Expression, two-dimensional crystallization, and electron cryo-crystallography of recombinant gap junction membrane channels

Conformational changes in the human Cx43/GJA1 gap junction channel visualized using cryo-EM

Connexin family proteins assemble into hexameric hemichannels in the cell membrane. The hemichannels dock together between two adjacent membranes to form gap junction intercellular channels (GJIChs). We report the cryo-electron microscopy structures of Cx43 GJICh, revealing the dynamic equilibrium state of various channel conformations in detergents and lipid nanodiscs. We identify three different N-terminal helix conformations of Cx43-gate-covering (GCN), pore-lining (PLN), and flexible intermediate (FIN)-that are randomly distributed in purified GJICh particles. The conformational equilibrium shifts to GCN by cholesteryl hemisuccinates and to PLN by C-terminal truncations and at varying pH. While GJIChs that mainly comprise GCN protomers are occluded by lipids, those containing conformationally heterogeneous protomers show markedly different pore sizes. We observe an α-to-π-helix transition in the first transmembrane helix, which creates a side opening to the membrane in the FIN and PLN conformations. This study provides basic structural information to understand the mechanisms of action and regulation of Cx43 GJICh.

An Overview of Microcrystal Electron Diffraction (MicroED)

The bedrock of drug discovery and a key tool for understanding cellular function and drug mechanisms of action is the structure determination of chemical compounds, peptides, and proteins. The development of new structure characterization tools, particularly those that fill critical gaps in existing methods, presents important steps forward for structural biology and drug discovery. The emergence of microcrystal electron diffraction (MicroED) expands the application of cryo-electron microscopy to include samples ranging from small molecules and membrane proteins to even large protein complexes using crystals that are one-billionth the size of those required for X-ray crystallography. This review outlines the conception, achievements, and exciting future trajectories for MicroED, an important addition to the existing biophysical toolkit.

Lens Connexin Channels Show Differential Permeability to Signaling Molecules

Gap junction channels mediate the direct intercellular passage of small ions as well as larger solutes such as second messengers. A family of proteins called connexins make up the subunits of gap junction channels in chordate animals. Each individual connexin forms channels that exhibit distinct permeability to molecules that influence cellular signaling, such as calcium ions, cyclic nucleotides, or inositol phosphates. In this review, we examine the permeability of connexin channels containing Cx43, Cx46, and Cx50 to signaling molecules and attempt to relate the observed differences in permeability to possible in vivo consequences that were revealed by studies of transgenic animals where these connexin genes have been manipulated. Taken together, these data suggest that differences in the permeability of individual connexin channels to larger solutes like 3',5'-cyclic adenosine monophosphate (cAMP) and inositol 1,4,5-trisphosphate (IP₃) could play a role in regulating epithelial cell division, differentiation, and homeostasis in organs like the ocular lens.

Disulfide-mediated reversible two-dimensional self-assembly of protein nanocages

Disulfide-mediated 2D protein self-assembly was achieved by single point mutation of hot spots at the C₄ interface of ferritin.

Cell communication across gap junctions: a historical perspective and current developments

Collaborative communication lies at the centre of multicellular life. Gap junctions (GJs) are surface membrane structures that allow direct communication between cells. They were discovered in the 1960s following the convergence of the detection of low-resistance electrical interactions between cells and anatomical studies of intercellular contact points. GJs purified from liver plasma membranes contained a 27 kDa protein constituent; it was later named Cx32 (connexin 32) after its full sequence was determined by recombinant technology. Identification of Cx43 in heart and later by a further GJ protein, Cx26 followed. Cxs have a tetraspan organization in the membrane and oligomerize during intracellular transit to the plasma membrane; these were shown to be hexameric hemichannels (connexons) that could interact end-to-end to generate GJs at areas of cell-to-cell contact. The structure of the GJ was confirmed and refined by a combination of biochemical and structural approaches. Progress continues towards obtaining higher atomic 3D resolution of the GJ channel. Today, there are 20 and 21 highly conserved members of the Cx family in the human and mouse genomes respectively. Model organisms such as Xenopus oocytes and zebra fish are increasingly used to relate structure to function. Proteins that form similar large pore membrane channels in cells called pannexins have also been identified in chordates. Innexins form GJs in prechordates; these two other proteins, although functionally similar, are very different in amino acid sequence to the Cxs. A time line tracing the historical progression of wide ranging research in GJ biology over 60 years is mapped out. The molecular basis of channel dysfunctions in disease is becoming evident and progress towards addressing Cx channel-dependent pathologies, especially in ischaemia and tissue repair, continues.

