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J A Spudich

Publications and source records attributed to J A Spudich.

At least 37 records · Page 2Linked to original sources
PubMed1998

Nucleotide-dependent conformational change near the fulcrum region in Dictyostelium myosin II.

In skeletal muscle myosin, the reactive thiols (SH1 and SH2) are close to a proposed fulcrum region that is thought to undergo a large conformational change. The reactive thiol region is thought to transmit the conformational changes induced by the actin-myosin-ATP interactions to the lever arm, which amplifies the power stroke. In skeletal muscle myosin, SH1 and SH2 can be chemically cross-linked in the presence of nucleotide, trapping the nucleotide in its pocket. Although the flexibility of the reactive thiol region has been well studied in skeletal muscle myosin, crystal structures of truncated nonmuscle myosin II from Dictyostelium in the presence of various ATP analogs do not show changes at the reactive thiol region that would be consistent with the SH1-SH2 cross-linking observed for muscle myosin. To examine the dynamics of the reactive thiol region in Dictyostelium myosin II, we have examined a modified myosin II that has cysteines at the muscle myosin SH1 and SH2 positions. This myosin is specifically cross-linked at SH1-SH2 by a chemical cross-linker in the presence of ADP, but not in its absence. Furthermore, the cross-linked species traps the nucleotide, as in the case of muscle myosin. Thus, the Dictyostelium myosin II shares the same dynamic behavior in the fulcrum region of the molecule as the skeletal muscle myosin. This result emphasizes the importance of nucleotide-dependent changes in this part of the molecule.

Actins↗
PubMed1998

MLCK-A, an unconventional myosin light chain kinase from Dictyostelium, is activated by a cGMP-dependent pathway.

Dictyostelium myosin II is activated by phosphorylation of its regulatory light chain by myosin light chain kinase A (MLCK-A), an unconventional MLCK that is not regulated by Ca2+/calmodulin. MLCK-A is activated by autophosphorylation of threonine-289 outside of the catalytic domain and by phosphorylation of threonine-166 in the activation loop by an unidentified kinase, but the signals controlling these phosphorylations are unknown. Treatment of cells with Con A results in quantitative phosphorylation of the regulatory light chain by MLCK-A, providing an opportunity to study MLCK-A's activation mechanism. MLCK-A does not alter its cellular location upon treatment of cells with Con A, nor does it localize to the myosin-rich caps that form after treatment. However, MLCK-A activity rapidly increases 2- to 13-fold when Dictyostelium cells are exposed to Con A. This activation can occur in the absence of MLCK-A autophosphorylation. cGMP is a promising candidate for an intracellular messenger mediating Con A-triggered MLCK-A activation, as addition of cGMP to fresh Dictyostelium lysates increases MLCK-A activity 3- to 12-fold. The specific activity of MLCK-A in cGMP-treated lysates is 210-fold higher than that of recombinant MLCK-A, which is fully autophosphorylated, but lacks threonine-166 phosphorylation. Purified MLCK-A is not directly activated by cGMP, indicating that additional cellular factors, perhaps a kinase that phosphorylates threonine-166, are involved.

Actomyosin↗
PubMed1998

Kinetic characterization of myosin head fragments with long-lived myosin.ATP states.

We have separately expressed the Dictyosteliumdiscoideum myosin II nonhydrolyzer point mutations E459V and E476K [Ruppel, K. M., and Spudich, J. A. (1996) Mol. Biol. Cell 7, 1123-1136] in the soluble myosin head fragment M761-1R [Anson et al. (1996) EMBO J. 15, 6069-6074] and performed transient kinetic analyses to characterize the ATPase cycles of the mutant proteins. While the mutations cause some changes in mantATP [2'(3')-O-(N-methylanthraniloyl)-ATP] and mantADP binding, the most dramatic effect is on the hydrolysis step of the ATPase cycle, which is reduced by 4 (E476K) and 6 (E459V) orders of magnitude. Thus, both mutant myosin constructs do in fact catalyze ATP hydrolysis but have very long-lived myosin.ATP states. The E459V mutation allowed for a direct measurement of the ATP off rate constant from myosin, which was found to be 2 x 10(-)5 s-1. Actin accelerated ATP release from this E459V construct by at least 100-fold. Additionally, we found that the affinity of the E476K construct for actin is significantly weaker than for the wild-type construct, while the E459V mutant interacts with actin normally. Their functional properties and the fact that they can be produced and purified in large amounts make the E476K and E459V constructs ideal tools to elucidate key structural features of the myosin ATPase cycle. These constructs should allow us to address important questions, including how binding of ATP to myosin heads results in a >3 order of magnitude reduction in actin affinity.

