The polymerization of actin: structural changes from small-angle neutron scattering

Visualizing the nucleating and capped states of f-actin by Ca2+-gelsolin: Saxs data based structures of binary and ternary complexes

Structural insight eludes on how full-length gelsolin depolymerizes and caps filamentous (F-)actin, while the same entity can nucleate polymerization of G-actins. Analyzing small angle X-ray scattering (SAXS) data, we deciphered assemblies which enable these contrasting processes. Mixing Ca2+-gelsolin with F-actin in high salt F-buffer resulted in depolymerization of ordered F-actin rods to smaller sized species which became monodispersed upon dialysis with low salt G-buffer. These entities were the ternary (GA2) and binary (GA) complexes of gelsolin and actin with radius of gyration and maximum linear dimension of 4.55 and 4.68 nm, and 15 and 16 nm, respectively. Using size exclusion chromatography in-line with SAXS, we confirmed that initially GA and GA2 species are formed as seen upon depolymerization of F-actin followed by dialysis. Interestingly, while GA2 could seed formation of native-like F-actin in both G- and F-buffer, GA failed in G-buffer. Thus, GA2 and GA are the central species formed via depolymerization or towards nucleation. SAXS profile referenced modeling revealed that: 1) in GA, actin is bound to the C-terminal half of gelsolin, and 2) in GA2, second actin binds to the open N-terminal half accompanied by dramatic rearrangements across g1-g2 and g3-g4 linkers.

Indirect Fourier transform in the context of statistical inference

Inferring structural information from the intensity of a small-angle scattering (SAS) experiment is an ill-posed inverse problem. Thus, the determination of a solution is in general non-trivial. In this work, the indirect Fourier transform (IFT), which determines the pair distance distribution function from the intensity and hence yields structural information, is discussed within two different statistical inference approaches, namely a frequentist one and a Bayesian one, in order to determine a solution objectively From the frequentist approach the cross-validation method is obtained as a good practical objective function for selecting an IFT solution. Moreover, modern machine learning methods are employed to suppress oscillatory behaviour of the solution, hence extracting only meaningful features of the solution. By comparing the results yielded by the different methods presented here, the reliability of the outcome can be improved and thus the approach should enable more reliable information to be deduced from SAS experiments.

pH-Induced Re-entrant Microstructural Transitions in Cationic Surfactant-Hydrotrope Mixtures

The structural transitions occurring with change in pH for aqueous mixtures of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) and a hydrotrope (sodium salicylate, NaSal) were investigated at various temperatures using dynamic light scattering and small-angle neutron scattering. Direct structural studies show a transition from rigid cylindrical micelles at neutral pH to spherical micelles at ∼ pH 2 upon protonation of salicylate molecules; however, an unanticipated reversion to flexible cylindrical micelles with further decrease in pH was observed. We also observed these microstructure transitions from cylinders at high pH to spherical micelles at intermediate pH to flexible cylindrical micelles at low pH were highly sensitive to temperature. Our results suggest that, in addition to the well-described electrostatic and hydrophobic interactions in cationic surfactant-hydrotrope mixtures, the pH-induced microstructural changes are potentially governed by complementary cation-π and hydrogen bonding interactions.

Investigating the structural impact of the glutamine repeat in huntingtin assembly

Acquiring detailed structural information about the various aggregation states of the huntingtin-exon1 protein (Htt-exon1) is crucial not only for identifying the true nature of the neurotoxic species responsible for Huntington's disease (HD) but also for designing effective therapeutics. Using time-resolved small-angle neutron scattering (TR-SANS), we followed the conformational changes that occurred during fibrillization of the pathologic form of Htt-exon1 (NtQ42P10) and compared the results with those obtained for the wild-type (NtQ22P10). Our results show that the aggregation pathway of NtQ22P10 is very different from that of NtQ42P10, as the initial steps require a monomer to 7-mer transition stage. In contrast, the earliest species identified for NtQ42P10 are monomer and dimer. The divergent pathways ultimately result in NtQ22P10 fibrils that possess a packing arrangement consistent with the common amyloid sterical zipper model, whereas NtQ42P10 fibrils present a better fit to the Perutz β-helix structural model. The structural details obtained by TR-SANS should help to delineate the key mechanisms that underpin Htt-exon1 aggregation leading to HD.

