Structural relationships of actin, myosin, and tropomyosin revealed by cryo-electron microscopy.

We have calculated three-dimensional maps from images of myosin subfragment-1 (S1)-decorated thin filaments and S1-decorated actin filaments preserved in frozen solution. By averaging many data sets we obtained highly reproducible maps that can be interpreted simply to provide a model for the native structure of decorated filaments. From our results we have made the following conclusions. The bulk of the actin monomer is approximately 65 X 40 X 40 A and is composed of two domains. In the filaments the monomers are strongly connected along the genetic helix with weaker connections following the long pitch helix. The long axis of the monomer lies roughly perpendicular to the filament axis. The myosin head (S1) approaches the actin filament tangentially and binds to a single actin, the major interaction being with the outermost domain of actin. In the map the longest chord of S1 is approximately 130 A. The region of S1 closest to actin is of high density, whereas the part furthest away is poorly defined and may be disordered. By comparing maps from decorated thin filaments with those from decorated actin, we demonstrate that tropomyosin is bound to the inner domain of actin just in front of the myosin binding site at a radius of approximately 40 A. A small change in the azimuthal position of tropomyosin, as has been suggested by others to occur during Ca2+-mediated regulation in vertebrate striated muscle, appears to be insufficient to eclipse totally the major site of interaction between actin and myosin.

Microtubule dynamics and microtubule caps: a time-resolved cryo-electron microscopy study.

Microtubules display the unique property of dynamic instability characterized by phase changes between growth and shrinkage, even in constant environmental conditions. The phases can be synchronized, leading to bulk oscillations of microtubules. To study the structural basis of dynamic instability we have examined growing, shrinking, and oscillating microtubules by time-resolved cryo-EM. In particular we have addressed three questions which are currently a matter of debate: (a) What is the relationship between microtubules, tubulin subunits, and tubulin oligomers in microtubule dynamics?; (b) How do microtubules shrink? By release of subunits or via oligomers?; and (c) Is there a conformational change at microtubule ends during the transitions from growth to shrinkage and vice versa? The results show that (a) oscillating microtubules coexist with a substantial fraction of oligomers, even at a maximum of microtubule assembly; (b) microtubules disassemble primarily into oligomers; and (c) the ends of growing microtubules have straight protofilaments, shrinking microtubules have protofilaments coiled inside out. This is interpreted as a transition from a tense to a relaxed conformation which could be used to perform work, as suggested by some models of poleward chromosome movement during anaphase.

Brush border myosin-I structure and ADP-dependent conformational changes revealed by cryoelectron microscopy and image analysis.

Brush border myosin-I (BBM-I) is a single-headed myosin found in the microvilli of intestinal epithelial cells, where it forms lateral bridges connecting the core bundle of actin filaments to the plasma membrane. Extending previous observations (Jontes, J.D., E.M. Wilson-Kubalek, and R.A. Milligan. 1995. Nature [Lond.]. 378:751-753), we have used cryoelectron microscopy and helical image analysis to generate three-dimensional (3D) maps of actin filaments decorated with BBM-I in both the presence and absence of 1 mM MgADP. In the improved 3D maps, we are able to see the entire light chain-binding domain, containing density for all three calmodulin light chains. This has enabled us to model a high resolution structure of BBM-I using the crystal structures of the chicken skeletal muscle myosin catalytic domain and essential light chain. Thus, we are able to directly measure the full magnitude of the ADP-dependent tail swing. The approximately 31 degrees swing corresponds to approximately 63 A at the end of the rigid light chain-binding domain. Comparison of the behavior of BBM-I with skeletal and smooth muscle subfragments-1 suggests that there are substantial differences in the structure and energetics of the biochemical transitions in the actomyosin ATPase cycle.

In vitro crystallization of ribosomes from chick embryos.

A new two-dimensional ribosome crystal, having the tetragonal space group P42(1)2 (a = 593 A), has been grown from ribosome tetramers extracted from hypothermic chick embryos. It is of particular interest because of its larger size (up to 3 x 3 micron2) and greater stability compared to other related polymorphic forms, and because it can easily be grown in large amounts. X-ray diffraction shows the order in the crystal to extend to a resolution of at least 60 A. The crystalline ribosomes appear to contain a full complement of small and large ribosomal subunit proteins and an additional four proteins not characteristic of chick embryo polysomes.

A large particle associated with the perimeter of the nuclear pore complex.

The three-dimensional structure of the nuclear pore complex has been determined to a resolution of approximately 90 A by electron microscopy using nuclear envelopes from Xenopus oocytes. It is shown to be an assembly of several discrete constituents arranged with octagonal symmetry about a central axis. There are apparent twofold axes perpendicular to the octad axis which suggest that the framework of the pore complex is constructed from two equal but oppositely facing halves. The half facing the cytoplasm is in some instances decorated by large particles, similar in appearance and size to ribosomes.

