Parallel actin monomers in the 8S complex of actin-INF2.

Polymerization and depolymerization of actin play an essential role in eukaryotic cells. Actin exists in cells in both monomeric (G-actin) and filamentous (polymer, F-actin) forms. Actin binding proteins (ABPs) facilitate the transition between these two states, and their interactions with these two states of actin are critical for actin-based cellular processes. Rapid depolymerization of actin is assisted in the brain and/or other cells by its oxidation by the enzyme Mical (yielding Mox-actin), and/or by the binding of Inverted Formin 2 (INF2) - which can also accelerate filaments formation. At their stoichiometric molar ratio INF2 and actin yield the 8S complex (consisting of 4 actin monomers: 2 INF2 dimer molecules). Using biochemical and biophysical methods, we investigate the structural arrangement of actin in the 8S particles and the interaction of INF2 with actin and Mox-actin. To that end, we show 2 D class averages of 8S particles obtained by negative staining electron microscopy. We also show that: (i) 8S particles can seed rapid actin assembly; (ii) Mox-actin and INF2 form 8S particles at proteins ratios similar to those of unoxidized actin; (iii) chemical crosslinkings suggest that actin monomers are in a parallel orientation in the 8S particles of both actin and Mox-actin; and (iv) INF2 accelerates the disassembly of Mox-F-actin. Our results provide better understanding of actin's arrangement in the 8S particles formed during actin depolymerization and in the early polymerization stages of both actin and Mox-actin.Communicated by Ramaswamy H. Sarma.

D-loop Dynamics and Near-Atomic-Resolution Cryo-EM Structure of Phalloidin-Bound F-Actin.

Detailed molecular information on G-actin assembly into filaments (F-actin), and their structure, dynamics, and interactions, is essential for understanding their cellular functions. Previous studies indicate that a flexible DNase I binding loop (D-loop, residues 40-50) plays a major role in actin's conformational dynamics. Phalloidin, a "gold standard" for actin filament staining, stabilizes them and affects the D-loop. Using disulfide crosslinking in yeast actin D-loop mutant Q41C/V45C, light-scattering measurements, and cryoelectron microscopy reconstructions, we probed the constraints of D-loop dynamics and its contribution to F-actin formation/stability. Our data support a model of residues 41-45 distances that facilitate G- to F-actin transition. We report also a 3.3-Å resolution structure of phalloidin-bound F-actin in the ADP-Pi-like (ADP-BeFx) state. This shows the phalloidin-binding site on F-actin and how the relative movement between its two protofilaments is restricted by it. Together, our results provide molecular details of F-actin structure and D-loop dynamics.

Structural Organization and Dynamics of Homodimeric Cytohesin Family Arf GTPase Exchange Factors in Solution and on Membranes.

Membrane dynamic processes require Arf GTPase activation by guanine nucleotide exchange factors (GEFs) with a Sec7 domain. Cytohesin family Arf GEFs function in signaling and cell migration through Arf GTPase activation on the plasma membrane and endosomes. In this study, the structural organization of two cytohesins (Grp1 and ARNO) was investigated in solution by size exclusion-small angle X-ray scattering and negative stain-electron microscopy and on membranes by dynamic light scattering, hydrogen-deuterium exchange-mass spectrometry and guanosine diphosphate (GDP)/guanosine triphosphate (GTP) exchange assays. The results suggest that cytohesins form elongated dimers with a central coiled coil and membrane-binding pleckstrin-homology (PH) domains at opposite ends. The dimers display significant conformational heterogeneity, with a preference for compact to intermediate conformations. Phosphoinositide-dependent membrane recruitment is mediated by one PH domain at a time and alters the conformational dynamics to prime allosteric activation by Arf-GTP. A structural model for membrane targeting and allosteric activation of full-length cytohesin dimers is discussed.

Structural Dynamics Control Allosteric Activation of Cytohesin Family Arf GTPase Exchange Factors.

