Interplay of entropy and enthalpy in peptide binding to zwitterionic phospholipid membranes as revealed from membrane thinning.

Membrane thinning that resulted from peptide-binding is observed via temperature dependent small-angle X-ray scattering (SAXS). The result reveals a mean thermal thinning rate of 0.038 Å K-1 for the neat unilamellar vesicles (ULVs) of a zwitterionic phospholipid of 1,2-dieicosenoyl-sn-glycero-3-phosphocholine (diC20:1PC) in the temperature range of 285-312 K. The thinning effect promotes greatly the association between a model antimicrobial peptide melittin and the ULV. Scaling the observed isothermal melittin-ULV bilayer thinning to that measured using low-angle X-ray diffraction from the melittin-multilamellar membranes of defined peptide-to-lipid ratios establishes temperature-dependent binding isotherms χb of the peptide-ULV as a function of free peptide concentration in solution. From the binding isotherms, temperature-dependent peptide-membrane binding constant K(T) is extracted on the basis of a modified Gouy-Chapman model. Changes in K(T) follow the linearized van't Hoff equation ln K(T) ∝ -ΔHT-1 with a constant enthalpy change ΔH = 9.6 kcal mol-1, suggesting an entropy-driven binding process prior to membrane pore formation. Correspondingly, a five-fold enhancement of K is observed in the temperature range studied. The peptide-binding strength is found to follow the growth trend of the membrane thermal thinning rate better than the lipid chain length of the three phosphocholine-based ULVs of diCn:1PC with n = 18, 20, and 22.

Oligomerization process of Bcl-2 associated X protein revealed from intermediate structures in solution.

Upon apoptotic stress, Bcl-2 associated X (BAX) protein undergoes conformational changes and oligomerizes, leading to the mitochondrial membrane permeabilization and cell death. While structures of the resultant oligomer have been extensively studied, little is known about the intermediates that describe the reaction pathway from the inactive monomers to activated oligomers. Here we characterize the intermediate structures of BAX using combined small-angle X-ray scattering (SAXS) with on-line gel-filtration and electron spin resonance (ESR). The intermediates, including monomers, dimers, and tetramers, are reconstructed via integrating the SAXS-envelopes and ESR-determined skeleton structures. The hence revealed structures suggest a linear oligomerization of BAX utilizing the extended dimers with the two flexible α6 chains protruded out as ditopic ligands. The results of molecular dynamics simulation also support the ditopic dimer conformation with mobile α6. The ditopic dimers could further wind into a helical rod structure with three dimers in one helical turn. Our results not only reveal the on-pathway intermediates, but also suggest a ditopic oligomerization mechanism that may bridge the observed intermediate structures in solution to the large BAX assemblies lately observed on mitochondria.

Peptide-induced bilayer thinning structure of unilamellar vesicles and the related binding behavior as revealed by X-ray scattering.

We have studied the bilayer thinning structure of unilamellar vesicles (ULV) of a phospholipid 1,2-dierucoyl-sn-glycero-3-phosphocholine (di22:1PC) upon binding of melittin, a water-soluble amphipathic peptide. Successive thinning of the ULV bilayers with increasing peptide concentration was monitored via small-angle X-ray scattering (SAXS). Results suggest that the two leaflets of the ULV of closed bilayers are perturbed and thinned asymmetrically upon free peptide binding, in contrast to the centro-symmetric bilayer thinning of the substrate-oriented multilamellar membranes (MLM) with premixed melittin. Moreover, thinning of the melittin-ULV bilayer associates closely with peptide concentration in solution and saturates at ~4%, compared to the ~8% maximum thinning observed for the correspondingly premixed peptide-MLM bilayers. Linearly scaling the thinning of peptide-ULV bilayers to that of the corresponding peptide-MLM of a calibrated peptide-to-lipid ratio, we have deduced the number of bound peptides on the ULV bilayers as a function of free peptide concentration in solution. The hence derived X-ray-based binding isotherm allows extraction of a low binding constant of melittin to the ULV bilayers, on the basis of surface partition equilibrium and the Gouy-Chapman theory. Moreover, we show that the ULV and MLM bilayers of di22:1PC share a same thinning constant upon binding of a hydrophobic peptide alamethicin; this result supports the linear scaling approach used in the melittin-ULV bilayer thinning for thermodynamic binding parameters of water-soluble peptides.

Revealing cholesterol effects on PEGylated HSPC liposomes using AF4-MALS and simultaneous small- and wide-angle X-ray scattering.

Liposome development is of great interest owing to increasing requirements for efficient drug carriers. The structural features and thermal stability of such liposomes are crucial in drug transport and delivery. Reported here are the results of the structural characterization of PEGylated liposomes via small- and wide-angle X-ray scattering and an asymmetric flow field-flow fractionation (AF4) system coupled with differential refractive-index detection, multi-angle light scattering (MALS) and dynamic light scattering. This integrated analysis of the exemplar PEGylated liposome formed from hydrogenated soy phosphatid-yl-choline (HSPC) with the addition of cholesterol reveals an average hydro-dynamic radius (R h) of 52 nm with 10% polydispersity, a comparable radius of gyration (R g) and a major liposome particle mass of 118 kDa. The local bilayer structure of the liposome is found to have asymmetric electronic density profiles in the inner and outer leaflets, sandwiched by two PEGylated outer layers ca 5 nm thick. Cholesterol was found to effectively intervene in lipid chain packing, resulting in the thickening of the liposome bilayer, an increase in the area per lipid and an increase in liposome size, especially in the fluid phase of the liposome. These cholesterol effects show signs of saturation at cholesterol concentrations above ca 1:5 cholesterol:lipid molar ratio.

