Three-dimensional cryoelectron microscopy of dimeric kinesin and ncd motor domains on microtubules.

Kinesin and ncd motor proteins are homologous in sequence yet move in opposite directions along microtubules. We have previously shown that monomeric kinesin and ncd bind in the same orientation on equivalent sites relative to the ends of tubulin sheets of known polarity. We now report cryoelectron microscope images of 16-protofilament microtubules decorated with both single- and double-headed kinesin and double-headed ncd. Three-dimensional density maps and difference maps show that, in adenosine 5'-[beta,gamma-imido]triphosphate, both dimeric motors bind tightly to microtubules via one head, leaving the other free, though apparently in a fixed position. The attached heads of dimers bind to tubulin in the same way as single kinesin heads. The second heads are connected to the tops of the first but, whereas the second kinesin head is closely associated with the first, pairs of ncd heads are splayed apart. There is also a distinct difference in orientation: the second kinesin head is tilted toward the microtubule plus end, while the second head of ncd points toward the minus end.

Conformational transitions monitored for single molecules in solution.

Phenomena that can be observed for a large number of molecules may not be understood if it is not possible to observe the events on the single-molecule level. We measured the fluorescence lifetimes of individual tetramethylrhodamine molecules, linked to an 18-mer deoxyribonucleotide sequence specific for M13 DNA, by time-resolved, single-photon counting in a confocal fluorescence microscope during Brownian motion in solution. When many molecules were observed, a biexponential fluorescence decay was observed with equal amplitudes. However, on the single-molecule level, the fraction of one of the amplitudes spanned from 0 to unity for a collection of single-molecule detections. Further analysis by fluorescence correlation spectroscopy made on many molecules revealed a process that obeys a stretched exponential relaxation law. These facts, combined with previous evidence of the quenching effect of guanosine on rhodamines, indicate that the tetramethylrhodamine molecule senses conformational transitions as it associates and dissociates to a guanosine-rich area. Thus, our results reveal conformational transitions in a single molecule in solution under conditions that are relevant for biological processes.

The kinesin walk: a dynamic model with elastically coupled heads.

Recently individual two-headed kinesin molecules have been studied in in vitro motility assays revealing a number of their peculiar transport properties. In this paper we propose a simple and robust model for the kinesin stepping process with elastically coupled Brownian heads that show all of these properties. The analytic and numerical treatment of our model results in a very good fit to the experimental data and practically has no free parameters. Changing the values of the parameters in the restricted range allowed by the related experimental estimates has almost no effect on the shape of the curves and results mainly in a variation of the zero load velocity that can be directly fitted to the measured data. In addition, the model is consistent with the measured pathway of the kinesin ATPase.

The role of surface loops (residues 204-216 and 627-646) in the motor function of the myosin head.

A characteristic feature of all myosins is the presence of two sequences which despite considerable variations in length and composition can be aligned with loops 1 (residues 204-216) and 2 (residues 627-646) in the chicken myosin-head heavy chain sequence. Recently, an intriguing hypothesis has been put forth suggesting that diverse performances of myosin motors are achieved through variations in the sequences of loops 1 and 2 [Spudich, J. (1994) Nature (London) 372, 515-518]. Here, we report on the study of the effects of tryptic digestion of these loops on the motor and enzymatic functions of myosin. Tryptic digestions of myosin, which produced heavy meromyosin (HMM) with different percentages of molecules cleaved at both loop 1 and loop 2, resulted in the consistent decrease in the sliding velocity of actin filaments over HMM in the in vitro motility assays, did not affect the Vmax, and increased the Km values for actin-activated ATPase of HMM. Selective cleavage of loop 2 on HMM decreased its affinity for actin but did not change the sliding velocity of actin in the in vitro motility assays. The cleavage of loop 1 and HMM decreased the mean sliding velocity of actin in such assays by almost 50% but did not alter its affinity for HMM. To test for a possible kinetic determinant of the change in motility, 1-N6-ethenoadenosine diphosphate (epsilon-ADP) release from cleaved and uncleaved myosin subfragment 1 (S1) was examined. Tryptic digestion of loop 1 slightly accelerated the release of epsilon-ADP from S1 but did not affect the rate of epsilon-ADP release from acto-S1 complex. Overall, the results of this work support the hypothesis that loop 1 can modulate the motor function of myosin and suggest that such modulation involves a mechanism other than regulation of ADP release from myosin.

Coupling the phosphotransferase system and the methyl-accepting chemotaxis protein-dependent chemotaxis signaling pathways of Escherichia coli.

Chemotactic responses in Escherichia coli are typically mediated by transmembrane receptors that monitor chemoeffector levels with periplasmic binding domains and communicate with the flagellar motors through two cytoplasmic proteins, CheA and CheY. CheA autophosphorylates and then donates its phosphate to CheY, which in turn controls flagellar rotation. E. coli also exhibits chemotactic responses to substrates that are transported by the phosphoenolpyruvate (PEP)-dependent carbohydrate phosphotransferase system (PTS). Unlike conventional chemoreception, PTS substrates are sensed during their uptake and concomitant phosphorylation by the cell. The phosphoryl groups are transferred from PEP to the carbohydrates through two common intermediates, enzyme I (EI) and phosphohistidine carrier protein (HPr), and then to sugar-specific enzymes II. We found that in mutant strains HPr-like proteins could substitute for HPr in transport but did not mediate chemotactic signaling. In in vitro assays, these proteins exhibited reduced phosphotransfer rates from EI, indicating that the phosphorylation state of EI might link the PTS phospho-relay to the flagellar signaling pathway. Tests with purified proteins revealed that unphosphorylated EI inhibited CheA autophosphorylation, whereas phosphorylated EI did not. These findings suggest the following model for signal transduction in PTS-dependent chemotaxis. During uptake of a PTS carbohydrate, EI is dephosphorylated more rapidly by HPr than it is phosphorylated at the expense of PEP. Consequently, unphosphorylated EI builds up and inhibits CheA autophosphorylation. This slows the flow of phosphates to CheY, eliciting an up-gradient swimming response by the cell.