Membrane dynamic processes including vesicle biogenesis depend on Arf guanosine triphosphatase (GTPase) activation by guanine nucleotide exchange factors (GEFs) containing a catalytic Sec7 domain and a membrane-targeting module such as a pleckstrin homology (PH) domain. The catalytic output of cytohesin family Arf GEFs is controlled by autoinhibitory interactions that impede accessibility of the exchange site in the Sec7 domain. These restraints can be relieved through activator Arf-GTP binding to an allosteric site comprising the PH domain and proximal autoinhibitory elements (Sec7-PH linker and C-terminal helix). Small-angle X-ray scattering and negative-stain electron microscopy were used to investigate the structural organization and conformational dynamics of cytohesin-3 (Grp1) in autoinhibited and active states. The results support a model in which hinge dynamics in the autoinhibited state expose the activator site for Arf-GTP binding, while subsequent C-terminal helix unlatching and repositioning unleash conformational entropy in the Sec7-PH linker to drive exposure of the exchange site.

Membrane Trafficking: An Endosome Tether Meets a Rab and Collapses.

Long-range tethering is a ubiquitous recognition event preceding membrane fusion. A new study shows that Rab GTPase binding causes 'entropic collapse' of the coiled-coil endosome tether EEA1, driving membrane apposition and facilitating short-range interactions required for fusion.

Dynamical features of the Plasmodium falciparum ribosome during translation.

Plasmodium falciparum, the mosquito-transmitted Apicomplexan parasite, causes the most severe form of human malaria. In the asexual blood-stage, the parasite resides within erythrocytes where it proliferates, multiplies and finally spreads to new erythrocytes. Development of drugs targeting the ribosome, the site of protein synthesis, requires specific knowledge of its structure and work cycle, and, critically, the ways they differ from those in the human host. Here, we present five cryo-electron microscopy (cryo-EM) reconstructions of ribosomes purified from P. falciparum blood-stage schizonts at sub-nanometer resolution. Atomic models were built from these density maps by flexible fitting. Significantly, our study has taken advantage of new capabilities of cryo-EM, in visualizing several structures co-existing in the sample at once, at a resolution sufficient for building atomic models. We have discovered structural and dynamic features that differentiate the ribosomes of P. falciparum from those of mammalian system. Prompted by the absence of RACK1 on the ribosome in our and an earlier study we confirmed that RACK1 does not specifically co-purify with the 80S fraction in schizonts. More extensive studies, using cryo-EM methodology, of translation in the parasite will provide structural knowledge that may lead to development of novel anti-malarials.

A new role for Rab GTPases during early mitotic stages.

A recent study revealed new roles for the Rab11 GTPase during mitosis. Rab11 is involved in recycling endosome localization to mitotic spindle poles via dynein-mediated transport. This process is in contrast to Golgi membranes, which disperse in mitosis and do not appear to directly contribute to mitotic functions. Rab11-depletion prevents recycling endosome organization at spindle poles, delays mitotic progression, and disrupts spindle pole protein recruitment, astral microtubule organization, and mitotic spindle orientation. However, Rab11 is not the only endocytic and/or trafficking protein that regulates mitotic progression. Clathrin and two small GTPases (Rab6A', Rab5) play key roles in spindle organization and function. In this commentary, we discuss the roles of all these canonical endocytic and membrane trafficking proteins during mitosis and speculate on possible cross-communication between them and their molecular pathways that ensure faithful progression through mitosis.

Affinity grid-based cryo-EM of PKC binding to RACK1 on the ribosome.

Affinity grids (AG) are specialized EM grids that bind macromolecular complexes containing tagged proteins to obtain maximum occupancy for structural analysis through single-particle EM. In this study, utilizing AG, we show that His-tagged activated PKC ßII binds to the small ribosomal subunit (40S). We reconstructed a cryo-EM map which shows that PKC ßII interacts with RACK1, a seven-bladed ß-propeller protein present on the 40S and binds in two different regions close to blades 3 and 4 of RACK1. This study is a first step in understanding the molecular framework of PKC ßII/RACK1 interaction and its role in translation.

Cryo-EM structure of the mammalian eukaryotic release factor eRF1-eRF3-associated termination complex.

Eukaryotic translation termination results from the complex functional interplay between two eukaryotic release factors, eRF1 and eRF3, and the ribosome, in which GTP hydrolysis by eRF3 couples codon recognition with peptidyl-tRNA hydrolysis by eRF1. Here, using cryo-electron microscopy (cryo-EM) and flexible fitting, we determined the structure of eRF1-eRF3-guanosine 5'-[ß,γ-imido]triphosphate (GMPPNP)-bound ribosomal pretermination complex (pre-TC), which corresponds to the initial, pre-GTP hydrolysis stage of factor attachment. Our results show that eukaryotic translation termination involves a network of interactions between the two release factors and the ribosome. Our structure provides mechanistic insight into the coordination between GTP hydrolysis by eRF3 and subsequent peptide release by eRF1.

