly understood. IRSp53 contains a membrane-binding BAR domain followed by an unconventional CRIB motif that overlaps with a proline-rich region (CRIB–PR) and an SH3 domain that recruits actin cytoskeleton effectors. Using a fluorescence reporter assay, we show that human IRSp53 adopts a closed inactive conformation that opens synergistically with the binding of human Cdc42 to the CRIB–PR and effector proteins, such as the tumor-promoting factor Eps8, to the SH3 domain. The crystal structure of Cdc42 bound to the CRIB–PR reveals a new mode of effector binding to Rho family GTPases. Structure-inspired mutations disrupt autoinhibition and Cdc42 binding in vitro and decouple Cdc42- and IRSp53-dependent filopodia formation in cells. The data support a combinatorial mechanism of IRSp53 activation.

Sca2 (surface cell antigen 2) is the only bacterial protein known to promote both actin filament nucleation and profilin-dependent elongation, mimicking eukaryotic formins to assemble actin comet tails for Rickettsia motility. We show that Sca2's functional mimicry of formins is achieved through a unique mechanism. Unlike formins, Sca2 is monomeric, but has N-and C-terminal repeat domains (NRD and CRD) that interact with each other for processive barbed-end elongation. The crystal structure of NRD reveals a previously undescribed fold, consisting of helix-loop-helix repeats arranged into an overall crescent shape. CRD is predicted to share this fold and might form together with NRD, a doughnut-shaped formin-like structure. In between NRD and CRD, proline-rich sequences mediate the incorporation of profilin-actin for elongation, and WASP-homology 2 (WH2) domains recruit actin monomers for nucleation. Sca2's α-helical fold is unusual among Gram-negative autotransporters, which overwhelmingly fold as β-solenoids. Rickettsia has therefore "rediscovered" formin-like actin nucleation and elongation.passenger domain | translocator domain | spotted fever M any bacterial pathogens use the actin cytoskeleton of host eukaryotic cells for invasion and motility (1, 2). In so doing, bacteria often resort to mimicry by expressing proteins that adopt core functions of key actin cytoskeletal components, particularly actin filament nucleation and elongation factors. However, bacterial proteins tend to bypass the elaborate regulatory networks characteristic of their eukaryotic counterparts, offering a rare opportunity to dissect their functions within a simplified system (2, 3), with implications for our understanding of pathogenicity and the eukaryotic actin cytoskeleton alike.Rickettsiae are obligate intracellular Gram-negative pathogens that are transmitted to humans via arthropod vectors, such as ticks, fleas, and lice (4). Rickettsia species are responsible for a number of severe human diseases, including typhus and spotted fever (5). The spotted fever group, including Rickettsia parkeri, Rickettsia conorii, Rickettsia rickettsii, and over 20 other species throughout the world, uses the host-cell actin cytoskeleton to spread inter-and intracellularly. Similar to Listeria and Shigella, Rickettsia forms actin comet tails to propel its movement. However, the actin tails of Rickettsia consist of long and unbranched actin filaments, whereas those of Listeria and Shigella contain shorter and densely branched filaments (6, 7). These morphological differences stem from different molecular mechanisms for comet tail formation by these pathogens. Listeria and Shigella rely heavily on the activity of the host Arp2/3 complex that localizes uniformly along their tails (6). Although the Arp2/3 complex, activated by either host nucleation promoting factors (8) or the Rickettsia surface protein RickA (9, 10), is necessary for Rickettsia invasion (11), it is absent from Rickettsia tails (6). Another protein, Sca2 (surface cell antigen 2), has ...

A SAXS-based structural model is described for PICK1, a key player in AMPA receptor trafficking. It is shown that the acidic C-terminal tail of PICK1 is involved in autoinhibition and motility of PICK1-associated vesicle-like structures, but, contrary to previous reports, PICK1 neither binds nor inhibits Arp2/3 complex.

Subdenaturing concentrations of guanidine hydrochloride (GdnHCl) stabilize proteins. For ferrocytochrome c the stabilization is detected at subglobal level with no measured change in global stability. These deductions are made by comparing observed rates of thermally driven ferrocytochrome cHCO reactions with global unfolding rates of ferrocytochrome c measured by stopped flow and NMR hydrogen exchange in the presence of a wide range of GdnHCl concentrations at pH 7, 22 degrees C.