Dynein mutations impair autophagic clearance of aggregate-prone proteins

Mutations that affect the dynein motor machinery are sufficient to cause motor neuron disease. It is not known why there are aggregates or inclusions in affected tissues in mice with such mutations and in most forms of human motor neuron disease. Here we identify a new mechanism of inclusion formation by showing that decreased dynein function impairs autophagic clearance of aggregate-prone proteins. We show that mutations of the dynein machinery enhanced the toxicity of the mutation that causes Huntington disease in fly and mouse models. Furthermore, loss of dynein function resulted in premature aggregate formation by mutant huntingtin and increased levels of the autophagosome marker LC3-II in both cell culture and mouse models, compatible with impaired autophagosome-lysosome fusion.

Actin-dependent organelle movement in squid axoplasm

Studies of organelle movement in axoplasm extruded from the squid giant axon have led to the basic discoveries of microtubule-dependent organelle motility and the characterization of the microtubule-based motor proteins kinesin and cytoplasmic dynein. Rapid organelle movement in higher animal cells, especially in neurons, is considered to be microtubule-based. The role of actin filaments, which are also abundant in axonal cytoplasm, has remained unclear. The inhibition of organelle movement in axoplasm by actin-binding proteins such as DNase I, gelsolin and synapsin I has been attributed to their ability to disorganize the microtubule domains where most of the actin-filaments are located. Here we provide evidence of a new type of organelle movement in squid axoplasm which is independent of both microtubules and microtubule-based motors. This movement is ATP-dependent, unidirectional, actin-dependent, and probably generated by a myosin-like motor. These results demonstrate that an actomyosin-like mechanism can be directly involved in the generation of rapid organelle transport in nerve cells.

Energy coupling in DNA gyrase and the mechanism of action of novobiocin

Escherichia coli DNA gyrase catalyzes negative supercoiling of closed duplex DNA at the expense of ATP. Two additional activities of the enzyme that have illuminated the energy coupling component of the supercoiling reaction are the DNA-dependent hydrolysis of ATP to ADP and P(i) and the alteration by ATP of the DNA site specificity of the gyrase cleavage reaction. This cleavage of both DNA strands results from treatment with sodium dodecyl sulfate of the stable gyrase-DNA complex that is trapped by the inhibitor oxolinic acid. Either ATP or a nonhydrolyzable analogue, adenyl-5'-yl-imidodiphosphate (App[NH]p), shifts the primary cleavage site on ColE1 DNA. The prevention by novobiocin and coumermycin A(1) of this cleavage rearrangement places the site of action of the antibiotics at a reaction step prior to ATP hydrolysis. The step blocked is the binding of ATP because coumermycin A(1) and novobiocin interact competitively with ATP in the ATPase and supercoiling assays; the K(i) values are more than four orders of magnitude less than the K(m) for ATP. This simple mechanism accounts for all effects of the drugs on DNA gyrase. Studies with App[NH]p, another potent competitive inhibitor of reactions catalyzed by gyrase, show that cleavage of a high energy bond is not required for driving DNA into the higher energy supercoiled form. With substrate levels of gyrase, App[NH]p induces supercoiling that is proportional to the amount of enzyme; a -0.3 superhelical turn was introduced per gyrase protomer A. We postulate that ATP and App[NH]p are allosteric effectors of a conformational change of gyrase that leads to one round of supercoiling. Nucleotide dissociation favored by hydrolysis of ATP returns gyrase to its original conformation and thereby permits enzyme turnover. Such cyclic conformational changes accompanying alteration in nucleotide affinity also seem to be a common feature of energy transduction in other diverse processes including muscle contraction, protein synthesis, and oxidative phosphorylation.