Actins↗
PubMed1998

Single molecule biochemistry using optical tweezers.

The use of optical trapping to create extremely compliant mechanical probes has ushered in a new field of biological inquiry, the mechanical and kinetic study of proteins at the single molecule level. This review focuses on three examples of such study and includes methods of extracting parameters of interest from the raw data such experiments generate.

Biochemistry↗
PubMed1998

Dictyostelium myosin 25-50K loop substitutions specifically affect ADP release rates.

While most of the sequence of myosin's motor domain is highly conserved among various organisms and tissue types, the junctions between the 25 and 50 kDa domains and the 50 and 20 kDa domains are strikingly divergent. The 50-20K loop is positioned to interact with actin, while the 25-50K loop is situated nearer the ATP binding site [Rayment, I., et al. (1993) Science 261, 50-58]. Chimeric studies of the 50-20K loop [Uyeda, T. Q.-P., et al. (1994) Nature 368, 567-569; Rovner, A. S., et al. (1995) J. Biol. Chem. 270 (51), 30260-30263] have shown that this loop affects actin activation of ATPase activity. Given the function of myosin as a molecular motor, it was proposed that the 25-50K loop might specifically alter ADP release [Spudich, J. A. (1994) Nature 374, 515-518]. Here we study the role of this loop by engineering chimeras containing the Dictyostelium myosin heavy chain with loops from two enzymatically diverse myosins, rabbit skeletal and Acanthamoeba. The chimeric myosins complement the myosin null phenotype in vivo, bind nucleotide normally, interact normally with actin, and display wild-type levels of actin-activated ATPase activity. However, the rate of ADP release from the myosins, normally the slowest step involved in motility, was changed in a manner that reflects the activity of the donor myosin. In summary, studies of Dictyostelium myosin heavy chain chimeras have shown that the 50-20K sequence specifically affects the actin-activated ATPase activity [Uyeda, T. Q.-P., et al. (1994)] while the 25-50K sequence helps determine the rate of ADP release.

Acanthamoeba↗
PubMed1998

Conditional loss-of-myosin-II-function mutants reveal a position in the tail that is critical for filament nucleation.

Myosin-II must be assembled into filaments to perform its cellular functions. Two conditional loss-of-myosin-II-function mutants were recovered from a previous genetic screen with defects that were mapped to the coiled-coil tail region of Dictyostelium myosin-II. Strikingly, both tail mutations affected the same arginine residue at position 1880. A single amino acid substitution, R1880P, disrupted both the dimerization and tetramerization steps of filament nucleation. Even a single charge reversal at this position, R1880D, was sufficient to inhibit filament assembly, while other single charge reversals in the region of antiparallel contract suppressed these filament assembly mutants. The considerable impact of small electrostatic forces on nucleation suggests that these steps are delicately balanced and easily reversible.

Actin Cytoskeleton↗
PubMed1997

Cold-sensitive mutants G680V and G691C of Dictyostelium myosin II confer dramatically different biochemical defects.

Cold-sensitive myosin mutants represent powerful tools for dissecting discrete deficiencies in myosin function. Biochemical characterization of two such mutants, G680V and G691C, has allowed us to identify separate facets of myosin motor function perturbed by each alteration. Compared with wild type, the G680V myosin exhibits a substantially enhanced affinity for several nucleotides, decreased ATPase activity, and overoccupancy or creation of a novel strongly actin-binding state. The properties of the novel strong binding state are consistent with a partial arrest or pausing at the onset of the mechanical stroke. The G691C mutant, on the other hand, exhibits an elevated basal ATPase indicative of premature phosphate release. By releasing phosphate without a requirement for actin binding, the G691C can bypass the part of the cycle involving the mechanical stroke. The two mutants, despite having alterations in glycine residues separated by only 11 residues, have dramatically different consequences on the mechanochemical cycle.

Actins↗
PubMed1997

Reversible, site-specific immobilization of polyarginine-tagged fusion proteins on mica surfaces.

A large variety of genes is expressed as fusion proteins for the purpose of characterization and purification in molecular biology. We have used this strategy to append polyarginine peptides in order to achieve specific binding of the Arg-tag to atomically flat, negatively charged mica surfaces. We show that the model protein, hexaarginine-tagged green fluorescent protein (GFP), binds to mica via its Arg-tag based on ion exchange of naturally occurring potassium cations. Only non-specific binding was observed with the control protein that is free of the Arg-tag. This novel technology will be widely applicable to orient functional proteins on flat surfaces.

Adsorption↗
PubMed1997

Detection of single-molecule interactions using correlated thermal diffusion.