Connexins in migration during development and cancer

Connexins, the gap junction proteins, through their multitude of actions are implicated in a variety of cell processes during animal development and cancer. They allow direct or paracrine/autocrine cell communication through their channel and hemi-channel functions. They enable adhesion and interact with a plethora of signalling molecules. Here, we review the common themes in developmental and pathological processes and we focus in their involvement in cell migration in four different systems: neurons, astrocytes, neural crest and cancer.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Structure and closure of connexin gap junction channels

Connexin gap junctions comprise assembled channels penetrating two plasma membranes for which gating regulation is associated with a variety of factors, including voltage, pH, Ca(2+), and phosphorylation. Functional studies have established that various parts of the connexin peptides are related to channel closure and electrophysiology studies have provided several working models for channel gating. The corresponding structural models supporting these findings, however, are not sufficient because only small numbers of closed connexin structures have been reported. To fully understand the gating mechanisms, the channels should be visualized in both the open and closed states. Electron crystallography and X-ray crystallography studies recently revealed three-dimensional structures of connexin channels in a couple of states in which the main difference is the conformation of the N-terminal domain, which have helped to clarify the structure in regard to channel closure. Here the closure models for connexin gap junction channels inferred from structural and functional studies are described in the context of each domain of the connexin protein associated with gating modulation.

The influence of lipids on voltage-gated ion channels

Voltage-gated ion channels are responsible for transmitting electrochemical signals in both excitable and non-excitable cells. Structural studies of voltage-gated potassium and sodium channels by X-ray crystallography have revealed atomic details on their voltage-sensor domains (VSDs) and pore domains, and were put in context of disparate mechanistic views on the voltage-driven conformational changes in these proteins. Functional investigation of voltage-gated channels in membranes, however, showcased a mechanism of lipid-dependent gating for voltage-gated channels, suggesting that the lipids play an indispensible and critical role in the proper gating of many of these channels. Structure determination of membrane-embedded voltage-gated ion channels appears to be the next frontier in fully addressing the mechanism by which the VSDs control channel opening. Currently electron crystallography is the only structural biology method in which a membrane protein of interest is crystallized within a complete lipid-bilayer mimicking the native environment of a biological membrane. At a sufficiently high resolution, an electron crystallographic structure could reveal lipids, the channel and their mutual interactions at the atomic level. Electron crystallography is therefore a promising avenue toward understanding how lipids modulate channel activation through close association with the VSDs.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Atomic force microscopy of Connexin40 gap junction hemichannels reveals calcium-dependent three-dimensional molecular topography and open-closed conformations of both the extracellular and cytoplasmic faces

Atomic force microscopy was used to study the three-dimensional molecular topography and calcium-sensitive conformational changes of Connexin40 hemichannels (connexons) reconstituted in 1,2-dioeloyl-sn-glycero-3-phosphatidylcholine lipid bilayers. Two classes of objects were observed that differed in their protrusion heights above the bilayer (2.6 versus 4.2 nm). Comparison to reconstituted connexons containing Connexin40 truncated to eliminate most of its C-terminal cytoplasmic domain showed that the two height classes corresponded to the shorter extracellular and taller cytoplasmic aspects of the hemichannels and that the C-terminal tail of Connexin40 contributes ∼1.6 nm in thickness. Hemichannels imaged in solutions containing < 10 μm Ca(2+) showed 3.1-3.2 nm depressions (openings) in 30% of the cytoplasmic faces and 65% of the extracellular faces, and high-resolution three-dimensional topography of extracellular or cytoplasmic aspects of some connexons was observed. After addition of 3.6 mm Ca(2+), > 75% of the connexons in either orientation adopted closed conformations. In contrast, hemichannels imaged in the presence of 0.1 mm EDTA showed large (5.6- to 5.8-nm diameter) openings in nearly all hemichannels regardless of orientation, and detailed topography was visible in many connexons. Real-time imaging following the addition of 3.6 mm Ca(2+) showed transitions of both extracellular and cytoplasmic orientations from "open" into "closed" conformations within several minutes. These studies provide the first high-resolution topographic information regarding a connexin with a large cytoplasmic domain and suggest that the extramembranous portions of Connexin40 contribute to a channel entrance that is relaxed by chelation of residual divalent cations.

