Biased signaling pathways in β2-adrenergic receptor characterized by 19F-NMR

Extracellular ligand binding to G protein-coupled receptors (GPCRs) modulates G protein and β-arrestin signaling by changing the conformational states of the cytoplasmic region of the receptor. Using site-specific (19)F-NMR (fluorine-19 nuclear magnetic resonance) labels in the β(2)-adrenergic receptor (β(2)AR) in complexes with various ligands, we observed that the cytoplasmic ends of helices VI and VII adopt two major conformational states. Changes in the NMR signals reveal that agonist binding primarily shifts the equilibrium toward the G protein-specific active state of helix VI. In contrast, β-arrestin-biased ligands predominantly impact the conformational states of helix VII. The selective effects of different ligands on the conformational equilibria involving helices VI and VII provide insights into the long-range structural plasticity of β(2)AR in partial and biased agonist signaling.     

Structure of Galphaq-p63RhoGEF-RhoA complex reveals a pathway for the activation of RhoA by GPCRs

The guanine nucleotide exchange factor p63RhoGEF is an effector of the heterotrimeric guanine nucleotide-binding protein (G protein) Galphaq and thereby links Galphaq-coupled receptors (GPCRs) to the activation of the small-molecular-weight G protein RhoA. We determined the crystal structure of the Galphaq-p63RhoGEF-RhoA complex, detailing the interactions of Galphaq with the Dbl and pleckstrin homology (DH and PH) domains of p63RhoGEF. These interactions involve the effector-binding site and the C-terminal region of Galphaq and appear to relieve autoinhibition of the catalytic DH domain by the PH domain. Trio, Duet, and p63RhoGEF are shown to constitute a family of Galphaq effectors that appear to activate RhoA both in vitro and in intact cells. We propose that this structure represents the crux of an ancient signal transduction pathway that is expected to be important in an array of physiological processes.     

The 2.6 angstrom crystal structure of a human A2A adenosine receptor bound to an antagonist

The adenosine class of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) mediates the important role of extracellular adenosine in many physiological processes and is antagonized by caffeine. We have determined the crystal structure of the human A2A adenosine receptor, in complex with a high-affinity subtype-selective antagonist, ZM241385, to 2.6 angstrom resolution. Four disulfide bridges in the extracellular domain, combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin receptor structures, define a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation, perpendicular to the membrane plane. The binding site highlights an integral role for the extracellular loops, together with the helical core, in ligand recognition by this class of GPCRs and suggests a role for ZM241385 in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.     

GPCR engineering yields high-resolution structural insights into beta2-adrenergic receptor function

The beta2-adrenergic receptor (beta2AR) is a well-studied prototype for heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) that respond to diffusible hormones and neurotransmitters. To overcome the structural flexibility of the beta2AR and to facilitate its crystallization, we engineered a beta2AR fusion protein in which T4 lysozyme (T4L) replaces most of the third intracellular loop of the GPCR ("beta2AR-T4L") and showed that this protein retains near-native pharmacologic properties. Analysis of adrenergic receptor ligand-binding mutants within the context of the reported high-resolution structure of beta2AR-T4L provides insights into inverse-agonist binding and the structural changes required to accommodate catecholamine agonists. Amino acids known to regulate receptor function are linked through packing interactions and a network of hydrogen bonds, suggesting a conformational pathway from the ligand-binding pocket to regions that interact with G proteins.     

Structure of a site-2 protease family intramembrane metalloprotease

Regulated intramembrane proteolysis by members of the site-2 protease (S2P) family is an important signaling mechanism conserved from bacteria to humans. Here we report the crystal structure of the transmembrane core domain of an S2P metalloprotease from Methanocaldococcus jannaschii. The protease consists of six transmembrane segments, with the catalytic zinc atom coordinated by two histidine residues and one aspartate residue approximately 14 angstroms into the lipid membrane surface. The protease exhibits two distinct conformations in the crystals. In the closed conformation, the active site is surrounded by transmembrane helices and is impermeable to substrate peptide; water molecules gain access to zinc through a polar, central channel that opens to the cytosolic side. In the open conformation, transmembrane helices alpha1 and alpha6 separate from each other by 10 to 12 angstroms, exposing the active site to substrate entry. The structure reveals how zinc embedded in an integral membrane protein can catalyze peptide cleavage.     

