Scanning tunneling microscopy imaging of Torpedo acetylcholine receptor

Observation of Giant Conductance Fluctuations in a Protein

Proteins are insulating molecular solids, yet even those containing easily reduced or oxidized centers can have single-molecule electronic conductances that are too large to account for with conventional transport theories. Here, we report the observation of remarkably high electronic conductance states in an electrochemically-inactive protein, the ~200 kD α_Vβ₃ extracelluar domain of human integrin. Large current pulses (up to nA) were observed for long durations (many ms, corresponding to many pC of charge transfer) at large gap (>5nm) distances in an STM when the protein was bound specifically by a small peptide ligand attached to the electrodes. The effect is greatly reduced when a homologous, weakly-binding protein (α₄β₁) is used as a control. In order to overcome the limitations of the STM, the time- and voltage-dependence of the conductance were further explored using a fixed-gap (5 nm) tunneling junction device that was small enough to trap a single protein molecule at any one time. Transitions to a high conductance (~ nS) state were observed, the protein being "on" for times from ms to tenths of a second. The high-conductance states only occur above ~ 100mV applied bias, and thus are not an equilibrium property of the protein. Nanoamp two-level signals indicate the specific capture of a single molecule in an electrode gap functionalized with the ligand. This offers a new approach to label-free electronic detection of single protein molecules. Electronic structure calculations yield a distribution of energy level spacings that is consistent with a recently proposed quantum-critical state for proteins, in which small fluctuations can drive transitions between localized and band-like electronic states.

Strychnine activates neuronal alpha7 nicotinic receptors after mutations in the leucine ring and transmitter binding site domains

Recent work has shown that strychnine, the potent and selective antagonist of glycine receptors, is also an antagonist of nicotinic acetylcholine (AcCho) receptors including neuronal homomeric alpha7 receptors, and that mutating Leu-247 of the alpha7 nicotinic AcCho receptor-channel domain (L247Talpha7; mut1) converts some nicotinic antagonists into agonists. Therefore, a study was made of the effects of strychnine on Xenopus oocytes expressing the chick wild-type alpha7 or L247Talpha7 receptors. In these oocytes, strychnine itself did not elicit appreciable membrane currents but reduced the currents elicited by AcCho in a reversible and dose-dependent manner. In sharp contrast, in oocytes expressing L247Talpha(7) receptors with additional mutations at Cys-189 and Cys-190, in the extracellular N-terminal domain (L247T/C189-190Salpha7; mut2), micromolar concentrations of strychnine elicited inward currents that were reversibly inhibited by the nicotinic receptor blocker alpha-bungarotoxin. Single-channel recordings showed that strychnine gated mut2-channels with two conductance levels, 56 pS and 42 pS, and with kinetic properties similar to AcCho-activated channels. We conclude that strychnine is a modulator, as well as an activator, of some homomeric nicotinic alpha7 receptors. After injecting oocytes with mixtures of cDNAs encoding mut1 and mut2 subunits, the expressed hybrid receptors were activated by strychnine, similar to the mut2, and had a high affinity to AcCho like the mut1. A pentameric symmetrical model yields the striking conclusion that two identical alpha7 subunits may be sufficient to determine the functional properties of alpha7 receptors.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Imaging of reconstituted biological channels at molecular resolution by atomic force microscopy

Using atomic force microscopy (AFM), we obtained high-resolution surface images of the bacterial outer membrane channels Escherichia coli OmpF porin and Bordetella pertussis porin that were reconstituted in artificial bilayer membranes as two-dimensional crystalline arrays. These porins were chosen because they are among the most extensively studied proteins of this type and are known for their well-defined crystalline nature in the native membrane. Such reconstituted membrane proteins are ideal specimens to assess the suitability and resolution of AFM for imaging biomembranes and associated proteins. Although OmpF porin often showed a mixed pattern of rectangular and hexagonal arrays with approximately 8.4 x 9.8- and approximately 7.2-nm-spacings, respectively, B. pertussis porin showed mostly a rectangular pattern with an approximately 7.9 x 13.8-nm spacing. The packing patterns of the E. coli OmpF porin in the membrane are very close to those found in electron-microscopic studies. When B. pertussis porin was imaged in a buffer solution, its trimeric subunits were apparently resolved, and the surface of each monomer revealed beadlike structures. This is the first report of such a high-resolution structural analysis of B. pertussis porin by any imaging method. We also imaged the lipid bilayer itself as an internal control for imaging and to further ascertain the resolution. Individual polar head groups of bilayer lipid molecules were resolved, suggesting the intrinsic resolution of AFM for bioimaging.

Atomic force microscopy of cloned nicotinic acetylcholine receptor expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) was expressed in Xenopus oocytes from in vitro transcribed mRNA and was imaged by atomic force microscopy. A characteristic pentameric structure of AChR was readily observed on the extracellular face of the cell membrane, with a central pore surrounded by protruding AChR subunits. These structures were seen only in mRNA-injected oocytes that also gave acetylcholine-induced membrane currents. The size of individual AChR channels, the angles between subunits, and the interchannel spacing were all compatible with the current model of AChR. In addition, localized patches of microscopic AChR clustering were observed, with packing density approaching that at the neuromuscular junction. These findings show the potential of studying cloned membrane proteins in oocytes for both their surface topography and their structure-function relationship in native membrane without the need for crystallization.