Unique properties of eukaryote-type actin and profilin horizontally transferred to cyanobacteria

A eukaryote-type actin and its binding protein profilin encoded on a genomic island in the cyanobacterium Microcystis aeruginosa PCC 7806 co-localize to form a hollow, spherical enclosure occupying a considerable intracellular space as shown by in vivo fluorescence microscopy. Biochemical and biophysical characterization reveals key differences between these proteins and their eukaryotic homologs. Small-angle X-ray scattering shows that the actin assembles into elongated, filamentous polymers which can be visualized microscopically with fluorescent phalloidin. Whereas rabbit actin forms thin cylindrical filaments about 100 µm in length, cyanobacterial actin polymers resemble a ribbon, arrest polymerization at 5-10 µm and tend to form irregular multi-strand assemblies. While eukaryotic profilin is a specific actin monomer binding protein, cyanobacterial profilin shows the unprecedented property of decorating actin filaments. Electron micrographs show that cyanobacterial profilin stimulates actin filament bundling and stabilizes their lateral alignment into heteropolymeric sheets from which the observed hollow enclosure may be formed. We hypothesize that adaptation to the confined space of a bacterial cell devoid of binding proteins usually regulating actin polymerization in eukaryotes has driven the co-evolution of cyanobacterial actin and profilin, giving rise to an intracellular entity.

Optical torques guiding cell motility

The main mechanism responsible for cell motility is the stochastic generation and breakup of actin filaments forming the cytoskeleton. However, the role of environmental factors in the migration and differentiation of cells is yet to be fully understood. Here we demonstrate that polarized optical fields can exert controllable torques on the actin network and therefore influence the treadmilling process responsible for cells motility. Through systematic experiments and stochastic modeling we demonstrate that actively guiding the dynamics of large groups of cells is possible in a noninvasive manner.

Clathrin triskelia show evidence of molecular flexibility

The clathrin triskelion, which is a three-legged pinwheel-shaped heteropolymer, is a major component in the protein coats of certain post-Golgi and endocytic vesicles. At low pH, or at physiological pH in the presence of assembly proteins, triskelia will self-assemble to form a closed clathrin cage, or "basket". Recent static light scattering and dynamic light scattering studies of triskelia in solution showed that an individual triskelion has an intrinsic pucker similar to, but differing from, that inferred from a high resolution cryoEM structure of a triskelion in a clathrin basket. We extend the earlier solution studies by performing small-angle neutron scattering (SANS) experiments on isolated triskelia, allowing us to examine a higher q range than that probed by static light scattering. Results of the SANS measurements are consistent with the light scattering measurements, but show a shoulder in the scattering function at intermediate q values (0.016 A(-1)), just beyond the Guinier regime. This feature can be accounted for by Brownian dynamics simulations based on flexible bead-spring models of a triskelion, which generate time-averaged scattering functions. Calculated scattering profiles are in good agreement with the experimental SANS profiles when the persistence length of the assumed semiflexible triskelion is close to that previously estimated from the analysis of electron micrographs.

The self-assembly, elasticity, and dynamics of cardiac thin filaments

Solutions of intact cardiac thin filaments were examined with transmission electron microscopy, dynamic light scattering (DLS), and particle-tracking microrheology. The filaments self-assembled in solution with a bell-shaped distribution of contour lengths that contained a population of filaments of much greater length than the in vivo sarcomere size ( approximately 1 mum) due to a one-dimensional annealing process. Dynamic semiflexible modes were found in DLS measurements at fast timescales (12.5 ns-0.0001 s). The bending modulus of the fibers is found to be in the range 4.5-16 x 10(-27) Jm and is weakly dependent on calcium concentration (with Ca2+ > or = without Ca2+). Good quantitative agreement was found for the values of the fiber diameter calculated from transmission electron microscopy and from the initial decay of DLS correlation functions: 9.9 nm and 9.7 nm with and without Ca2+, respectively. In contrast, at slower timescales and high polymer concentrations, microrheology indicates that the cardiac filaments act as short rods in solution according to the predictions of the Doi-Edwards chopsticks model (viscosity, eta approximately c(3), where c is the polymer concentration). This differs from the semiflexible behavior of long synthetic actin filaments at comparable polymer concentrations and timescales (elastic shear modulus, G' approximately c(1.4), tightly entangled) and is due to the relative ratio of the contour lengths ( approximately 30). The scaling dependence of the elastic shear modulus on the frequency (omega) for cardiac thin filaments is G' approximately omega(3/4 +/- 0.03), which is thought to arise from flexural modes of the filaments.