Structure of the gap junction channel and its implications for its biological functions

Gap junctions consist of arrays of intercellular channels composed of integral membrane proteins called connexin in vertebrates. Gap junction channels regulate the passage of ions and biological molecules between adjacent cells and, therefore, are critically important in many biological activities, including development, differentiation, neural activity, and immune response. Mutations in connexin genes are associated with several human diseases, such as neurodegenerative disease, skin disease, deafness, and developmental abnormalities. The activity of gap junction channels is regulated by the membrane voltage, intracellular microenvironment, interaction with other proteins, and phosphorylation. Each connexin channel has its own property for conductance and molecular permeability. A number of studies have tried to reveal the molecular architecture of the channel pore that should confer the connexin-specific permeability/selectivity properties and molecular basis for the gating and regulation. In this review, we give an overview of structural studies and describe the structural and functional relationship of gap junction channels.

Optimizing the solution conditions to solve the structure of the Connexin43 carboxyl terminus attached to the 4(th) transmembrane domain in detergent micelles

pH-mediated gating of Cx43 channels following an ischemic event is believed to contribute to the development of lethal cardiac arrhythmias. Studies using a soluble version of the Cx43 carboxyl-terminal domain (Cx43CT; S255-I382) have established the central role it plays in channel regulation; however, research in the authors' laboratory suggests that this construct may not be the ideal model system. Therefore, we have developed a more 'native-like' construct (Cx43CT attached to the 4th transmembrane domain [TM4-Cx43CT; G178-I382]) than the soluble Cx43CT to further investigate the mechanism(s) governing this regulation. Here, we utilize circular dichroism and nuclear magnetic resonance (NMR) were used to validate the TM4-Cx43CT for studying channel gating and optimize solution conditions for structural studies. The data indicate that, unlike the soluble Cx43CT, the TM4-Cx43CT is structurally responsive to changes in pH, suggesting the presence of the TM4 facilitates pH-induced structural alterations. Additionally, the optimal solution conditions for solving the NMR solution structure include 10% 2,2,2 trifluoroethanol and removal of the 2nd extracellular loop (G178-V196).

Pannexins, distant relatives of the connexin family with specific cellular functions?

Intercellular communication (IC) is mediated by gap junctions (GJs) and hemichannels, which consist of proteins. This has been particularly well documented for the connexin (Cx) family. Initially, Cxs were thought to be the only proteins capable of GJ formation in vertebrates. About 10 years ago, however, a new GJ-forming protein family related to invertebrate innexins (Inxs) was discovered in vertebrates, and named the pannexin (Panx) family. Panxs, which are structurally similar to Cxs, but evolutionarily distinct, have been shown to be co-expressed with Cxs in vertebrates. Both protein families show distinct properties and have their own particular function. Identification of the mechanisms that control Panx channel gating is a major challenge for future work. In this review, we focus on the specific properties and role of Panxs in normal and pathological conditions.

Projection Structure of Full Length Connexin 43 by Electron Cryo-Crystallography

We previously used electron cryo-crystallography to determine the three-dimensional structure of recombinant gap junction channels formed by a C-terminal truncation mutant of Cx43 (11). The dodecameric channel is formed by the end-to-end docking of two hexameric connexons, each comprised of 24 transmembrane alpha-helices. We have now generated two-dimensional crystals of the recombinant, full-length channel, as well as crystals in which the C-tail has been completely removed by trypsin digestion. Projection density maps at 7.5 A resolution closely resemble our previous analysis of the C-terminal truncation mutant (9). A difference map between the full length and trypsin-treated channels suggests that there are small but significant shifts in protein density upon removal of the C-tail.