Identification of kinesin in sea urchin eggs, and evidence for its localization in the mitotic spindle

To understand the molecular basis of microtubule-associated motility during mitosis, the mechanochemical factors that generate the relevant motile force must be identified. Myosin, the ATPase that interacts with actin to produce the force for muscle contraction and other forms of cell motility, is believed to be involved in cytokinesis but not in mitosis. Dynein, the mechanochemical enzyme that drives microtubule sliding in eukaryotic cilia and flagella, has been identified in the cytoplasm of sea urchin eggs, but the evidence that it is involved in cytoplasmic microtubule-based motility (rather than serving as a precursor for embryonic cilia) is equivocal. Microtubule-associated ATPases have been prepared from other tissues, but their role in cytoplasmic motility is also unknown. Recent work on axoplasmic transport, however, has led to the identification of a novel mechanochemical protein called kinesin, which is thought to generate the force for moving vesicles along axonal microtubules. These results suggest that kinesin may also be a mechanochemical factor for non-axoplasmic forms of microtubule-based motility, such as mitosis. We describe here the identification and isolation of a kinesin-like protein from the cytoplasm of sea urchin eggs. We present evidence that this protein is localized in the mitotic spindle, and propose that it may be a mechanochemical factor for some form of motility associated with the mitotic spindle.

Overlapping hand-over-hand mechanism of single molecular motility of cytoplasmic dynein

Structural differences between dynein and kinesin suggest a unique molecular mechanism of dynein motility. Measuring the mechanical properties of a single molecule of dynein is crucial for revealing the mechanisms underlying its movement. We measured the step size and force produced by single molecules of active cytoplasmic dynein by using an optical trap and fluorescence imaging with a high temporal resolution. The velocity of dynein movement, 800 nm/s, is consistent with that reported in cells. The maximum force of 7-8 pN was independent of the ATP concentration and similar to that of kinesin. Dynein exhibited forward and occasional backwards steps of approximately 8 nm, independent of load. It is suggested that the large dynein heads take 16-nm steps by using an overlapping hand-over-hand mechanism.

Proteolytic cleavage of human p53 by calpain: a potential regulator of protein stability

The p53 tumor suppressor protein is activated in cells in response to DNA damage and prevents the replication of cells sustaining genetic damage by inducing a cell cycle arrest or apoptosis. Activation of p53 is accompanied by stabilization of the protein, resulting in accumulation to high levels within the cell. p53 is normally degraded through the proteasome following ubiquitination, although the mechanisms which regulate this proteolysis in normal cells and how the p53 protein becomes stabilized following DNA damage are not well understood. We show here that p53 can also be a substrate for cleavage by the calcium-activated neutral protease, calpain, and that a preferential site for calpain cleavage exists within the N terminus of the p53 protein. Treatment of cells expressing wild-type p53 with an inhibitor of calpain resulted in the stabilization of the p53 protein. By contrast, in vitro or in vivo degradation mediated by human papillomavirus E6 protein was unaffected by the calpain inhibitor, indicating that the stabilization did not result from inhibition of the proteasome. These results suggest that calpain cleavage plays a role in regulating p53 stability.

The two nucleotide-binding domains of cystic fibrosis transmembrane conductance regulator (CFTR) have distinct functions in controlling channel activity

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel contains two cytoplasmic nucleotide-binding domains (NBDs). After phosphorylation of the R domain, ATP interacts with the NBDs to regulate channel activity. To learn how the NBDs regulate channel function, we used the patch-clamp technique to study CFTR and variants which contained site-directed mutations in the conserved Walker A motif lysine residues in either NBD1 (K464A), NBD2 (K1250A and K1250M), or both NBDs simultaneously (K464A/K1250A). Studies in related proteins suggest that such mutations slow the rate of ATP hydrolysis. These mutations did not alter the conductive properties of the channel or the requirement for phosphorylation and ATP to open the channel. However, all mutations decreased open state probability. Mutations in NBD1 decreased the frequency of bursts of activity, whereas mutations in NBD2 and mutations in both NBDs simultaneously prolonged bursts of activity, as well as decreased the frequency of bursts. These results could not be attributed to altered binding of nucleotide because none of the mutants studied had reduced 8-N3ATP binding. These data suggest that the two NBDs have distinct functions in channel gating; ATP hydrolysis at NBD1 initiates a burst of activity, and hydrolysis at NBD2 terminates a burst.

