Slo3, a novel pH-sensitive K+ channel from mammalian spermatocytes

Potassium channels have evolved to play specialized roles in both excitable and inexcitable tissues. Here we describe the cloning and expression of Slo3, a novel potassium channel abundantly expressed in mammalian spermatocytes. Slo3 represents a new and unique type of potassium channel regulated by both intracellular pH and membrane voltage. Reverse transcription-polymerase chain reaction, Northern analysis, and in situ hybridization show that Slo3 is primarily expressed in testis in both mouse and human. Because of its sensitivity to both pH and voltage, Slo3 could be involved in sperm capacitation and/or the acrosome reaction, essential steps in fertilization where changes in both intracellular pH and membrane potential are known to occur. The protein sequence of mSlo3 (the mouse Slo3 homologue) is similar to Slo1, the large conductance, calcium- and voltage-gated potassium channel. These results suggest that Slo channels comprise a multigene family, defined by a combination of sensitivity to voltage and a variety of intracellular factors. Northern analysis from human testis indicates that a Slo3 homologue is present in humans and conserved with regard to sequence, transcript size, and tissue distribution. Because of its high testis-specific expression, pharmacological agents that target human Slo3 channels may be useful in both the study of fertilization as well as in the control or enhancement of fertility.

Direct object-shape comparison by light-in-flight speckle interferometry

A method is proposed for direct optical comparison of the three-dimensional shapes of objects by light-in-flight speckle holography. The basic idea of this technique is to use an ultrashort laser pulse with a short coherence length to produce interference patterns that present a single contouring of the object. A simple experiment using plane diffuse objects was performed to verify the method.

Behavioral defects in C. elegans egl-36 mutants result from potassium channels shifted in voltage-dependence of activation

Mutations in the C. elegans egl-36 gene result in defective excitation of egg-laying and enteric muscles. Dominant gain-of-function alleles inhibit enteric and egg-laying muscle contraction, whereas a putative null mutation has no observed phenotype. egl-36 encodes a Shaw-type (Kv3) voltage-dependent potassium channel subunit. In Xenopus oocytes, wild-type egl-36 expresses noninactivating channels with slow activation kinetics. One gain-of-function mutation causes a single amino acid substitution in S6, and the other causes a substitution in the cytoplasmic amino terminal domain. Both mutant alleles produce channels dramatically shifted in their midpoints of activation toward hyperpolarized voltages. An egl-36::gfp fusion is expressed in egg-laying muscles and in a pair of enteric muscle motor neurons. The mutant egl-36 phenotypes can thus be explained by expression in these cells of potassium channels that are inappropriately opened at hyperpolarized potentials, causing decreased excitability due to increased potassium conductance.

Choline's phosphorylation in rat striatal slices is regulated by the activity of cholinergic neurons

The mechanism by which populations of brain cells regulate the flux of choline (Ch) into membrane or neurotransmitter biosynthesis was investigated using electrically stimulated superfused slices of rat corpus striatum. [Me-14C]Ch placed in the superfusion medium for 30 min during a 1-h stimulation period was incorporated into tissue [14C] phosphorylcholine (PCh) and [14C]phosphatidylcholine (PtdCh). Stimulation also caused a profound inhibition of PCh synthesis and a 10-fold increase in [14C]ACh release into the medium; it failed to affect tissue [14C]ACh levels. This effect was not explained by changes in ATP levels nor in the kinetic properties of Ch kinase (E.C. 2.7.1.32) or Ch acetyltransferase (ChAT) (E.C.2.3.1.7). To investigate the mechanism of these effects, Ch uptake studies were performed with and without hemicholinium-3 (HC3), a selective inhibitor of high affinity Ch uptake. A two-compartment model accurately fit the observed data and yielded a K(m) for Ch uptake of 5 microM into cholinergic structures and 72 microM into all other cells. Using this model it was estimated that cholinergic neurons account for 60% of observed uptake of Ch at physiologic Ch concentrations, even though they represent fewer than 1% of the total cells in the slice. The model also predicts that an increase in Ch uptake within cholinergic neurons, reported to be associated with depolarization [4,27,32], would significantly inhibit Ch uptake into all other cells, and would account for the observed decrease in PCh synthesis.