Cx43 has distinct mobility within plasma-membrane domains, indicative of progressive formation of gap-junction plaques

Connexin 43 (Cx43) is a dynamic molecule, having a short half-life of only a few hours. In this study, we use fluorescent-protein-tagged Cx43 variants to examine Cx43 delivery to the cell surface, its residency status in various cell-surface membrane domains and its mobility characteristics. Rapid time-lapse imaging led to the identification of Cx43 being delivered to cell-surface domains that lacked a contacting cell, and also to its localization within membrane protrusions. Fluorescence recovery after photobleaching (FRAP) was used to investigate the mobility state of cell-surface-localized Cx43. Cx43 mobility within clustered cell-surface profiles of Cx43 could be categorized into those with generally a high degree of lateral mobility and those with generally a low degree of lateral mobility. Cx43 mobility was independent of cluster size, yet the C-terminal domain of Cx43 regulated the proportion of gap-junction-like clusters that acquired a low Cx43 mobility state. Collectively, these studies show that Cx43 establishes residency at all cell-surface membrane domains, and progressively acquires assembly states that probably reflect differences in either channel packing and/or its interactions with Cx43-binding proteins.

Structural and immunocytochemical alterations in eye lens fiber cells from Cx46 and Cx50 knockout mice

In the current study we describe the changes of overall organization of lens fiber cells in connexin 46 (Cx46) and connexin 50 (Cx50) knockout mice. Morphometric analyses and the application of immunocytochemical techniques revealed that in Cx46 knockout lens (Cx46 -/-), where Cx50 is expressed alone, the postnatal differentiation of secondary fiber cells proceeds faster and is characterized by an increased number of smaller fiber cells. Conversely, in Cx50 knockout mice (Cx50 -/-), the lenticular mass is considerably reduced and characterized by a small number of fiber cells added during the postnatal period. The process of terminal differentiation was impaired and generated larger fiber cells still possessing cytoplasmic organelles. Freeze-fracture and fracture labeling revealed that the junctional assembly, packing organization and topographic interactions between connexons and MP26 differed when Cx46 and Cx50 were co-assembled in the wild-type or expressed separately in the two distinct knockout phenotypes. Filipin cytochemistry provided indirect evidence that Cx46 and Cx50 expressed alone are recruited into different lipid environments. Our results represent the structural proof that interaction of connexins and MP26 contributes to the overall organization of the fiber cells.

The gap junction cellular internet: connexin hemichannels enter the signalling limelight

Cxs (connexins), the protein subunits forming gap junction intercellular communication channels, are transported to the plasma membrane after oligomerizing into hexameric assemblies called connexin hemichannels (CxHcs) or connexons, which dock head-to-head with partner hexameric channels positioned on neighbouring cells. The double membrane channel or gap junction generated directly couples the cytoplasms of interacting cells and underpins the integration and co-ordination of cellular metabolism, signalling and functions, such as secretion or contraction in cell assemblies. In contrast, CxHcs prior to forming gap junctions provide a pathway for the release from cells of ATP, glutamate, NAD+ and prostaglandin E2, which act as paracrine messengers. ATP activates purinergic receptors on neighbouring cells and forms the basis of intercellular Ca2+ signal propagation, complementing that occuring more directly via gap junctions. CxHcs open in response to various types of external changes, including mechanical, shear, ionic and ischaemic stress. In addition, CxHcs are influenced by intracellular signals, such as membrane potential, phosphorylation and redox status, which translate external stresses to CxHc responses. Also, recent studies demonstrate that cytoplasmic Ca2+ changes in the physiological range act to trigger CxHc opening, indicating their involvement under normal non-pathological conditions. CxHcs not only respond to cytoplasmic Ca2+, but also determine cytoplasmic Ca2+, as they are large conductance channels, suggesting a prominent role in cellular Ca2+ homoeostasis and signalling. The functions of gap-junction channels and CxHcs have been difficult to separate, but synthetic peptides that mimic short sequences in the Cx subunit are emerging as promising tools to determine the role of CxHcs in physiology and pathology.

Connexins and their environment: effects of lipids composition on ion channels

Intercellular communication is mediated through paired connexons that form an aqueous pore between two adjacent cells. These membrane proteins reside in the plasma membrane of their respective cells and their activity is modulated by the composition of the lipid bilayer. The effects of the bilayer on connexon structure and function may be direct or indirect, and may arise from specific binding events or the physicochemical properties of the bilayer. While the effects of the bilayer and its constituent lipids on gap junction activity have been described in the literature, the underlying mechanisms of the interaction of connexin with its lipidic microenvironment are not as well characterized. Given that the information regarding connexons is limited, in this review, the specific roles of lipids and the properties of the bilayer on membrane protein structure and function are described for other ion channels as well as for connexons.

