The pore domain outer helix contributes to both activation and inactivation of the HERG K+ channel

Ion flow in many voltage-gated K(+) channels (VGK), including the (human ether-a-go-go-related gene) hERG channel, is regulated by reversible collapse of the selectivity filter. hERG channels, however, exhibit low sequence homology to other VGKs, particularly in the outer pore helix (S5) domain, and we hypothesize that this contributes to the unique activation and inactivation kinetics in hERG K(+) channels that are so important for cardiac electrical activity. The S5 domain in hERG identified by NMR spectroscopy closely corresponded to the segment predicted by bioinformatics analysis of 676 members of the VGK superfamily. Mutations to approximately every third residue, from Phe(551) to Trp(563), affected steady state activation, whereas mutations to approximately every third residue on an adjacent face and spanning the entire S5 segment perturbed inactivation, suggesting that the whole span of S5 experiences a rearrangement associated with inactivation. We refined a homology model of the hERG pore domain using constraints from the mutagenesis data with residues affecting inactivation pointing in toward S6. In this model the three residues with maximum impact on activation (W563A, F559A, and F551A) face out toward the voltage sensor. In addition, the residues that when mutated to alanine, or from alanine to valine, that did not express (Ala(561), His(562), Ala(565), Trp(568), and Ile(571)), all point toward the pore helix and contribute to close hydrophobic packing in this region of the channel.

Mutation of a single TMVI residue, Phe(282), in the beta(2)-adrenergic receptor results in structurally distinct activated receptor conformations

We showed previously that Phe(303) in transmembrane segment (TM) VI of the alpha(1B)-adrenergic receptor (alpha(1B)-AR), a residue conserved in many G protein-coupled receptors (GPCRs), is critically involved in coupling agonist binding with TM helical movement and G protein activation. Here the equivalent residue, Phe(282), in the beta(2)-AR was evaluated by mutation to glycine, asparagine, alanine, or leucine. Except for F282N, which exhibits attenuated basal and maximal isoproterenol stimulation, the Phe(282) mutants display varying degrees of constitutive activity (F282L > F282A > F282G), and as shown by the results of substituted cysteine accessibility method (SCAM) studies, induce movement of endogenous cysteine(s) into the water-accessible ligand-binding pocket. For F282A, movement is confined to Cys(285) in TMVI, whereas F282L induces movement of both Cys(285) in TMVI and Cys(327) in TMVII. Further, engineered cysteine-sensor studies indicate that F282L causes movement of TMVI, both above and below an apparent kink-inducing TMVI proline (Pro(288)), whereas that due to F282A is confined to the domain below Pro(288). A plausible interpretation of these data is that receptor activation involves rigid body movement of TMVI which, because of its Pro(288)-induced kink, acts as a pivot to transduce and amplify the agonist-induced conformational change in the upper domain, to a change in the lower domain required for productive receptor-G protein coupling.

Non-alpha-helical elements modulate polytopic membrane protein architecture

In "all alpha-fold" transmembrane proteins, including ion channels, G-protein-coupled receptors (GPCRs), bacterial rhodopsins and photosynthetic reaction centers, relatively long alpha-helices, straight, curved or kinked, pack into compact elliptical or circular domains. Using both existing and newly developed tools to analyze transmembrane segments of all available membrane protein three-dimensional structures, including that very recently elucidated for the GPCR, rhodopsin, we report here the finding of frequent non-alpha-helical components, i.e. 3(10)-helices ("tight turns"), pi-helices ("wide turns") and intrahelical kinks (often due to residues other than proline). Often, diverse helical types and kinks concatenate over long segments and produce complex inclinations of helical axis, and/or diverse frame shifts in the "canonical", alpha-helical side-chain pattern. Marked differences in transmembrane architecture exist even between seemingly structurally related proteins, such as bacteriorhodopsin and rhodopsin. Deconvolution of these non-canonical features into their composite elements is essential for understanding the pleiotropy of polytopic protein structure and function, and must be considered in developing valid macromolecular models.

Evolutionary conservation of both the hydrophilic and hydrophobic nature of transmembrane residues

An algorithm (HRG), developed to allow the pairwise comparisons of the aligned residues of several members of large gene families of polytopic integral membrane proteins is described. Using hydrophobicity scales, application of this algorithm allows the number and size of the membrane-spanning domains of bacteriorhodopsin, a polytopic protein whose structure has been partially determined, to be predicted with a high degree of accuracy (sensitivity 94%, specificity 82% for predicting the membrane embedded or extramembranous location of residues). As opposed to previously reported structure-prediction algorithms, delineation of putative transmembrane segments from connecting loops is also more clearly evident with the application of the HRG algorithm, even with proteins from widely divergent species. This indicates strong evolutionary pressure for the conservation of both the hydrophobic and hydrophilic character of residues in membrane-embedded regions of polytopic proteins, such as those of the G-protein-coupled receptor superfamily. These and other structural and functional implications evident from the application of the HRG algorithm are considered.