Orientation of spin-labeled myosin heads in glycerinated muscle fibers

We have used electron paramagnetic resonance (EPR) spectra to study spin labels selectively and rigidly attached to myosin heads in glycerinated rabbit psoas muscle fibers. Because the angle between the magnetic field and the principal axis of the probe determines the position of the EPR absorption line, spectra from labeled fibers oriented parallel to the magnetic field yielded directly the distribution of spin label orientations relative to the fiber axis. Two spin labels, having reactivities resembling iodoacetamide (IASL) and maleimide (MSL), were used. In rigor fibers with complete filament overlap, both labels displayed a narrow angular distribution, full width at half maximum approximately 15 degrees, centered at angles of 68 degrees (IASL) and 82 degrees (MSL). Myosin subfragments (heavy meromyosin and subfragment-1) were labeled and allowed to diffuse into fibers. The resulting spectra showed the same sharp angular distribution that was found for the labeled fibers. Thus is appears that virtually all myosin heads in a rigor fiber have the same orientation relative to the fiber axis, and this orientation is determined by the actomyosin bond. Experiments with stretched fibers indicated that the spin labels on the fraction of heads not interacting with actin filaments had a broad angular distribution. Addition of ATP to unstretched fibers under relaxing conditions produced orientational disorder, resulting in a spectrum almost indistinguishable from that of an isotropic distribution of probes. Addition of either an ATP analog (AMPPNP) or pyrophosphate produced partial disorder. That is a fraction of the probes remained sharply oriented as in rigor while a second fraction was in a disordered distribution similar to that of relaxed fibers.

Newly identified brain potassium channels gated by the guanine nucleotide binding protein Go

Potassium channels in neurons are linked by guanine nucleotide binding (G) proteins to numerous neurotransmitter receptors. The ability of Go, the predominant G protein in the brain, to stimulate potassium channels was tested in cell-free membrane patches of hippocampal pyramidal neurons. Four distinct types of potassium channels, which were otherwise quiescent, were activated by both isolated brain G0 and recombinant Go alpha. Hence brain Go can couple diverse brain potassium channels to neurotransmitter receptors.

Role of actin cortex in the subplasmalemmal transport of secretory granules in PC-12 cells

In neuroendocrine PC-12 cells, evanescent-field fluorescence microscopy was used to track motions of green fluorescent protein (GFP)-labeled actin or GFP-labeled secretory granules in a thin layer of cytoplasm where cells adhered to glass. The layer contained abundant filamentous actin (F-actin) locally condensed into stress fibers. More than 90% of the granules imaged lay within the F-actin layer. One-third of the granules did not move detectably, while two-thirds moved randomly; the average diffusion coefficient was 23 x 10(-4) microm(2)/s. A small minority (<3%) moved rapidly and in a directed fashion over distances more than a micron. Staining of F-actin suggests that such movement occurred along actin bundles. The seemingly random movement of most other granules was not due to diffusion since it was diminished by the myosin inhibitor butanedione monoxime, and blocked by chelating intracellular Mg(2+) and replacing ATP with AMP-PNP. Mobility was blocked also when F-actin was stabilized with phalloidin, and was diminished when the actin cortex was degraded with latrunculin B. We conclude that the movement of granules requires metabolic energy, and that it is mediated as well as limited by the actin cortex. Opposing actions of the actin cortex on mobility may explain why its degradation has variable effects on secretion.

Antibodies to the kinesin motor domain and CENP-E inhibit microtubule depolymerization-dependent motion of chromosomes in vitro