Effect of Polyelectrolyte Structure on Protein−Polyelectrolyte Coacervates: Coacervates of Bovine Serum Albumin with Poly(diallyldimethylammonium chloride) versus Chitosan

Electrostatic interactions between synthetic polyelectrolytes and proteins can lead to the formation of dense, macroion-rich liquid phases, with equilibrium microheterogeneities on length scales up to hundreds of nanometers. The effects of pH and ionic strength on the rheological and optical properties of these coacervates indicate microstructures sensitive to protein-polyelectrolyte interactions. We report here on the properties of coacervates obtained for bovine serum albumin (BSA) with the biopolyelectrolyte chitosan and find remarkable differences relative to coacervates obtained for BSA with poly(diallyldimethylammonium chloride) (PDADMAC). Coacervation with chitosan occurs more readily than with PDADMAC. Viscosities of coacervates obtained with chitosan are more than an order of magnitude larger and, unlike those with PDADMAC, show temperature and shear rate dependence. For the coacervates with chitosan, a fast relaxation time in dynamic light scattering, attributable to relatively unrestricted protein diffusion in both systems, is diminished in intensity by a factor of 3-4, and the consequent dominance by slow modes is accompanied by a more heterogeneous array of slow apparent diffusivities. In place of a small-angle neutron scattering Guinier region in the vicinity of 0.004 A-1, a 10-fold increase in scattering intensity is observed at lower q. Taken together, these results confirm the presence of dense domains on length scales of hundreds of nanometers to micrometers, which in coacervates prepared with chitosan are less solidlike, more interconnected, and occupy a larger volume fraction. The differences in properties are thus correlated with differences in mesophase structure.

Assessment of cellular actin dynamics by measurement of fluorescence anisotropy

To study cellular actin dynamics, a cell-free assay based on fluorescence anisotropy was developed. Using G-actin-Alexa as a probe, we found that anisotropy enhancement reflects F-actin elongation. Anisotropy enhancement varies with the concentration of magnesium and calcium cations and with ethylenediaminetetraacetate or well-known effectors of the polymerization. This assay gives the overall status of actin dynamics in cell extracts which are the closest conditions to in vivo, implying most of the regulating proteins that are missing in purified actin measurements. It can be used in a large-scale screening for chemical compounds which modulate actin polymerization.

Actin polymerization under pressure: A theoretical study

An extended Flory-Huggins-type equilibrium polymerization theory for compressible systems is used to describe experimental data for the unusual pressure and temperature dependence of the equilibrium polymerization of G-actin to F-actin. The calculations provide rich insights into the reaction mechanism and the thermodynamics of actin polymerization at the molecular level. Volume changes associated with individual steps of the mechanism are calculated to be DeltaVactiv=(s1*-s1)upsilon0=+1553 mlmol for the activation reaction, DeltaVdim=(s2-s1*)upsilon0=-3810 mlmol for dimerization, and DeltaVprop=(sP-s1)upsilon0=+361 mlmol for the propagation reaction, where s1upsilon0, s1*upsilon0, s2upsilon0, and sPupsilon0 are the monomer volumes in the G-actin monomer, the activated G-action, the dimer, and higher polymers, respectively. Comparison with experimental measurements is made, and discrepancies are discussed.

Crystal structure of polymerization-competent actin

All actin crystal structures reported to date represent actin complexed or chemically modified with molecules that prevent its polymerization. Actin cleaved with ECP32 protease at a single site between Gly42 and Val43 is virtually non-polymerizable in the Ca-ATP bound form but remains polymerization-competent in the Mg-bound form. Here, a crystal structure of the true uncomplexed ECP32-cleaved actin (ECP-actin) solved to 1.9 A resolution is reported. In contrast to the much more open conformation of the ECP-actin's nucleotide binding cleft in solution, the crystal structure of uncomplexed ECP-actin contains actin in a typical closed conformation similar to the complexed actin structures. This unambiguously demonstrates that the overall structure of monomeric actin is not significantly affected by a multitude of actin-binding proteins and toxins. The invariance of actin crystal structures suggests that the salt and precipitants necessary for crystallization stabilize actin in only one of its possible conformations. The asymmetric unit cell contains a new type of antiparallel actin dimer that may correspond to the "lower dimer" implicated in F-actin nucleation and branching. In addition, symmetry-related actin-actin contacts form a head to tail dimer that is strikingly similar to the longitudinal dimer predicted by the Holmes F-actin model, including a rotation of the monomers relative to each other not observed previously in actin crystal structures.