The crystal structure of a sodium galactose transporter reveals mechanistic insights into Na+/sugar symport

Membrane transporters that use energy stored in sodium gradients to drive nutrients into cells constitute a major class of proteins. We report the crystal structure of a member of the solute sodium symporters (SSS), the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT). The approximately 3.0 angstrom structure contains 14 transmembrane (TM) helices in an inward-facing conformation with a core structure of inverted repeats of 5 TM helices (TM2 to TM6 and TM7 to TM11). Galactose is bound in the center of the core, occluded from the outside solutions by hydrophobic residues. Surprisingly, the architecture of the core is similar to that of the leucine transporter (LeuT) from a different gene family. Modeling the outward-facing conformation based on the LeuT structure, in conjunction with biophysical data, provides insight into structural rearrangements for active transport.     

Structure. Rhodopsin sees the light

Members of the seven transmembrane receptor superfamily bind a remarkable variety of ligands, from neurotransmitters to odorants, and activate a spectacular array of G protein signaling molecules. These G-protein coupled receptors (GPCRs) are important in many cellular functions and so there has been great interest in elucidating how they transmit their signals to the interior of the cell after activation by ligand. As Bourne and Meng explain in their Perspective, the molecular movements of activated GPCRs are becoming clear now that the first crystal structure of a GPCR (rhodopsin, the light-trapping receptor found in the retina of the eye) has been reported (Palczweski et al.).     

Crystal structure of Escherichia coli MscS,a voltage-modulated and mechanosensitive channel

The mechanosensitive channel of small conductance (MscS) responds both to stretching of the cell membrane and to membrane depolarization. The crystal structure at 3.9 angstroms resolution demonstrates that Escherichia coli MscS folds as a membrane-spanning heptamer with a large cytoplasmic region. Each subunit contains three transmembrane helices (TM1, -2, and -3), with the TM3 helices lining the pore, while TM1 and TM2, with membrane-embedded arginines, are likely candidates for the tension and voltage sensors. The transmembrane pore, apparently captured in an open state, connects to a large chamber, formed within the cytoplasmic region, that connects to the cytoplasm through openings that may function as molecular filters. Although MscS is likely to be structurally distinct from other ion channels, similarities in gating mechanisms suggest common structural elements.     

Reexamination of the Three-Dimensional Structure of the Small Subunit of RuBisCo from Higher Plants

The structure of L(8)S(8) RuBisCo (where L is the large subunit and S is the small subunit) from spinach has been determined to a resolution of 2.8 ?ngstrom by using fourfold averaging of an isomorphous electron density map based on three heavy-atom derivatives. The structure of the S subunit is different from that previously reported for the tobacco S subunit in spite of 75 percent sequence identity. The elements of secondary structure, four antiparallel beta strands and two alpha helices, are the same, but the topology and direction of the polypeptide chain through these elements differ completely. One of these models is clearly wrong. The spinach model has hydrophobic residues in the core between the alpha helices and beta sheet as well as conserved residues in the subunit interactions. The deletion of residues 49 to 62 that is present in the Anabaena sequence removes a loop region in the spinach model. The positions of three mercury atoms in the heavy-atom derivatives agree with the assignment of side chains in the spinach structure.     

大豆C4H基因克隆及生物信息学分析

为了从分子水平上揭示大豆C4H的结构特点,并为提高其表达活性提供理论依据,利用RT-PCR技术克隆了大豆C4H基因,并已登录GenBank(登录号为FJ770468),长度为1 766 bp,编码区1 521 bp,编码一个含有506个氨基酸残基的多肽。生物信息学分析显示,C4H理论PI=5.30,Mw=39.092 ku;C4H是易溶、亲水性较强的蛋白,同时有两个明显的疏水峰,有2个跨膜肽段;通过HNN分析表明,C4H各个氨基酸残基对应的二级结构分别为α螺旋(Alpha helix)(Hh):251,49.60%,β折叠(Extended strand)(Ee):52,10.28%,无规卷曲(Random coil)(Cc):203,40.12%;Geno3d模建预测,C4H蛋白中β折叠区有57个折叠,折叠区间有24个α螺旋,此外还有14个卷曲结构。氨基酸序列和结构分析显示C4H蛋白包含了一个保守区,即P450 domain。利用SignalP分析得到信号肽存在几率、信号肽锚定几率均为0.498,切割位点位于28和29位氨基酸之间。     

