Helix orientations in membrane-associated Bcl-XL determined by 15N-solid-state NMR spectroscopy

Solid-State NMR Approaches to Study Protein Structure and Protein-Lipid Interactions

Solid-state NMR spectroscopy has been developed for the investigation of membrane-associated polypeptides and remains one of the few techniques to reveal high-resolution structural information in liquid-disordered phospholipid bilayers. In particular, oriented samples have been used to investigate the structure, dynamics and topology of membrane polypeptides. Much of the previous solid-state NMR work has been developed and performed on peptides but the technique is constantly expanding towards larger membrane proteins. Here, a number of protocols are presented describing among other the reconstitution of membrane proteins into oriented membranes, monitoring membrane alignment by ³¹P solid-state NMR spectroscopy, investigations of the protein by one- and two-dimensional ¹⁵N solid-state NMR and measurements of the lipid order parameters using ²H solid-state NMR spectroscopy. Using such methods solid-state NMR spectroscopy has revealed a detailed picture of the ensemble of both lipids and proteins and their mutual interdependence in the bilayer environment.

Solid-State NMR Investigations of the MHC II Transmembrane Domains: Topological Equilibria and Lipid Interactions

The major histocompatibility complex class II (MHC II) membrane proteins are key players in the adaptive immune response. An aberrant function of these molecules is associated with a large number of autoimmune diseases such as diabetes type I and chronic inflammatory diseases. The MHC class II is assembled from DQ alpha 1 and DQ beta 1 which come together as a heterodimer through GXXXG-mediated protein-protein interactions and a highly specific protein-sphingomyelin-C18 interaction motif located on DQA1. This association can have important consequences in regulating the function of these membrane proteins. Here, we investigated the structure and topology of the DQA1 and DQB1 transmembrane helical domains by CD-, oriented ²H and ¹⁵N solid-state NMR spectroscopies. The spectra at peptide-to-lipid ratios of 0.5 to 2 mol% are indicative of a topological equilibrium involving a helix crossing the membrane with a tilt angle of about 20° and another transmembrane topology with around 30° tilt. The latter is probably representing a dimer. Furthermore, at the lowest peptide-to-lipid ratio, a third polypeptide population becomes obvious. Interestingly, the DQB1 and to a lesser extent the DQA1 transmembrane helical domains exhibit a strong fatty acyl chain disordering effect on the inner segments of the ²H-labelled palmitoyl chain of POPC bilayers. This phosphatidylcholine disordering requires the presence of sphingomyelin-C18 suggesting that the ensemble of transmembrane polypeptide and sphingolipid exerts positive curvature strain.

A Minimal Functional Complex of Cytochrome P450 and FBD of Cytochrome P450 Reductase in Nanodiscs

Structural interactions that enable electron transfer to cytochrome-P450 (CYP450) from its redox partner CYP450-reductase (CPR) are a vital prerequisite for its catalytic mechanism. The first structural model for the membrane-bound functional complex to reveal interactions between the full-length CYP450 and a minimal domain of CPR is now reported. The results suggest that anchorage of the proteins in a lipid bilayer is a minimal requirement for CYP450 catalytic function. Akin to cytochrome-b₅ (cyt-b₅ ), Arg 125 on the C-helix of CYP450s is found to be important for effective electron transfer, thus supporting the competitive behavior of redox partners for CYP450s. A general approach is presented to study protein-protein interactions combining the use of nanodiscs with NMR spectroscopy and SAXS. Linking structural details to the mechanism will help unravel the xenobiotic metabolism of diverse microsomal CYP450s in their native environment and facilitate the design of new drug entities.

Solid-State NMR/Dynamic Nuclear Polarization of Polypeptides in Planar Supported Lipid Bilayers

Dynamic nuclear polarization has been developed to overcome the limitations of the inherently low signal intensity of NMR spectroscopy. This technique promises to be particularly useful for solid-state NMR spectroscopy where the signals are broadened over a larger frequency range and most investigations rely on recording low gamma nuclei. To extend the range of possible investigations, a triple-resonance flat-coil solid-state NMR probe is presented with microwave irradiation capacities allowing the investigation of static samples at temperatures of 100 K, including supported lipid bilayers. The probe performance allows for two-dimensional separated local field experiments with high-power Lee-Goldberg decoupling and cross-polarization under simultaneous irradiation from a gyrotron microwave generator. Efficient cooling of the sample turned out to be essential for best enhancements and line shape and necessitated the development of a dedicated cooling chamber. Furthermore, a new membrane-anchored biradical is presented, and the geometry of supported membranes was optimized not only for good membrane alignment, handling, stability, and filling factor of the coil but also for heat and microwave dissipation. Enhancement factors of 17-fold were obtained, and a two-dimensional PISEMA spectrum of a transmembrane helical peptide was obtained in less than 2 h.

Functions of the C-terminal domains of apoptosis-related proteins of the Bcl-2 family

Bcl-2 family proteins are involved in cell homeostasis, where they regulate cell death. Some of these proteins are pro-apoptotic and others pro-survival. Moreover, many of them share a similar domain composition with several of the so-called BH domains, although some only have a BH3 domain. A C-terminal domain is present in all the multi-BH domain proteins and in some of the BH3-only ones. This C-terminal domain is hydrophobic or amphipathic, for which reason it was thought when they were discovered that they were membrane anchors. Although this is indeed one of their functions, it has since been observed that they may also serve as regulators of the function of some members of this family, such as Bax. They may also serve to recognize the target membrane of some of these proteins, which only after an apoptotic signal, are incorporated into a membrane. It has been shown that peptides that imitate the sequence of C-terminal domains can form pores and may serve as a model to design cytotoxic molecules.