Functional characterization of ribozymes expressed using U1 and T7 vectors for the intracellular cleavage of ANF mRNA

Hammerhead ribozymes targeted to various GUC or GUA sites on rat atrial natriuretic factor (ANF) mRNA were developed. The catalytic activity of ribozymes to four of these sites, synthesized by transcription off synthetic oligodeoxynucleotide duplexes, was studied in detail. In vitro, ribozyme-mediated cleavage was highly Mg(2+)-dependent, and at concentrations approaching those found intracellularly, the rate but not the extent of cleavage was markedly reduced. To test for cellular activity, synthetic genes encoding the ribozymes were cloned between the initiation and termination sequences of the U1snRNA gene or between the T7RNA polymerase promoter and terminator sequences in pSP64. Both constructs had defined initiation and termination sequences to minimize transcript size and for message stability. In vitro the addition of T7 or U1 terminator sequences had variable effects on catalytic activity, presumably due to structural interactions between the ribozyme and the added sequence. The ribozyme-encoding plasmids were cotransfected with an expression plasmid containing a rat ANF cDNA into COS-1 cells using a liposome method, which provided high-level transfection efficiency. Quantitation of ANF mRNA by RNase protection showed marked decreases in ANF transcript levels with both the U1- and the T7-expressed ribozymes directed at three of the four sites on ANF mRNA. With all constructs, target accessibility, determined in vitro, was a more important determinant of intracellular ANF mRNA cleavage than catalytic activity per se. ANF mRNA cleavage was not merely due to an antisense effect, since a mutant construct that was catalytically inactive but could still bind produced less cleavage than the corresponding wild-type ribozyme construct. These findings indicate that both U1 and T7 vector systems provide efficient ribozyme expression for the intracellular cleavage of target mRNA.

Genomic organization and expression of the human alpha 1B-adrenergic receptor

alpha 1-Adrenergic receptors (ARs) are members of the guanine nucleotide-binding protein-coupled receptor superfamily. The genes for all ARs described thus far are intronless. We report here the cloning and the nucleotide sequence of the gene for the human alpha 1B-AR. It consists of two exons and a single large intron of at least 20 kilobases which interrupts the coding region at the end of the putative sixth transmembrane domain. The deduced amino acid sequence of the encoded receptor has a high degree of homology to the cloned hamster, rat, and dog alpha 1B-ARs. To characterize the encoded protein, a fusion gene constructed by splicing together exon 1 and exon 2 was expressed transiently in COS-1 cells. The transfected gene fusion product resulted in the production of an alpha 1B-AR with ligand binding characteristics indistinguishable from those of the expressed hamster alpha 1B cDNA. Evidence that the human alpha 1B-AR gene we have isolated is indeed transcribed is the finding of similar sized (2.8-kilobase) transcripts in human heart and other tissues by Northern blot analysis when either exon 1 or exon 2 is used as a probe. Moreover, using primers designed to span the exon 1/exon 2 boundary, a polymerase chain reaction product generated from single-stranded DNA prepared from human heart mRNA had the exact size and nucleotide sequence predicted for a transcript in which exon 1 is spliced to exon 2. The 5'-flanking region (924 base pairs (bp)) of exon 1 contains neither a TATA box nor a CAAT box but is high in GC content (70%) and contains several Sp1 binding sites (GC boxes), consistent with promoters described for housekeeping genes. The 5'-untranslated region also contains a putative cyclic AMP response element. Primer extension studies and RNase protection assays suggested that there are several potential transcription start sites in most tissues with a predominant site located 173 bp upstream from the translation start site. The 3'-flanking region contains a putative polyadenylation signal (ATTAAA) 492 bp downstream from the stop codon. The genomic organization of the human alpha 1B-AR with a single large intron interrupting its coding region differs from those of other ARs as well as muscarinic and 5-hydroxy-tryptamine receptors, which are intronless. The location of the intron in the human alpha 1B-AR gene is also unique among those members of the G-protein-coupled receptor family that do possess introns.(ABSTRACT TRUNCATED AT 400 WORDS)

A novel guanine nucleotide-binding protein coupled to the alpha 1-adrenergic receptor. II. Purification, characterization, and reconstitution