Observation of discrete, single-molecule binding events allows one to bypass assumptions required to infer single-molecule properties from studies of ensembles of molecules. Optically trapped beads and glass microneedles have been applied to detect single-molecule binding events, but it remains difficult to identify signs of binding events given the large displacements induced by thermal forces. Here, we exploit thermal diffusion by using correlation between motion of optically trapped beads attached to both ends of a single actin filament to track binding events of individual myosin molecules. We use correlated diffusion to measure the stiffness of a single myosin molecule and estimate its thermal fluctuation in a poststroke state as comparable in amplitude to the measured stroke distance. The use of correlated diffusion to measure kinetics of single-molecule interactions and the stiffness of the interacting moieties should be applicable to any pair of interacting molecules, and not limited to biological motors.

Actins↗
PubMed1997

Phenotypically selected mutations in myosin's actin binding domain demonstrate intermolecular contacts important for motor function.

Here, we biochemically characterize Dictyostelium myosin II mutants that were previously phenotypically selected following random mutagenesis and shown to lie in the actin binding domain [Patterson, B., & Spudich, J. A. (1996) Genetics 143, 801-810]. We show that the conditional loss of myosin-dependent activity in vivo, which results from the mutations E531Q, P536R, and R562L, is likely due to the loss of important contacts with actin. Purified wild-type and mutant myosin subfragments 1 (S1), expressed in Dictyostelium, are alike in binding to actin and releasing it in an ATP-dependent manner. Furthermore, the rates of ATP hydrolysis without actin are similar for the mutant and wild-type S1s. Thus, the mutations in the actin binding site have little effect on ATP binding or product release in the absence of actin. All three mutants, however, have impaired actin-activated ATPase activity, with apparent second-order rate constants for actin interactions that are 4-25-fold smaller than that of wild-type S1 at 30 degrees C. The mutations also cause defects in the ability to move actin, as measured by in vitro motility assays of full-length myosins. On the basis of motility of a mixture of wild-type and mutant myosins, there appears to be at least two classes of mutations, with the primary defect in either a weak or a strong actin binding state. In summary, the activities in vitro of myosins with mutations in the actin binding site suggest losses of important contacts with actin.

Actins↗
PubMed1997

Bioreactive self-assembled monolayers on hydrogen-passivated Si(111) as a new class of atomically flat substrates for biological scanning probe microscopy.

This is the first report of bioreactive self-assembled monolayers, covalently bound to atomically flat silicon surfaces and capable of binding biomolecules for investigation by scanning probe microscopy and other surface-related assays and sensing devices. These monolayers are stable under a wide range of conditions and allow tailor-made functionalization for many purposes. We describe the substrate preparation and present an STM and SFM characterization, partly performed with multiwalled carbon nanotubes as tapping-mode supertips. Furthermore, we present two strategies of introducing in situ reactive headgroup functionalities. One method entails a free radical chlorosulfonation process with subsequent sulfonamide formation. A second method employs singlet carbenemediated hydrogen-carbon insertion of a heterobifunctional, amino-reactive trifluoromethyl-diazirinyl crosslinker. We believe that this new substrate is advantageous to others, because it (i) is atomically flat over large areas and can be prepared in a few hours with standard equipment, (ii) is stable under most conditions, (iii) can be modified to adjust a certain degree of reactivity and hydrophobicity, which allows physical adsorption or covalent crosslinking of the biological specimen, (iv) builds the bridge between semiconductor microfabrication and organic/biological molecular systems, and (v) is accessible to nanopatterning and applications requiring conductive substrates.

Alkanes↗
PubMed1997

Myosin heavy chain phosphorylation sites regulate myosin localization during cytokinesis in live cells.

Conventional myosin II plays a fundamental role in the process of cytokinesis where, in the form of bipolar thick filaments, it is thought to be the molecular motor that generates the force necessary to divide the cell. In Dictyostelium, the formation of thick filaments is regulated by the phosphorylation of three threonine residues in the tail region of the myosin heavy chain. We report here on the effects of this regulation on the localization of myosin in live cells undergoing cytokinesis. We imaged fusion proteins of the green-fluorescent protein with wild-type myosin and with myosins where the three critical threonines had been changed to either alanine or aspartic acid. We provide evidence that thick filament formation is required for the accumulation of myosin in the cleavage furrow and that if thick filaments are overproduced, this accumulation is markedly enhanced. This suggests that myosin localization in dividing cells is regulated by myosin heavy chain phosphorylation.

Actins↗
PubMed1997

On the role of myosin-II in cytokinesis: division of Dictyostelium cells under adhesive and nonadhesive conditions.