Chromosomes can move with the ends of depolymerizing microtubules (MTs) in vitro, even in the absence of nucleotide triphosphates (Coue, M., V. A. Lombillo, and J. R. McIntosh. 1991. J. Cell Biol. 112:1165-1175.) Here, we describe an immunological investigation of the proteins important for this form of motility. Affinity-purified polyclonal antibodies to kinesin exert a severe inhibitory effect on depolymerization-dependent chromosome motion. These antibodies predominantly recognize a polypeptide of M(r) approximately 250 kD on immunoblots of CHO chromosomes and stain kinetochores as well as some vesicles that are in the chromosome preparation. Antibodies to CENP-E, a kinetochore-associated kinesin-like protein, also recognize a 250-kD electrophoretic component, but they stain only the kinetochroe region of isolated chromosomes. Polyclonal antibodies that recognize specific domains of the CENP-E polypeptide affect MT disassembly-dependent chromosome motion in different ways; antibodies to the head or tail portions slow motility threefold, while those raised against the neck region stop motion completely. Analogous antibodies that block conventional, ATP-dependent motility of cytoplasmic dynein (Vaisberg, G., M. P. Koonce, and J. R. McIntosh. 1993. J. Cell Biol. 123:849-858) have no effect on disassembly-dependent chromosome motion, even though they bind to kinetochores. These observations suggest that CENP-E helps couple chromosomes to depolymerizing MTs. A similar coupling activity may allow spindle MTs to remain kinetochore-bound while their lengths change during both prometaphase and anaphase A.

Dual presynaptic control by ATP of glutamate release via facilitatory P2X1, P2X2/3, and P2X3 and inhibitory P2Y1, P2Y2, and/or P2Y4 receptors in the rat hippocampus

ATP is released in a vesicular manner from nerve terminals mainly at higher stimulation frequencies. There is a robust expression of ATP (P2) receptors in the brain, but their role is primarily unknown. We report that ATP analogs biphasically modulate the evoked release of glutamate from purified nerve terminals of the rat hippocampus, the facilitation being mediated by P2X1, P2X2/3, and P2X3 [antagonized by 8-(benzamido)naphthalene-1,3,5-trisulfonate and 2',3'-O-(2,4,6-trinitrophenyl)-ATP] and the inhibition by P2Y1, P2Y2, and/or P2Y4 [antagonized by reactive blue 2 and 2'deoxy-N6-methyladenosine-3',5'-bisphosphate and mimicked by P1-(urinine 5'-),P4-(inosine 5'-) tetraphosphate and 2-methylthio-ADP] receptors. The combination of single-cell PCR analysis of rat hippocampal pyramidal neurons, Western blot analysis of purified presynaptic active zone fraction, and immunocytochemical analysis of hippocampal glutamatergic terminals revealed that the P2 receptors expressed in glutamatergic neurons, located in the active zone and in glutamatergic terminals, were precisely P2X1, P2X2, and P2X3 subunits and P2Y1, P2Y2, and P2Y4 receptors. This provides coincident functional and molecular evidence that P2 receptors are present and act presynaptically as a modulatory system controlling hippocampal glutamate release.

Beta-adrenergic agonists regulate KCa channels in airway smooth muscle by cAMP-dependent and -independent mechanisms

Stimulation of calcium-activated potassium (KCa) channels in airway smooth muscle cells by phosphorylation-dependent and membrane-delimited, G protein actions has been reported (Kume, H. A. Takai, H. Tokuno, and T. Tomita. 1989. Nature [Lond.]. 341:152-154; Kume, H., M. P. Graziano, and M. I. Kotlikoff. 1992. Proc. Natl. Acad. Sci. USA. 89:11051-11055). We show that beta-adrenergic receptor/channel coupling is not affected by inhibition of endogenous ATP, and that activation of KCa channels is stimulated by both alpha S and cAMP-dependent protein kinase (PKA). PKA stimulated channel activity in a dose-dependent fashion with an EC50 of 0.12 U/ml and maximum stimulation of 7.38 +/- 2.04-fold. Application of alpha S to patches near maximally stimulated by PKA significantly increased channel activity to 15.1 +/- 3.65-fold above baseline, providing further evidence for dual regulatory mechanisms and suggesting that the stimulatory actions are independent. Analysis of channel open-time kinetics indicated that isoproterenol and alpha S stimulation of channel activity primarily increased the proportion of longer duration events, whereas PKA stimulation had little effect on the proportion of short and long duration events, but resulted in a significant increase in the duration of the long open-state. cAMP formation during equivalent relaxation of precontracted muscle strips by isoproterenol and forskolin resulted in significantly less cAMP formation by isoproterenol than by forskolin, suggesting that the degree of activation of PKA is not the only determinant of tissue relaxation. We conclude that beta-adrenergic stimulation of KCa channel activity and relaxation of tone in airway smooth muscle occurs, in part, by means independent of cyclic AMP formation.