Meiosis in Drosophila: seeing is believing.

Recently many exciting advances have been achieved in our understanding of Drosophila meiosis due to combined cytological and genetic approaches. New techniques have permitted the characterization of chromosome position and spindle formation in female meiosis I. The proteins encoded by the nod and ncd genes, two genes known to be needed for the proper partitioning of chromosomes lacking exchange events, have been identified and found to be kinesin-like motors. The effects of mutations in these genes on the spindle and chromosomes, together with the localization of the proteins, have yielded a model for the mechanism of female meiosis I. In male meiosis I, the chromosomal regions responsible for homolog pairing have been resolved to the level of specific DNA sequences. This provides a foundation for elucidating the molecular basis of meiotic pairing. The cytological techniques available in Drosophila also have permitted inroads into the regulation of sister-chromatid segregation. The products of two genes (mei-S332 and ord) essential for sister-chromatid cohesion have been identified recently. Additional advances in understanding Drosophila meiosis are the delineation of a functional centromere by using minichromosome derivatives and the identification of several regulatory genes for the meiotic cell cycle.

Expression in cochlea and retina of myosin VIIa, the gene product defective in Usher syndrome type 1B.

Myosin VIIa is a newly identified member of the myosin superfamily of actin-based motors. Recently, the myosin VIIa gene was identified as the gene defective in shaker-1, a recessive deafness in mice [Gibson, F., Walsh, J., Mburu, P., Varela, A., Brown, K.A., Antonio, M., Beisel, K.W., Steel, K.P. & Brown, S.D.M. (1995) Nature (London) 374, 62-64], and in human Usher syndrome type 1B, an inherited disease characterized by congenital deafness, vestibular dysfunction, and retinitis pigmentosa [Weil, D., Blanchard, S., Kaplan, J., Guilford, P., Gibson, F., Walsh, J., Mburu, P., Varela, A., Levilliers, J., Weston, M.D., Kelley, P.M., Kimberling, W.J., Wagenaar, M., Levi-Acobas, F., Larget-Piet, D., Munnich, A., Steel, K.P., Brown, S.D.M. & Petit, C. (1995) Nature (London) 374, 60-61]. To understand the normal function of myosin VIIa and how it could cause these disease phenotypes when defective, we generated antibodies specific to the tail portion of this unconventional myosin. We found that myosin VIIa was expressed in cochlea, retina, testis, lung, and kidney. In cochlea, myosin VIIa expression was restricted to the inner and outer hair cells, where it was found in the apical stereocilia as well as the cytoplasm. In the eye, myosin VIIa was expressed by the retinal pigmented epithelial cells, where it was enriched within the apical actin-rich domain of this cell type. The cell-specific localization of myosin VIIa suggests that the blindness and deafness associated with Usher syndrome is due to lack of proper myosin VIIa function within the cochlear hair cells and the retinal pigmented epithelial cells.

Features of MotA proton channel structure revealed by tryptophan-scanning mutagenesis.

The MotA protein of Escherichia coli is a component of the flagellar motors that functions in transmembrane proton conduction. Here, we report several features of MotA structure revealed by use of a mutagenesis-based approach. Single tryptophan residues were introduced at many positions within the four hydrophobic segments of MotA, and the effects on function were measured. Function was disrupted according to a periodic pattern that implies that the membrane-spanning segments are alpha-helices and that identifies the lipid-facing parts of each helix. The results support a hypothesis for MotA structure and mechanism in which water molecules form most of the proton-conducting pathway. The success of this approach in studying MotA suggests that it could be useful in structure-function studies of other integral membrane proteins.

ncd and kinesin motor domains interact with both alpha- and beta-tubulin.

Motor domains of the Drosophila minus-end-directed microtubule (MT) motor protein ncd, were found to saturate microtubule binding sites at a stoichiometry of approximately one motor domain per tubulin dimer. To determine the tubulin subunit(s) involved in binding to ncd, mixtures of ncd motor domain and MTs were treated with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl-carbodiimide) (EDC). EDC treatment generated covalently cross-linked products of ncd and alpha-tubulin and of ncd and beta-tubulin, indicating that the ncd motor domain interacts with both alpha- and beta-tubulin. When the Drosophila kinesin motor domain protein was substituted for the ncd motor domain, cross-linked products of kinesin and alpha-tubulin and of kinesin and beta-tubulin were produced. EDC treatment of mixtures of ncd motor domain and unassembled tubulin dimers or of kinesin motor domain and unassembled tubulin dimers produced the same motor-tubulin products generated in the presence of MTs. These results indicate that kinesin family motors of opposite polarity interact with both tubulin monomers and support a model in which some portion of each protein's motor domain overlaps adjacent alpha- and beta-tubulin subunits.

Fluctuation analysis of rotational speeds of the bacterial flagellar motor.

We measured the dependence of the variance in the rotation rate of tethered cells of Escherichia coli on the mean rotation rate over a regime in which the motor generates constant torque. This dependence was compared with that of broken motors. In either case, motor torque was augmented with externally applied torque. We show that, in contrast to broken motors, functioning motors in this regime do not freely rotationally diffuse and that the variance measurements are consistent with the predicted values of a stepping mechanism with exponentially distributed waiting times (a Poisson stepper) that steps approximately 400 times per revolution.