Endocytosis of connexin protein in adrenal cells

The ability of adrenocorticotropin (ACTH) to affect gap junctions was examined in adrenal cells in vivo and in vitro. Treatment with ACTH increased the size and number of gap junction plaques on the cell membranes in hypophysectomized animals and in adrenal culture. Intracellular (cytoplasmic) annular gap junctions were observed in cells of the inner adrenal cortical zones and in adrenal cell cultures. To investigate the relationship of annular gap junctions to surface junctions, adrenal cells in culture were transfected with cDNA encoding a green fluorescent protein tagged connexin 43 construct (Cx43-GFP), and subsequently studied by time-lapse video microscopy, immunocytochemistry, and transmission electron microscopy (TEM). Internalization of part or all of a surface gap junction plaque resulted in annular gap junction formation. These studies support the hypothesis that cytoplasmic vesicles, initially described with TEM methods, can result from removal of gap junction plaques from the cell surface. It is suggested that hormones can play a regulatory role in cell-cell communication by influencing the availability of gap junction protein at the cell surface and that hormonally-sensitive processes might serve as a means of altering intercellular communication.

Lens connexins alpha3Cx46 and alpha8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin alpha1Cx43 has previously been shown to bind to the PDZ domain-containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins alpha3Cx46 and alpha8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with alpha3Cx46 and alpha8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with alpha3Cx46 and alpha8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed alpha3Cx46 and alpha8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins alpha3Cx46 and alpha8Cx50 in a manner similar to that previously described for alpha1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association.

The highly conserved Gln49 and Ser50 of mammalian connexin43 are present in chick connexin43 and essential for functional gap junction channels

In an attempt to compare the regulation of chick connexin43 channels to those of mammalian connexin43, we found that the nucleotide sequence reported for chick connexin43 differs from that of the chick connexin gene by two codons that had been entered as histidine49 (H49) and valine50 (V50) (accession no. M29003), but are in fact glutamine49 (Q49) and serine50 (S50). Neuro2A cells were transfected with corrected wild-type (Q49/S50) chick connexin43 (accession no. AF233738), the double-replacement Q49H/S50V connexin43, or the single replacement of Q49H or S50V. All clones had gap junctions in membrane based on immunocytochemistry and immunoblots of the triton-resistant membrane fraction. Wild-type transfectants had three conductance states with a predominant channel conductance of 85 +/- 5 pS. Cells producing the Q49H-Cx43 or the double-replacement Q49H/S50V-Cx43 protein had no detectable connexin43 channels. In contrast, cells expressing S50V-Cx43 gap junctions had channels with reduced conductances (75 +/- 8 pS) compared to wild-type controls. Low or high pH of the bathing solution had no effect on the Q49H-Cx43 channels. We conclude that glutamine49 is important for channel function, and replacement of this residue with histidine most likely distorts secondary structure of the first extracellular loop, possibly by changing the orientation of conserved cysteines, and this inhibits channel function. The S50V substitution may also cause similar but less severe structural changes.

Ser364 of connexin43 and the upregulation of gap junction assembly by cAMP

The assembly of gap junctions (GJs) is a process coordinated by growth factors, kinases, and other signaling molecules. GJ assembly can be enhanced via the elevation of cAMP and subsequent stimulation of connexon trafficking to the plasma membrane. To study the positive regulation of GJ assembly, fibroblasts derived from connexin (Cx)43 knockout (KO) and wild-type (WT) mice were transfected with WT Cx43 (WTCx43) or mutant Cx43. GJ assembly between untransfected WT fibroblasts or stably transfected WTCx43/KO fibroblasts was increased two- to fivefold by 8Br-cAMP, and this increase could be blocked by inhibition of cAMP-dependent protein kinase (PKA) or truncation of the Cx43 COOH terminus (CT). Although serine 364 (S364) of the Cx43 CT was determined to be a major site of phosphorylation, the molar ratio of Cx43 phosphorylation was not increased by 8Br-cAMP. Importantly, GJ assembly between either S364ECx43/KO or S364ECx43/WT fibroblasts was stimulated by 8Br-cAMP, but that between S364ACx43/KO or S364PCx43/KO fibroblasts was not stimulated, indicating that phosphorylation or a negative charge at S364 is required for enhancement of GJ assembly by cAMP. Furthermore, GJ assembly between S364ACx43/WT fibroblasts could be stimulated by 8Br-cAMP, but could not be between S364PCx43/WT fibroblasts. Thus, S364PCx43 interferes with enhanced GJ assembly when coexpressed with WTCx43.