Eight potassium channel families revealed by the C. elegans genome project

The wealth of accumulating data from the Caenorhabditis elegans genome sequencing project has rapidly accelerated the discovery of novel potassium channel genes and now places within reach the possibility of describing the total complement of potassium channels used by an individual species. Using annotated GenBank sequences, BLAST searches of unfinished sequences and degenerate oligonucleotide polymerase chain reaction (PCR) screens, we have identified and compiled genes for 38 C. elegans potassium channel and two cyclic nucleotide-gated cation channel subunits, representing eight conserved multigene families. Novel families of potassium channel genes were revealed, as well as conserved homologues of all known vertebrate families. Two separate families represent C. elegans homologues for human potassium channels recently implicated in hereditary long QT arrhythmias. Of particular note is an exceptionally large class of at least 23 genes with a novel subunit structure having two tandem 'P' domains; these channels may form as dimers in contrast to all other potassium channel types which form as tetramers. The 40 potassium channel genes are evenly distributed on all six C. elegans chromosomes, with the exception of the instances of gene clustering on the fifth and X chromosomes.

[Karyological studies on the fruit of Gardenia jasminoides Ellis]

This paper reports the karyological studies on the fruit of Gardenia jasminoides and G. jasminoides var. grandiflora. The chromosome numbers are 2n = 22 and the places of centromere are metacentric and submedian. The fruit of G. jarminoides is more primary than Gardenia jarminoidce var. grandiflora in karyotype.

Alcohols inhibit a cloned potassium channel at a discrete saturable site. Insights into the molecular basis of general anesthesia

The molecular basis of general anesthetic action on membrane proteins that control ion transport is not yet understood. In a previous report (Covarrubias, M., and Rubin, E. (1993) Proc. Natl. Acad. Sci. 90, 6957-6960), we found that low concentrations of ethanol (17-170mM) selectively inhibited a noninactivating cloned K+ channel encoded by Drosophila Shaw2. Here, we have conducted equilibrium dos-inhibition experiments, single channel recording, and mutagenesis in vitro to study the mechanism underlying the inhibition of Shaw2K+ channels by a homologous series of n-alkanols (ethanol to 1-hexanol). The results showed that: (i) these alcohols inhibited Shaw2 whole-cell currents, the equilibrium dose-inhibition relations were hyperbolic, and competition experiments revealed the presence of a discrete site of action, possibly a hydrophobic pocket; (ii) this pocket may be part of the protein because n-alkanol sensitivity can be transferred to novel hybrid K+ channels composed of Shaw2 subunits and homologous ethanol-insensitive subunits: (iii) moreover, a hydrophobic point mutation within a cytoplasmic loop of an ethanol-insensitive K+ channel (human Kv3.4) was sufficient to allow significant inhibition by n-alkanols, with a dose-inhibition relation that closely resembled that of wildtype Shaw2 channels; and (iv) 1-butanol selectively inhibited long duration single channel openings in a manner consistent with a direct effect on channel gating. These results strongly suggest that a discrete site within the ion channel protein is the primary locus of alcohol and general anesthetic action.

Elimination of rapid potassium channel inactivation by phosphorylation of the inactivation gate

The effect of protein kinase C (PKC) on rapid N-type inactivation of K+ channels has not been reported previously. We found that PKC specifically eliminates rapid inactivation of a cloned human A-type K+ channel (hKv3.4), converting this channel from a rapidly inactivating A type to a noninactivating delayed rectifier type. Biochemical analysis showed that the N-terminal domain of hKv3.4 is phosphorylated in vitro by PKC, and mutagenesis experiments revealed that two serines within the inactivation gate at the N-terminus are sites of direct PKC action. Moreover, mutating one of these serines to aspartic acid mimics the action of PKC. Serine phosphorylation may thus prevent rapid inactivation by shielding basic residues known to be critical to the function of the inactivation gate. The regulatory mechanism reported here may have substantial effects on signal coding in the nervous system.