Anti-proliferative effect of Flos Albiziae flavonoids on the human gastric cancer SGC-7901 cell line

The flavonoids found in many foods may have a protective effect against human gastric cancer. However, little information is available concerning the effects of flavonoids on the SGC-7901 cell line. Therefore, we first evaluated the effects of purified Flos Albiziae flavonoids (FAFs) on the proliferation of the SGC-7901 human gastric cancer cell line and investigated its possible anti-proliferative mechanisms. When SGC-7901 cells were treated with FAFs for various time periods (12-72 h) and at various doses (0-32 μg/ml), cell growth decreased significantly in a time- and dose-dependent manner. Morphological observations with fluorescence microscopy and transmission electron microscopy (TEM) yielded clear evidence of cell shrinkage, formation of cytoplasmic filaments, condensation of nuclear chromatin, and cell apoptosis in the presence of FAFs. Treatment with FAFs changed the expression levels of Bcl-2, P65, Bax and caspase. The anti-apoptotic protein expression of Bcl-2 and p65 decreased gradually with the increase in FAF concentration, compared with control cells (P<0.05). FAFs contributed to the increase in Bax and caspase expression. The expression of pro-apoptotic proteins Bax and caspase were upregulated by FAFs compared with control cells (P<0.01). These results demonstrated that FAFs effectively induced apoptosis in the SGC-7901 cell line. This indicates that FAFs are likely to possess anticancer activity.

Computational studies of colicin insertion into membranes: the closed state

Colicins are water-soluble toxins that, upon interaction with membranes, undergo a conformational change, insert, and form pores in them. Pore formation activity is localized in a bundle of 10 α-helices named the pore-forming domain (PFD). There is evidence that colicins attach to the membrane via a hydrophobic hairpin embedded in the core of the PFD. Two main models have been suggested for the membrane-bound state: penknife and umbrella, differing in regard to the orientation of the hydrophobic hairpin with respect to the membrane. The arrangement of the amphipathic helices has been described as either a compact three-dimensional structure or a two-dimensional array of loosely interacting helices on the membrane surface. Using molecular dynamics simulations with an implicit membrane model, we studied the structure and stability of the conformations proposed earlier for four colicins. We find that colicins are initially driven towards the membrane by electrostatic interactions between basic residues and the negatively charged membrane surface. They do not have a unique binding orientation, but in the predominant orientations the central hydrophobic hairpin is parallel to the membrane. In the inserted state, the estimated free energy tends to be lower for the compact arrangements of the amphipathic helix, but the more expanded ones are in better agreement with experimental distance distributions. The difference in energy between penknife and umbrella conformations is small enough for equilibrium to exist between them. Elongation of the hydrophobic hairpin helices and membrane thinning were found unable to produce stabilization of the transmembrane configuration of the hydrophobic hairpin.

Active fragments from pro- and antiapoptotic BCL-2 proteins have distinct membrane behavior reflecting their functional divergence

BACKGROUND

The BCL-2 family of proteins includes pro- and antiapoptotic members acting by controlling the permeabilization of mitochondria. Although the association of these proteins with the outer mitochondrial membrane is crucial for their function, little is known about the characteristics of this interaction.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we followed a reductionist approach to clarify to what extent membrane-active regions of homologous BCL-2 family proteins contribute to their functional divergence. Using isolated mitochondria as well as model lipid Langmuir monolayers coupled with Brewster Angle Microscopy, we explored systematically and comparatively the membrane activity and membrane-peptide interactions of fragments derived from the central helical hairpin of BAX, BCL-xL and BID. The results show a connection between the differing abilities of the assayed peptide fragments to contact, insert, destabilize and porate membranes and the activity of their cognate proteins in programmed cell death.

CONCLUSION/SIGNIFICANCE

BCL-2 family-derived pore-forming helices thus represent structurally analogous, but functionally dissimilar membrane domains.

Role of membrane lipids for the activity of pore forming peptides and proteins

Bilayer lipids, far from being passive elements, have multiple roles in polypeptide-dependent pore formation. Lipids participate at all stages of the formation of pores by providing the binding site for proteins and peptides, conditioning their active structure and modulating the molecular reorganization of the membrane complex. Such general functions of lipids superimpose to other particular roles, from electrostatic and curvature effects to more specific actions in cases like cholesterol, sphingolipids or cardiolipin. Pores are natural phenomena in lipid membranes. Driven by membrane fluctuations and packing defects, transient water pores are related to spontaneous lipid flip-flop and non-assisted ion permeation. In the absence ofproteins or peptides, these are rare short living events, with properties dependent on the lipid composition of the membrane. Their frequency increases under conditions of internal membrane disturbance of the lipid packing, like in the presence of membrane-bound proteins or peptides. These latter molecules, in fact, form dynamic supramolecular assemblies together with the lipids and transmembrane pores are one of the possible structures of the complex. Active peptides and proteins can thus be considered inducers or enhancers of pores which increase their probability and lifetime by modifying the thermodynamic membrane balance. This includes destabilizing the membrane lamellar structure, lowering the activation energy for pore formation and stabilizing the open pore structure.

Disparate proteins use similar architectures to damage membranes

Membrane disruption can efficiently alter cellular function; indeed, pore-forming toxins (PFTs) are well known as important bacterial virulence factors. However, recent data have revealed that structures similar to those found in PFTs are found in membrane active proteins across disparate phyla. Many similarities can be identified only at the 3D-structural level. Of note, domains found in membrane-attack complex proteins of complement and perforin (MACPF) resemble cholesterol-dependent cytolysins from Gram-positive bacteria, and the Bcl family of apoptosis regulators share similar architectures with Escherichia coli pore-forming colicins. These and other correlations provide considerable help in understanding the structural requirements for membrane binding and pore formation.