Structure of the major membrane protein complex from urinary bladder epithelial cells by cryo-electron crystallography

Numerous protein plaques cover the apical surface of mammalian urinary bladder epithelial cells. These plaques contain four integral membrane proteins, called uroplakins, which form a well-ordered array of hexameric complexes. The 3D structure of these naturally occurring 2D crystals was studied by cryo-electron-crystallographic methods using a slow-scan charged-coupled device (CCD) camera to record the electron micrographs. A 1.2 nm projection map calculated from untilted crystals shows that each hexamer comprises a ring of six inner and six outer domains at a radius of 5.7 nm and 9.2 nm respectively. The 3D structure shows that the mass is distributed strongly asymmetrically with respect to the membrane, with most of the mass protruding from the luminal face. Both domains in the asymmetric unit traverse the membrane and protrude from the membrane on the cytoplasmic side. On the luminal side, the two domains are bridged forming a stretched arc. The total thickness of the complex is about 13.2 nm. A model of the urothelial plaque reveals that contacts between the hexamers are much less extended than within the hexamers.

Two-dimensional crystallogenesis of transmembrane proteins

Two-dimensional crystallogenesis is a crucial step in the long road that leads to the determination of macromolecules structure via electron crystallography. The necessity of having large and highly ordered samples can hold back the resolution of structural works for a long time, and this, despite improvements made in electron microscopes or image processing. Today, finding good conditions for growing two-dimensional crystals still rely on either "biocrystallo-cooks" or on lucky ones. The present review presents the field by first describing the different crystals that one can encounter and the different crystallisation methods used. Then, the effects of different components (such as protein, lipids, detergent, buffer, and temperature) and the different methods (dialysis, hydrophobic adsorption) are discussed. This discussion is punctuated by correspondences made to the world of three-dimensional crystallogenesis. Finally, a guide for setting up 2D crystallogenesis experiments, built on the discussion mentioned before, is proposed to the reader. More than giving recipes, this review is meant to open up the discussions in this field.

Transmembrane alpha-helices in the gap junction membrane channel: systematic search of packing models based on the pair potential function

Recent progress in the field of electron cryo-microscopy and image analysis has shown that there is an overwhelming need to interpret medium resolution (5 to 10 A) three-dimensional maps. Traditional methods of fitting amino acid residues into electron density using molecular modeling programs must be supplemented with further analysis. We have used a potential of mean force (PMF) method, derived from Boltzmann statistics in protein structure, to generate models for the packing of alpha-helices, using pairwise potentials between amino acid residues. The approach was tested using the three-dimensional map of a recombinant cardiac gap junction membrane channel provided by electron cryo-crystallography (Unger et al., 1997; 1999a, 1999b) which had a resolution of 7.5 A in the membrane plane and 21 A in the vertical direction. The dodecameric channel was formed by the end-to-end docking of two hexamers, each of which displayed 24 rods of density in the membrane interior, which was consistent with an alpha-helical conformation for the four transmembrane domains of each connexin subunit. Based on the three-dimensional map and the amino acid sequence for the 4 transmembrane domains determined by hydropathy analysis, we used the modeling utility SymServ (Macke et al., 1998) to build hexameric connexons with 24 transmembrane alpha-helices. Canonical alpha-helices were aligned to the axes of the rods of density and translated along the density so that the center of masses coincided. The PMF function was used to evaluate 162,000 conformations for each of the 24 possible alpha-helical packing models. Since the different packing models yielded different energy distributions, the pair potential function appears to be a promising tool for evaluating the packing of alpha-helices in membrane proteins. The analysis will be refined by energy calculations based on the expectations that the outer boundary of the channel will be formed by hydrophobic residues in contact with the lipids.