The variable subdomain of Escherichia coli SecA functions to regulate SecA ATPase activity and ADP release.

Bacterial SecA proteins can be categorized by the presence or absence of a variable subdomain (VAR) located within nucleotide-binding domain II of the SecA DEAD motor. Here we show that VAR is dispensable for SecA function, since the VAR deletion mutant secAΔ519-547 displayed a wild-type rate of cellular growth and protein export. Loss or gain of VAR is extremely rare in the history of bacterial evolution, indicating that it appears to contribute to secA function within the relevant species in their natural environments. VAR removal also results in additional secA phenotypes: azide resistance (Azi(r)) and suppression of signal sequence defects (PrlD). The SecAΔ(519-547) protein was found to be modestly hyperactive for SecA ATPase activities and displayed an accelerated rate of ADP release, consistent with the biochemical basis of azide resistance. Based on our findings, we discuss models whereby VAR allosterically regulates SecA DEAD motor function at SecYEG.

Mapping of the SecA·SecY and SecA·SecG interfaces by site-directed in vivo photocross-linking.

The two major components of the Eubacteria Sec-dependent protein translocation system are the heterotrimeric channel-forming component SecYEG and its binding partner, the SecA ATPase nanomotor. Once bound to SecYEG, the preprotein substrate, and ATP, SecA undergoes ATP-hydrolytic cycles that drive the stepwise translocation of proteins. Although a previous site-directed in vivo photocross-linking study (Mori, H., and Ito, K. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 16159-16164) elucidated residues of SecY needed for interaction with SecA, no reciprocal study for SecA protein has been reported to date. In the present study we mapped residues of SecA that interact with SecY or SecG utilizing this approach. Our results show that distinct domains of SecA on two halves of the molecule interact with two corresponding SecY partners as well as with the central cytoplasmic domain of SecG. Our data support the in vivo relevance of the Thermotoga maritima SecA·SecYEG crystal structure that visualized SecYEG interaction for only one-half of SecA as well as previous studies indicating that SecA normally binds two molecules of SecYEG.

Reexamination of the role of the amino terminus of SecA in promoting its dimerization and functional state.

The SecA nanomotor promotes protein translocation in eubacteria by binding both protein cargo and the protein-conducting channel and by undergoing ATP-driven conformation cycles that drive this process. There are conflicting reports about whether SecA functions as a monomer or dimer during this dynamic process. Here we reexamined the roles of the amino and carboxyl termini of SecA in promoting its dimerization and functional state by examining three secA mutants and the corresponding proteins: SecADelta8 lacking residues 2 to 8, SecADelta11 lacking residues 2 to 11, and SecADelta11/N95 lacking both residues 2 to 11 and the carboxyl-terminal 70 residues. We demonstrated that whether SecADelta11 or SecADelta11/N95 was functional for promoting cell growth depended solely on the vivo level of the protein, which appeared to govern residual dimerization. All three SecA mutant proteins were defective for promoting cell growth unless they were highly overproduced. Cell fractionation revealed that SecADelta11 and SecADelta11/N95 were proficient in membrane association, although the formation of integral membrane SecA was reduced. The presence of a modestly higher level of SecADelta11/N95 in the membrane and the ability of this protein to form dimers, as detected by chemical cross-linking, were consistent with the higher level of secA expression and better growth of the SecADelta11/N95 mutant than of the SecADelta11 mutant. Biochemical studies showed that SecADelta11 and SecADelta11/N95 had identical dimerization defects, while SecADelta8 was intermediate between these proteins and wild-type SecA in terms of dimer formation. Furthermore, both SecADelta11 and SecADelta11/N95 were equally defective in translocation ATPase specific activity. Our studies showed that the nonessential carboxyl-terminal 70 residues of SecA play no role in its dimerization, while increasing the truncation of the amino-terminal region of SecA from 8 to 11 residues results in increased defects in SecA dimerization and poor in vivo function unless the protein is highly overexpressed. They also clarified a number of conflicting previous reports and support the essential nature of the SecA dimer.