Functional recovery from desensitization of vanilloid receptor TRPV1 requires resynthesis of phosphatidylinositol 4,5-bisphosphate

Capsaicin and other naturally occurring pungent molecules have long been used as topical analgesics to treat a variety of chronic pain conditions. The analgesic effects of these compounds involve long-term desensitization of nociceptors after strong stimulation. To elucidate the underlying mechanisms, we studied the recovery from desensitization of the vanilloid receptor TRPV1. We showed that prolonged applications of capsaicin led to nearly complete desensitization of the channel and that its functional recovery from desensitization required a high concentration of intracellular ATP. Nonhydrolyzable ATP analogs did not substitute for ATP to promote recovery. Neither inhibition nor activation of protein kinases prevented recovery of the channel from desensitization. In contrast, blockade of lipid kinases, in particular phosphatidylinositol-4-kinase, abolished recovery, as did activation of membrane receptors that stimulate hydrolysis of phosphatidylinositol 4,5-biphosphate (PIP2). Additional experiments using the PIP2-sensitive inward rectifier potassium channel Kir2.1 as a biosensor showed a high degree of temporal correlation between the two channels on both functional suppression after capsaicin stimulation and subsequent recovery. These data suggest that depletion of PIP2 occurs concomitantly with activation of TRPV1 and its replenishment in the membrane determines recovery of the channel from desensitization. In addition to revealing a new role of phosphoinositide signaling in regulation of nociception, our results provide novel insight into the topical mechanisms of the analgesic effects of capsaicin and the strategies to improve its effectiveness.

Attachment of transported vesicles to microtubules in axoplasm is facilitated by AMP-PNP

Axoplasm extruded from the squid giant axon has been used to analyse the molecular mechanisms of intracellular vesicle transport. Using video-enhanced light microscopy, vesicle transport can be observed directly on individual microtubules at the edge of the axoplasm. Here we report that AMP-PNP (adenyl-5'-yl imidodiphosphate), a non-hydrolysable analogue of ATP, reversibly inhibited vesicle transport. Moreover, vesicles in solution attach to the microtubules and form relatively stable complexes. AMP-PNP may produce this effect by binding to an ATP-binding site on the transport machinery, thereby stabilizing the motility complex that is normally formed by a transported vesicle, an ATPase and a microtubule. The effects of AMP-PNP on the vesicle transport system indicate that the enzymatic machinery of this system differs significantly from that of the actomyosin system or the dynein-microtubule system.

DNA transport by a type II topoisomerase: direct evidence for a two-gate mechanism

Recent biochemical and crystallographic results suggest that a type II DNA topoisomerase acts as an ATP-modulated clamp with two sets of jaws at opposite ends: a DNA-bound enzyme can admit a second DNA through one set of jaws; upon binding ATP, this DNA is passed through an enzyme-mediated opening in the first DNA and expelled from the enzyme through the other set of jaws. Experiments based on the introduction of reversible disulfide links across one dimer interface of yeast DNA topoisomerase II have confirmed this mechanism. The second DNA is found to enter the enzyme through the gate formed by the N-terminal parts of the enzyme and leave it through the gate close to the C termini.

ATP- and ADP-dependent modulation of RNA unwinding and strand annealing activities by the DEAD-box protein DED1

DEAD-box RNA helicases, which are involved in virtually all aspects of RNA metabolism, are generally viewed as enzymes that unwind RNA duplexes or disrupt RNA-protein interactions in an ATP-dependent manner. Here, we show in vitro that the DEAD-box protein DED1 from Saccharomyces cerevisiae promotes not only RNA unwinding but also strand annealing, the latter in such a profound fashion that the physical limit for a bimolecular association rate constant is approached. We further demonstrate that DED1 establishes an ATP-dependent steady state between unwinding and annealing, which enables the enzyme to modulate the balance between the two opposing activities through ATP and ADP concentrations. The ratio between unwinding and annealing and the degree to which both activities are ATP- and ADP-modulated are strongly influenced by structured as well as unstructured regions in the RNA substrate. Collectively, these findings expand the known functional repertoire of DEAD-box proteins and reveal the capacity of DED1 to remodel RNA in response to ADP and ATP concentrations by facilitating not only disruption but also formation of RNA duplexes.