Activation of integrin alphaIIbbeta3 by modulation of transmembrane helix associations

Transmembrane helices of integrin alpha and beta subunits have been implicated in the regulation of integrin activity. Two mutations, glycine-708 to asparagine-708 (G708N)and methionine-701 to asparagine-701, in the transmembrane helix of the beta3 subunit enabled integrin alphaIIbbeta3 to constitutively bind soluble fibrinogen. Further characterization of the G708N mutant revealed that it induced alphaIIbbeta3 clustering and constitutive phosphorylation of focal adhesion kinase. This mutation also enhanced the tendency of the transmembrane helix to form homotrimers. These results suggest that homomeric associations involving transmembrane domains provide a driving force for integrin activation. They also suggest a structural basis for the coincidence of integrin activation and clustering.     

NMR studies of the structure and dynamics of membrane-bound bacteriophage Pf1 coat protein

Filamentous bacteriophage coat protein undergoes a remarkable structural transition during the viral assembly process as it is transferred from the membrane environment of the cell, where it spans the phospholipid bilayer, to the newly extruded virus particles. Nuclear magnetic resonance (NMR) studies show the membrane-bound form of the 46-residue Pf1 coat protein to be surprisingly complex with five distinct regions. The secondary structure consists of a long hydrophobic helix (residues 19 to 42) that spans the bilayer and a short amphipathic helix (residues 6 to 13) parallel to the plane of the bilayer. The NH2-terminus (residues 1 to 5), the COOH-terminus (residues 43 to 46), and residues 14 to 18 connecting the two helices are mobile. By comparing the structure and dynamics of the membrane-bound coat protein with that of the viral form as determined by NMR and neutron diffraction, essential features of assembly process can be identified.     

Calcium-induced peptide association to form an intact protein domain: 1H NMR structural evidence

The 70-residue carboxyl-terminal domain of the muscle contractile protein troponin-C contains two helix-loop-helix calcium (Ca)-binding sites that are related to each other by approximate twofold rotational symmetry. Hydrophobic residues from the helices and a short three residue beta sheet at the interface of the two sites act to stabilize the protein domain in the presence of Ca. A synthetic 34-residue peptide representing one of these sites (site III) has been synthesized and studied by H-1 nuclear magnetic resonance (NMR) spectroscopy. In solution this peptide undergoes a Ca-induced conformational change to form the helix-loop-helix Ca-binding motif. Two-dimensional nuclear Overhauser effect spectra have provided evidence for the formation of a beta sheet and interactions between several hydrophobic residues from opposing helices as found in troponin-C. It is proposed that a symmetric two-site dimer similar in tertiary structure to the carboxyl-terminal domain of troponin-C forms from the assembly of two site III peptides in the Ca-bound form.     

Residue-specific vibrational echoes yield 3D structures of a transmembrane helix dimer

Two-dimensional (2D) vibrational echo spectroscopy has previously been applied to structural determination of small peptides. Here we extend the technique to a more complex, biologically important system: the homodimeric transmembrane dimer from the α chain of the integrin α(IIb)β(3). We prepared micelle suspensions of the pair of 30-residue chains that span the membrane in the native structure, with varying levels of heavy ((13)C=(18)O) isotopes substituted in the backbone of the central 10th through 20th positions. The constraints derived from vibrational coupling of the precisely spaced heavy residues led to determination of an optimized structure from a range of model candidates: Glycine residues at the 12th, 15th, and 16th positions form a tertiary contact in parallel right-handed helix dimers with crossing angles of -58° ± 9° and interhelical distances of 7.7 ± 0.5 angstroms. The frequency correlation established the dynamical model used in the analysis, and it indicated the absence of mobile water associated with labeled residues. Delocalization of vibrational excitations between the helices was also quantitatively established.     

Rhodopsin mutants that bind but fail to activate transducin

Rhodopsin is a member of a family of receptors that contain seven transmembrane helices and are coupled to G proteins. The nature of the interactions between rhodopsin mutants and the G protein, transduction (Gt), was investigated by flash photolysis in order to monitor directly Gt binding and dissociation. Three mutant opsins with alterations in their cytoplasmic loops bound 11-cis-retinal to yield pigments with native rhodopsin absorption spectra, but they failed to stimulate the guanosine triphosphatase activity of Gt. The opsin mutations included reversal of a charged pair conserved in all G protein-coupled receptors at the cytoplasmic border of the third transmembrane helix (mutant CD1), replacement of 13 amino acids in the second cytoplasmic loop (mutant CD2), and deletion of 13 amino acids from the third cytoplasmic loop (mutant EF1). Whereas mutant CD1 failed to bind Gt, mutants CD2 and EF1 showed normal Gt binding but failed to release Gt in the presence of guanosine triphosphate. Therefore, it appears that at least the second and third cytoplasmic loops of rhodopsin are required for activation of bound Gt.     