In the previous paper, we reported the identification of a 74-kDa G-protein that co-purifies with the alpha 1-adrenergic receptor following ternary complex formation. We report here on the purification and characterization of this 74-kDa G-protein (termed Gh) isolated de novo from rat liver membranes. After solubilization of rat liver membranes with the detergent sucrose monolaurate, Gh was isolated by sequential chromatography using heparin-agarose, Ultrogel AcA 34, hydroxylapatite, and heptylamine-Sepharose columns. The protein, thus isolated, is not a substrate for cholera or pertussis toxin but displays GTPase activity (turnover number, 3-5 min-1) and high-affinity guanosine 5'-O-3-thiotriphosphate (GTP gamma S) binding (half-maximal binding = 0.25-0.3 microM), which is Mg2(+)-dependent and saturable. The relative order of nucleotide binding by Gh is GTP gamma S greater than GTP greater than GDP greater than ITP much much greater than ATP greater than or equal to adenyl-5'-yl imidodiphosphate, which is similar to that observed for other heterotrimeric G-proteins involved in receptor signaling. Moreover, specific alpha 1-agonist-stimulated GTPase (turnover number, 10-15 min-1) and GTP gamma S binding activity could be demonstrated after reconstitution of purified Gh with partially purified alpha 1-adrenergic receptor into phospholipid vesicles. The alpha 1-agonist stimulation of GTP gamma S binding and GTPase activity was inhibited by the alpha-antagonist phentolamine. A 50-kDa protein co-purifies with the 74-kDa G-protein. This protein does not bind guanine nucleotides and may be a subunit (beta-subunit) of Gh. These findings indicate that Gh is a G-protein that functionally couples to the alpha 1-adrenergic receptor.

Identification and structural characterization of alpha 1-adrenergic receptor subtypes

Rat liver and brain membrane alpha 1-adrenergic receptors were purified greater than 500-fold by successive chromatographic steps using heparin-agarose, an affinity matrix constructed by coupling a novel derivative of the alpha 1-selective antagonist prazosin to Affigel-102 and wheat germ agglutinin-agarose. Several lines of evidence were obtained for the existence in brain of an alpha 1-adrenergic receptor subtype that is structurally distinct from that previously characterized in liver and other tissues using photoaffinity labeling, protein purification, and DNA cloning techniques. The alpha 1-selective ligand chlorethylclonidine (CEC) (an alkylating agent) irreversibly inactivates 100% of [3H]prazosin binding sites in partially purified preparations of rat liver. Under identical conditions, only 50% of brain receptors are irreversibly inactivated. Computer modeling of data obtained from the competition by the alpha antagonists WB4101 and phentolamine for [3H]prazosin binding to partially purified preparations of rat liver is best fit by assuming a single class of low affinity sites for both ligands. However, analysis of partially purified brain preparations indicates the presence of two binding sites with different affinities for these antagonists. Additionally, prior alkylation of brain receptors with CEC results in the loss of low affinity phentolamine and WB4101 binding sites. The CEC-insensitive site in brain, which displays high affinity for phentolamine and WB4101, is resistant to photoaffinity labeling by [125I]azidoprazosin. This is not due to a markedly lower affinity of the CEC-insensitive sites for the photoaffinity label, because competition studies with [127I]azidoprazosin revealed a single class of high affinity sites in partially purified brain samples. Photoaffinity labeling of partially purified liver and brain samples not treated with CEC results in the specific labeling of a single protein of Mr 80,000. No specifically labeled protein is observed for partially purified brain samples that had previously been incubated with CEC. Treatment of photoaffinity-labeled liver and brain receptors with N-glycanase to cleave N-linked oligosaccharides results in a single Mr 55,000 protein. Taken together, these data provide evidence for the existence of a single receptor subtype (alpha 1b) in rat liver and for two subtypes (alpha 1a and alpha 1b) in rat brain. Furthermore, the insensitivity of the alpha 1a subtype to CEC and the resistance of the alpha 1a subtype to covalent labeling by an alpha 1b-selective photoaffinity probe suggest that the primary structures of the two receptor subtypes differ, such that an amino acid(s) in the alpha 1b subtype that incorporates CEC and the photoaffinity label is lacking in the alpha 1a subtype.

Renal failure in infancy due to over-production of urate

This report concerns a three month old infant who was failing to thrive. Renal insufficiency was demonstrated and attributed to aortic coarctation. However, surgical correction of the coarctation failed to correct the renal insufficiency completely and disproportionate hyperuricemia was noted. Excessive urinary excretion of uric acid was found and a moderate deficiency (6% of normal) of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) was demonstrated. When the urate over-production was corrected with allopurinol, renal function returned to normal and the child became well. The importance of over-production of urate and the resultant excessive urinary excretion of uric acid as a treatable cause of acute or persistent renal insufficiency is stressed.