Dual regulation of Ca2+/calmodulin-dependent kinase II activity by membrane voltage and by calcium influx

Calcium entry through voltage-gated Ca2+ channels is critical in cardiac excitation-contraction coupling and calcium metabolism. In this report, we demonstrate both spatially resolved and temporally distinct effects of Ca2+/calmodulin-dependent protein kinase II (CaMKII) on L-type Ca2+ channel current (ICa) in rat cardiac myocytes. Either depolarization alone or calcium influx can increase the amplitude and slow the inactivation of ICa. The distinct voltage- and Ca(2+)-dependent effects persist with time constants of approximately 1.7 sec and 9 sec, respectively. Both effects are completely abolished by a specific peptide inhibitor of CaMKII. This CaMKII inhibitor also suppresses the prolongation of ICa induced by depolarizing holding potentials. Furthermore, using an antibody specific for the autophosphorylated (activated) CaMKII, we find that this kinase is localized close to sarcolemmal membranes and that the profile of CaMKII activation correlates qualitatively with the changes in ICa under various conditions. Therefore, we conclude that the action of CaMKII on ICa is dually regulated by membrane depolarization and by Ca2+ influx; the latter directly activates CaMKII, whereas the former likely promotes the interaction between constitutive CaMKII and the membrane-channel proteins. These regulatory mechanisms provide positive-feedback control of Ca2+ channels and are probably important in the regulation of cardiac contractility and other intracellular Ca(2+)-regulated processes.

Severing of stable microtubules by a mitotically activated protein in Xenopus egg extracts

Eukaryotic cells disassemble and reorganize their cytoskeleton during the cell cycle and in response to environmental cues. Disassembly of the actin cytoskeleton is aided by proteins that sever filamentous actin, but microtubule-severing proteins thus far have not been identified. Here, we describe an activity in extracts from Xenopus eggs that rapidly severs stable microtubules along their length. Severing is elicited by a protein(s) whose activity is greatly stimulated during mitosis through a posttranslational mechanism. The microtubule-severing factor may be involved in disassembling the interphase microtubule network prior to constructing the mitotic spindle.

The structure of bovine F1-ATPase complexed with the antibiotic inhibitor aurovertin B

In the structure of bovine mitochondrial F1-ATPase that was previously determined with crystals grown in the presence of adenylyl-imidodiphosphate (AMP-PNP) and ADP, the three catalytic beta-subunits have different conformations and nucleotide occupancies. Adenylyl-imidodiphosphate is bound to one beta-subunit (betaTP), ADP is bound to the second (betaDP), and no nucleotide is bound to the third (betaE). Here we show that the uncompetitive inhibitor aurovertin B binds to bovine F1 at two equivalent sites in betaTP and betaE, in a cleft between the nucleotide binding and C-terminal domains. In betaDP, the aurovertin B pocket is incomplete and is inaccessible to the inhibitor. The aurovertin B bound to betaTP interacts with alpha-Glu399 in the adjacent alphaTP subunit, whereas the aurovertin B bound to betaE is too distant from alphaE to make an equivalent interaction. Both sites encompass betaArg-412, which was shown by mutational studies to be involved in binding aurovertin. Except for minor changes around the aurovertin pockets, the structure of bovine F1-ATPase is the same as determined previously. Aurovertin B appears to act by preventing closure of the catalytic interfaces, which is essential for a catalytic mechanism involving cyclic interconversion of catalytic sites.