Structural insights into the regulation, ligand recognition, and oligomerization of bacterial STING.

The cyclic GMP-AMP synthase (cGAS)/stimulator of interferon gene (STING) signaling pathway plays a critical protective role against viral infections. Metazoan STING undergoes multilayers of regulation to ensure specific signal transduction. However, the mechanisms underlying the regulation of bacterial STING remain unclear. In this study, we determined the crystal structure of anti-parallel dimeric form of bacterial STING, which keeps itself in an inactive state by preventing cyclic dinucleotides access. Conformational transition between inactive and active states of bacterial STINGs provides an on-off switch for downstream signaling. Some bacterial STINGs living in extreme environment contain an insertion sequence, which we show codes for an additional long lid that covers the ligand-binding pocket. This lid helps regulate anti-phage activities. Furthermore, bacterial STING can bind cyclic di-AMP in a triangle-shaped conformation via a more compact ligand-binding pocket, forming spiral-shaped protofibrils and higher-order fibril filaments. Based on the differences between cyclic-dinucleotide recognition, oligomerization, and downstream activation of different bacterial STINGs, we proposed a model to explain structure-function evolution of bacterial STINGs.

Membrane Charging and Swelling upon Calcium Adsorption as Revealed by Phospholipid Nanodiscs.

Direct binding of calcium ions (Ca2+) to phospholipid membranes is an unclarified yet critical signaling pathway in diverse Ca2+-regulated cellular phenomena. Here, high-pressure-liquid-chromatography, small-angle X-ray scattering (SAXS), UV-vis absorption, and differential refractive index detections are integrated to probe Ca2+-binding to the zwitterionic lipid membranes in nanodiscs. The responses of the membranes upon Ca2+-binding, in composition and conformation, are quantified through integrated data analysis. The results indicate that Ca2+ binds specifically into the phospholipid headgroup zone, resulting in membrane charging and membrane swelling, with a saturated Ca2+-lipid binding ratio of 1:8. A Ca2+-binding isotherm to the nanodisc is further established and yields an unexpectedly high binding constant K = 4260 M-1 and a leaflet potential of ca. 100 mV based on a modified Gouy-Chapman model. The calcium-lipid binding ratio, however, drops to 40% when the nanodisc undergoes a gel-to-fluid phase transition, leading to an effective charge capacity of a few µF/cm2.

Probing the Acid-Induced Packing Structure Changes of the Molten Globule Domains of a Protein near Equilibrium Unfolding.

Using simultaneously scanning small-angle X-ray scattering (SAXS) and UV-vis absorption with integrated online size exclusion chromatography, supplemental with molecular dynamics simulations, we unveil the long-postulated global structure evolution of a model multidomain protein bovine serum albumin (BSA) during acid-induced unfolding. Our results differentiate three global packing structures of the three molten globule domains of BSA, forming three intermediates I1, I2, and E along the unfolding pathway. The I1-I2 transition, overlooked in all previous studies, involves mainly coordinated reorientations across interconnected molten globule subdomains, and the transition activates a critical pivot domain opening of the protein for entering into the E form, with an unexpectedly large unfolding free energy change of -9.5 kcal mol-1, extracted based on the observed packing structural changes. The revealed local packing flexibility and rigidity of the molten globule domains in the E form elucidate how collective motions of the molten globule domains profoundly influence the folding-unfolding pathway of a multidomain protein.

Correlated changes in structure and viscosity during gelatinization and gelation of tapioca starch granules.

Melting of native tapioca starch granules in aqueous pastes upon heating is observed in situ using simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) and solution viscometry. Correlated structure and viscosity changes suggest closely associated amylose and amylopectin chains in the semicrystalline layers, and the release of amylose chains for enhanced solution viscosity occurs largely after melting of the semicrystalline structure. Before melting, WAXS results reveal mixed crystals of A- and B-types (∼4:1 by weight), whereas SAXS results indicate that the semicrystalline layers are composed of lamellar blocklets of ca 43 nm domain size, with polydisperse crystalline (≃7.5 nm) and amorphous (≃1.1 nm) layers alternatively assembled into a lamellar spacing of ≃8.6 nm with 20% polydispersity. Upon melting, the semicrystalline lamellae disintegrate into disperse and molten amylopectin nanoclusters with dissolved and partially untangled amylose chains in the aqueous matrix which leads to increased solution viscosity. During subsequent cooling, gelation starts at around 347 K; successively increased solution viscosity coincides with the development of nanocluster aggregation to a fractal dimension ≃2.3 at 303 K, signifying increasing intercluster association through collapsed amylose chains owing to decreased solvency of the aqueous medium with decreasing temperature.

A dedicated small-angle X-ray scattering beamline with a superconducting wiggler source at the NSRRC.

At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5 GeV storage ring, a dedicated small-angle X-ray scattering (SAXS) beamline has been installed with an in-achromat superconducting wiggler insertion device of peak magnetic field 3.1 T. The vertical beam divergence from the X-ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (DeltaE/E approximately 2 x 10(-4)) in the energy range 5-23 keV, or by a double Mo/B4C multilayer monochromator for 10-30 times higher flux ( approximately 10(11) photons s(-1)) in the 6-15 keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of approximately 0.9 mm x 0.3 mm (horizontal x vertical) at the focus point located 26.5 m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing-incidence SAXS (GISAXS) from liquid surfaces. Two online beam-position monitors separated by 8 m provide an efficient feedback control for an overall beam-position stability in the 10 microm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray-tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core-shell quantum dots) and GISAXS from liquid surfaces.