Partial symmetrization of the photosynthetic reaction center

The bacterial photosynthetic reaction center (RC) is a pigmented intrinsic membrane protein that performs the primary charge separation event of photosynthesis, thereby converting light to chemical energy. The RC pigments are bound primarily by two homologous peptides, the L and M subunits, each containing five transmembrane helices. These alpha helices and pigments are arranged in an approximate C2 symmetry and form two possible electron transfer pathways. Only one of these pathways is actually used. In an attempt to identify nonhomologous residues that are responsible for functional differences between the two branches, homologous helical regions that interact extensively with the pigments were genetically symmetrized (that is, exchanged). For example, replacement of the fourth transmembrane helix (D helix) in the M subunit with the homologous helix from the L subunit yields photosynthetically inactive RCs lacking a critical photoactive pigment. Photosynthetic revertants have been isolated in which single amino acid substitutions (intragenic suppressors) compensate for this partial symmetrization.     

石榴等观赏植物DFR基因生物信息学分析

根据GenBank中已登录的红花石榴、粉花石榴、姜荷花、芍药、水母雪莲、大丽花、瓜叶菊和兰花等植物二氢黄酮醇4-还原酶(DFR)基因的核苷酸序列和氨基酸序列,应用生物信息学软件对其核苷酸序列的外显子、氨基酸序列的理化性质、疏水性/亲水性和跨膜结构等进行了预测。结果表明,这几种植物DFR基因均存在1个外显子,含量最丰富的氨基酸是Ala、Gly、Cys和Thr;除红花石榴和粉花石榴DFR属于不稳定蛋白外,其余植物均属于稳定蛋白。这几种植物DFR蛋白均为亲水性蛋白,存在明显的疏水区和亲水区以及跨膜结构等。     

The structure of a pH-sensing mycobacterial adenylyl cyclase holoenzyme

Class III adenylyl cyclases contain catalytic and regulatory domains, yet structural insight into their interactions is missing. We show that the mycobacterial adenylyl cyclase Rv1264 is rendered a pH sensor by its N-terminal domain. In the structure of the inhibited state, catalytic and regulatory domains share a large interface involving catalytic residues. In the structure of the active state, the two catalytic domains rotate by 55 degrees to form two catalytic sites at their interface. Two alpha helices serve as molecular switches. Mutagenesis is consistent with a regulatory role of the structural transition, and we suggest that the transition is regulated by pH.     

油桐ACPase基因克隆及生物信息学分析

【目的】克隆油桐酸性磷酸酶(ACPase)基因的全长序列,为进一步分析油桐ACPase的功能奠定基础。【方法】根据油桐转录组酸性磷酸酶蛋白全长序列设计引物,利用RT-PCR从油桐种子中克隆ACPase基因,对其cDNA序列、核苷酸序列的相似性、理化性质、疏水性、跨膜结构、二级结构及三级结构进行分析预测,同时进行多序列比对并构建系统树。【结果】克隆获得油桐ACPase基因,其开放阅读框为1152 bp,编码383个氨基酸残基,相对分子质量为40.94 kDa,理论pI为5.30,属可溶性不稳定蛋白。同源性和进化树的预测分析结果显示,油桐ACPase与毛果杨的ACPase蛋白同源性达84%。油桐ACPase存在磷酸酶基因家族的保守区域,ACPase编码蛋白的大部分氨基酸属于亲水性氨基酸。其蛋白二级结构主要由不规则卷曲、α-螺旋和β-折叠构成。ACPase蛋白包含1个跨膜螺旋区域,其相应氨基酸位置在136~156;包含1个疑似跨膜域,其相应氨基酸位置在255~275;每个跨膜结构域为由20个氨基酸残基组成的螺旋,同时跨膜蛋白的N端和C端位于细胞膜胞质的一侧。【结论】油桐ACPase基因的表达可能与其体内磷营养的分解利用等过程有关。