Calcium sensitivity of BK-type KCa channels determined by a separable domain

High conductance, Ca(2+)-activated (BK-type) K+ channels from mouse (mSlo) and Drosophila (dSlo) differ in their functional properties but share a conserved core resembling voltage-gated K+ channels and a tail appended to the core by a nonconserved linker. We have found that the channel subunit is physically divisible into these two conserved domains and that the core determines such properties as channel open time, conductance, and, probably, voltage dependence, whereas the tail determines apparent Ca2+ sensitivity. Both domains are required for function. We demonstrated the different roles of the core and tail by taking advantage of the functional differences between mSlo and dSlo. Heterologous pairing of cores and tails from mSlo and dSlo showed that single-channel properties were always characteristic of the core species, but that apparent Ca2+ sensitivity was adjusted up or down depending on the species of the tail. Thus, the tail is implicated in the Ca(2+)-sensing role of BK channels.

Crystal structure of an uncleaved serpin reveals the conformation of an inhibitory reactive loop

The three-dimensional structure of an uncleaved serpin, a variant of human antichymotrypsin engineered to be an inhibitor of human neutrophil elastase, has been determined by X-ray crystallographic methods and is currently being refined at 2.5 A resolution. It contains an intact reactive loop in a distorted helical conformation. A comparison of the current model with that of its cleaved counterpart suggests that the conformational 'stress' of the serpin in its uncleaved and uncomplexed state may not be confined solely to the reactive loop or beta-sheet A. It is intriguing that strand s4A is not pre-inserted into beta-sheet A of the native serpin, and this has profound implications for the mechanism of serpin function.

Crystallization and atomic resolution X-ray diffraction analysis of antichymotrypsin variants

Crystals of two recombinant antichymotrypsin (rACT) variants have been prepared: variant rACT-T345R crystallizes in space group P2(1) (a = 109.2 A, b = 79.4 A, c = 111.9 A, beta = 116.3 degrees, with 2 molecules in the asymmetric unit), and variant ACT' crystallizes in space group P2(1)22(1) (a = 69.7 A, b = 77.2 A, c = 83.8 A, with one molecule in the asymmetric unit). The latter variant is an engineered dimer having the P3-P3' hexapeptide sequence of the related serpin, alpha 1-proteinase inhibitor, substituted for the corresponding wild-type sequence. Crystals of each variant diffract to a limiting resolution of 2.5 A, which represents the best diffraction yet achieved for a crystalline, inhibitory serpin. The exceptional quality of ACT' crystals probably arises from favorable protein-protein interactions as well as a stabilizing disulfide crosslink engineered between the monomers.

mSlo, a complex mouse gene encoding "maxi" calcium-activated potassium channels

Complementary DNAs (cDNAs) from mSlo, a gene encoding calcium-activated potassium channels, were isolated from mouse brain and skeletal muscle, sequenced, and expressed in Xenopus oocytes. The mSlo-encoded channel resembled "maxi" or BK (high conductance) channel types; single channel conductance was 272 picosiemens with symmetrical potassium concentrations. Whole cell and single channel currents were blocked by charybdotoxin, iberiotoxin, and tetraethylammonium ion. A large number of variant mSlo cDNAs were isolated, indicating that several diverse mammalian BK channel types are produced by a single gene.

Crystallization, activity assay and preliminary X-ray diffraction analysis of the uncleaved form of the serpin antichymotrypsin

Crystals of recombinant wild-type antichymotrypsin have been prepared by the method of vapor diffusion with polyethylene glycol 4000 as a precipitant at pH 5.7. Two crystal forms are observed. One form belongs to tetragonal space group P4(3)2(1)2 (or P4(1)2(1)2) and has unit cell dimensions a = b = 126 A, c = 243 A, with two molecules in the asymmetric unit. The other crystal form belongs to orthorhombic space group P2(1)2(1)2(1) and has unit cell parameters of a = 73 A, b = 78 A and c = 80 A, with one molecular in the asymmetric unit. Diffraction intensity measurements have been made on the tetragonal crystal form to a limiting resolution of 4.1 A, and reflections have been observed on X-ray still photographs to a limiting resolution of 2.5 A for the orthorhombic form. An activity assay of redissolved tetragonal form crystals indicates that the uncleaved, functional serpin has been crystallized.