Allosteric effects of nucleotide cofactors on Escherichia coli Rep helicase-DNA binding

The Escherichia coli Rep helicase unwinds duplex DNA during replication. The functional helicase appears to be a dimer that forms only on binding DNA. Both protomers of the dimer can bind either single-stranded or duplex DNA. Because binding and hydrolysis of adenosine triphosphate (ATP) are essential for helicase function, the energetics of DNA binding and DNA-induced Rep dimerization were studied quantitatively in the presence of the nucleotide cofactors adenosine diphosphate (ADP) and the nonhydrolyzable ATP analog AMPP(NH)P. Large allosteric effects of nucleotide cofactors on DNA binding to Rep were observed. Binding of ADP favored Rep dimers in which both protomers bound single-stranded DNA, whereas binding of AMPP(NH)P favored simultaneous binding of both single-stranded and duplex DNA to the Rep dimer. A rolling model for the active unwinding of duplex DNA by the dimeric Rep helicase is proposed that explains vectorial unwinding and predicts that helicase translocation along DNA is coupled to ATP binding, whereas ATP hydrolysis drives unwinding of multiple DNA base pairs for each catalytic event.

Regulated cycling of mitochondrial Hsp70 at the protein import channel

Hsp70 of the mitochondrial matrix (mtHsp70) provides a critical driving force for the import of proteins into mitochondria. Tim44, a peripheral inner-membrane protein, tethers it to the import channel. Here, regulated interactions were found to maximize occupancy of the active, adenosine 5'-triphosphate (ATP)-bound mtHsp70 at the channel through its intrinsic high affinity for Tim44, as well as through release of adenosine diphosphate (ADP)-bound mtHsp70 from Tim44 by the cofactor Mge1. A model peptide substrate rapidly released mtHsp70 from Tim44, even in the absence of ATP hydrolysis. In vivo, the analogous interaction of translocating polypeptide would release mtHsp70 from the channel. Consistent with the ratchet model of translocation, subsequent hydrolysis of ATP would trap the polypeptide, driving import by preventing its movement back toward the cytosol.

Bovine brain kinesin is a microtubule-activated ATPase

Recently, a protein called kinesin was described, which is capable of inducing movement of inert particles along microtubules. To purify this protein from bovine brain, we used the ability of kinesin to bind to taxol-stabilized microtubules in the presence of inorganic tripolyphosphate. The brain kinesin preparation contained one major polypeptide of 135 kDa and four minor polypeptides of 45-70 kDa. The minor polypeptides were eluted from a gel-permeation chromatography column at the same position as the major component. All the polypeptides of the preparation were capable of binding to the microtubules under identical conditions. The kinesin molecule is most probably a complex of these polypeptides. Brain kinesin had a very low ATPase activity (0.06-0.08 mumol X min-1 X mg-1 in 3 mM Mg2+ at pH 6.7). ATPase activity was strongly stimulated by microtubules (Vmax = 4.6 mumol per min per mg of kinesin). Microtubule-activated kinesin ATPase had a Km for ATP between 10 and 12 X 10(-6) M and a Kapp for microtubules (i.e., polymerized tubulin concentration required for a half-maximal activation) of 12-14 X 10(-6) M. Kinesin had a significant ATPase activity even without microtubules if 2 mM Ca2+ was substituted for Mg2+ (Vmax = 1.6 mumol X min-1 X mg-1; Km = 800 X 10(-6) M). Kinesin is therefore a mechanochemical ATPase that is activated by microtubules.

S-nitrosylation of N-ethylmaleimide sensitive factor mediates surface expression of AMPA receptors

Postsynaptic AMPA receptor (AMPAR) trafficking mediates some forms of synaptic plasticity that are modulated by NMDA receptor (NMDAR) activation and N-ethylmaleimide sensitive factor (NSF). We report that NSF is physiologically S-nitrosylated by endogenous, neuronally derived nitric oxide (NO). S-nitrosylation of NSF augments its binding to the AMPAR GluR2 subunit. Surface insertion of GluR2 in response to activation of synaptic NMDARs requires endogenous NO, acting selectively upon the binding of NSF to GluR2. Thus, AMPAR recycling elicited by NMDA neurotransmission is mediated by a cascade involving NMDA activation of neuronal NO synthase to form NO, leading to S-nitrosylation of NSF which is thereby activated, enabling it to bind to GluR2 and promote the receptor's surface expression.