We have investigated the role of myosin in cytokinesis in Dictyostelium cells by examining cells under both adhesive and nonadhesive conditions. On an adhesive surface, both wild-type and myosin-null cells undergo the normal processes of mitotic rounding, cell elongation, polar ruffling, furrow ingression, and separation of daughter cells. When cells are denied adhesion through culturing in suspension or on a hydrophobic surface, wild-type cells undergo these same processes. However, cells lacking myosin round up and polar ruffle, but fail to elongate, furrow, or divide. These differences show that cell division can be driven by two mechanisms that we term Cytokinesis A, which requires myosin, and Cytokinesis B, which is cell adhesion dependent. We have used these approaches to examine cells expressing a myosin whose two light chain-binding sites were deleted (DeltaBLCBS-myosin). Although this myosin is a slower motor than wild-type myosin and has constitutively high activity due to the abolition of regulation by light-chain phosphorylation, cells expressing DeltaBLCBS-myosin were previously shown to divide in suspension (Uyeda et al., 1996). However, we suspected their behavior during cytokinesis to be different from wild-type cells given the large alteration in their myosin. Surprisingly, DeltaBLCBS-myosin undergoes relatively normal spatial and temporal changes in localization during mitosis. Furthermore, the rate of furrow progression in cells expressing a DeltaBLCBS-myosin is similar to that in wild-type cells.

Animals↗
PubMed1996

Activation of Dictyostelium myosin light chain kinase A by phosphorylation of Thr166.

Phosphorylation of the regulatory light chain is an important mechanism for the activation of myosin in non-muscle cells. Unlike most myosin light chain kinases (MLCKs), MLCK-A from Dictyostelium is not activated by Ca2+/calmodulin. Autophosphorylation increases activity, but only to a low level, suggesting that there is an additional activation mechanism. Here, we show that MLCK-A is autophosphorylated on Thr289, which is C-terminal to the catalytic domain. Phosphorylation of MLCK-A increases in response to concanavalin A (conA) treatment of cells, which was previously shown to activate MLCK-A. However, a mutant kinase with an alanine at position 289 (T289A) is also phosphorylated in vivo, indicating that there is an additional phosphorylated residue. Based on comparisons with other protein kinases, we tested whether phosphorylation of Thr166 drives activation of MLCK-A. Our data indicate that phosphorylation of Thr289 occurs in vivo, but is not associated with conA-induced activation, whereas phosphorylation of Thr166 by some as yet unidentified kinase is associated with activation. Replacement of Thrl66 with glutamate results in a 12-fold increase in activity as compared with the wild-type enzyme, supporting the idea that phosphorylation of Thr166 increases MLCK-A activity.

Amino Acid Sequence↗
PubMed1996

Myosin light chain kinase (MLCK) gene disruption in Dictyostelium: a role for MLCK-A in cytokinesis and evidence for multiple MLCKs.

We have created a strain of Dictyostelium that is deficient for the Ca2+/calmodulin-independent MLCK-A. This strain undergoes cytokinesis less efficiently than wild type, which results in an increased frequency of multinucleate cells when grown in suspension. The MLCK-A-cells are able, however, to undergo development and to cap crosslinked surface receptors, processes that require myosin heavy chain. Phosphorylated regulatory light chain (RLC) is still present in MLCK-A-cells, indicating that Dictyostelium has one or more additional protein kinases capable of phosphorylating RLC. Concanavalin A treatment was found to induce phosphorylation of essentially all of the RLC in wild-type cells, but RLC phosphorylation levels in MLCK-A-cells are unaffected by concanavalin A. Thus MLCK-A is regulated separately from the other MLCK(s) in the cell.

Animals↗
PubMed1996

The neck region of the myosin motor domain acts as a lever arm to generate movement.

The myosin head consists of a globular catalytic domain that binds actin and hydrolyzes ATP and a neck domain that consists of essential and regulatory light chains bound to a long alpha-helical portion of the heavy chain. The swinging neck-level model assumes that a swinging motion of the neck relative to the catalytic domain is the origin of movement. This model predicts that the step size, and consequently the sliding velocity, are linearly related to the length of the neck. We have tested this point by characterizing a series of mutant Dictyostelium myosins that have different neck lengths. The 2xELCBS mutant has an extra binding site for essential light chain. The delta RLCBS mutant myosin has an internal deletion that removes the regulatory light chain binding site. The delta BLCBS mutant lacks both light chain binding sites. Wild-type myosin and these mutant myosins were subjected to the sliding filament in vitro motility assay. As expected, mutants with shorter necks move slower than wild-type myosin in vitro. Most significantly, a mutant with a longer neck moves faster than the wild type, and the sliding velocities of these myosins are linearly related to the neck length, as predicted by the swinging neck-lever model. A simple extrapolation to zero speed predicts that the fulcrum point is in the vicinity of the SH1-SH2 region in the catalytic domain.

Actins↗