An essential 'set' of K+ channels conserved in flies, mice and humans

The molecular genetic approach to studying K+ channels has revealed that at least four subfamilies of voltage-gated K+ channels originally discovered in Drosophila are conserved in mice and humans. This conservation of the K+ channel subfamilies Shaker, Shal, Shab, and Shaw suggests that not only the broad outlines of membrane electrical properties but also many molecular details as well evolved in the parent species ancestral to both invertebrate and vertebrate life. Shaker, Shal, Shab, and Shaw K+ channels have similar structures, but appear to be independent channel systems: when co-expressed in Xenopus oocytes, all four function independently. These four K+ channel subfamilies may be part of an essential 'set' of excitable channels required by most nervous systems. The task now remaining is to understand the functions of each member of the set.

K+ current diversity is produced by an extended gene family conserved in Drosophila and mouse

The Drosophila Shaker gene on the X chromosome has three sister genes, Shal, Shab, and Shaw, which map to the second and third chromosomes. This extended gene family encodes voltage-gated potassium channels with widely varying kinetics (rate of macroscopic current activation and inactivation) and voltage sensitivity of steady-state inactivation. The differences in the currents of the various gene products are greater than the differences produced by alternative splicing of the Shaker gene. In Drosophila, the transient (A current) subtype of the potassium channel (Shaker and Shal) and the delayed-rectifier subtype (Shab and Shaw) are encoded by homologous genes, and there is more than one gene for each subtype of channel. Homologs of Shaker, Shal, Shab, and Shaw are present in mammals; each Drosophila potassium-channel gene may be represented as a multigene subfamily in mammals.

Genomic organization and deduced amino acid sequence of a putative sodium channel gene in Drosophila

The deduced amino acid sequence of a Drosophila gene isolated with a vertebrate sodium channel complementary DNA probe revealed an organization virtually identical to the vertebrate sodium channel protein; four homologous domains containing all putative membrane-spanning regions are repeated in tandem with connecting linkers of various sizes. All areas of the protein presumed to be critical for channel function show high evolutionary conservation. These include those proposed to function in voltage-sensitive gating, inactivation, and ion selectivity. All 24 putative gating charges of the vertebrate protein are in identical positions in the Drosophila gene. Ten introns interrupt the coding regions of the four homology units; introns with positions conserved among homology units bracket a region hypothesized to be the selectivity filter for the channel. The Drosophila gene maps to the right arm of the second chromosome in region 60D-E. This position does not coincide with any known mutations that confer behavioral phenotypes, but is close to the seizure locus (60A-B), which has been hypothesized to code for a voltage-sensitive sodium channel.

Occult Drosophila calcium channels and twinning of calcium and voltage-activated potassium channels

In the membrane of the flight muscle cells of developing Drosophila a large calcium-sensitive potassium current, IKc, was found. It was present before the development of voltage-activated potassium channels and seems to be the first potassium current to develop in the membrane. Also present in these early cells were large numbers of occult (hidden) calcium channels, which remained inactive until the end of pupal development. These inactive calcium channels could be made to function by injecting adenosine triphosphate or ethyleneglycol tetraacetic acid into the early cells. IKc has kinetic properties resembling the later developing voltage-sensitive current IKv, and is distinct from the fast, transient calcium-dependent outward current IAc, which appears much later in development. IAc closely resembles the voltage-sensitive current IAv, also present in these cells. Thus, both of the voltage-sensitive potassium channel types, IAv and IKv, have similar calcium-sensitive counterparts, IAc and IKc, that are present in the same cells.