Kinesin follows the microtubule's protofilament axis.

We tested the hypothesis that kinesin moves parallel to the microtubule's protofilament axis. We polymerized microtubules with protofilaments that ran either parallel to the microtubule's long axis or that ran along shallow helical paths around the cylindrical surface of the microtubule. When gliding across a kinesin-coated surface, the former microtubules did not rotate. The latter microtubules, those with supertwisted protofilaments, did rotate; the pitch and handedness of the rotation accorded with the supertwist measured by electron cryo-microscopy. The results show that kinesin follows a path parallel to the protofilaments with high fidelity. This implies that the distance between consecutive kinesin-binding sites along the microtubule must be an integral multiple of 4.1 nm, the tubulin monomer spacing along the protofilament, or a multiple of 8.2 nm, the dimer spacing.

Three-dimensional structure of Acanthamoeba castellanii myosin-IB (MIB) determined by cryoelectron microscopy of decorated actin filaments.

The Acanthamoeba castellanii myosin-Is were the first unconventional myosins to be discovered, and the myosin-I class has since been found to be one of the more diverse and abundant classes of the myosin superfamily. We used two-dimensional (2D) crystallization on phospholipid monolayers and negative stain electron microscopy to calculate a projection map of a "classical" myosin-I, Acanthamoeba myosin-IB (MIB), at approximately 18 A resolution. Interpretation of the projection map suggests that the MIB molecules sit upright on the membrane. We also used cryoelectron microscopy and helical image analysis to determine the three-dimensional structure of actin filaments decorated with unphosphorylated (inactive) MIB. The catalytic domain is similar to that of other myosins, whereas the large carboxy-terminal tail domain differs greatly from brush border myosin-I (BBM-I), another member of the myosin-I class. These differences may be relevant to the distinct cellular functions of these two types of myosin-I. The catalytic domain of MIB also attaches to F-actin at a significantly different angle, approximately 10 degrees, than BBM-I. Finally, there is evidence that the tails of adjacent MIB molecules interact in both the 2D crystal and in the decorated actin filaments.

Characterization of two related Drosophila gamma-tubulin complexes that differ in their ability to nucleate microtubules.

gamma-tubulin exists in two related complexes in Drosophila embryo extracts (Moritz, M., Y. Zheng, B.M. Alberts, and K. Oegema. 1998. J. Cell Biol. 142:1- 12). Here, we report the purification and characterization of both complexes that we name gamma-tubulin small complex (gammaTuSC; approximately 280,000 D) and Drosophila gammaTuRC ( approximately 2,200,000 D). In addition to gamma-tubulin, the gammaTuSC contains Dgrip84 and Dgrip91, two proteins homologous to the Spc97/98p protein family. The gammaTuSC is a structural subunit of the gammaTuRC, a larger complex containing about six additional polypeptides. Like the gammaTuRC isolated from Xenopus egg extracts (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578-583), the Drosophila gammaTuRC can nucleate microtubules in vitro and has an open ring structure with a diameter of 25 nm. Cryo-electron microscopy reveals a modular structure with approximately 13 radially arranged structural repeats. The gammaTuSC also nucleates microtubules, but much less efficiently than the gammaTuRC, suggesting that assembly into a larger complex enhances nucleating activity. Analysis of the nucleotide content of the gammaTuSC reveals that gamma-tubulin binds preferentially to GDP over GTP, rendering gamma-tubulin an unusual member of the tubulin superfamily.

Myosin-I nomenclature.

We suggest that the vertebrate myosin-I field adopt a common nomenclature system based on the names adopted by the Human Genome Organization (HUGO). At present, the myosin-I nomenclature is very confusing; not only are several systems in use, but several different genes have been given the same name. Despite their faults, we believe that the names adopted by the HUGO nomenclature group for genome annotation are the best compromise, and we recommend universal adoption.

The way things move: looking under the hood of molecular motor proteins.

The microtubule-based kinesin motors and actin-based myosin motors generate motions associated with intracellular trafficking, cell division, and muscle contraction. Early studies suggested that these molecular motors work by very different mechanisms. Recently, however, it has become clear that kinesin and myosin share a common core structure and convert energy from adenosine triphosphate into protein motion using a similar conformational change strategy. Many different types of mechanical amplifiers have evolved that operate in conjunction with the conserved core. This modular design has given rise to a remarkable diversity of kinesin and myosin motors whose motile properties are optimized for performing distinct biological functions.

Structure of the actin-myosin complex and its implications for muscle contraction.

Muscle contraction consists of a cyclical interaction between myosin and actin driven by the concomitant hydrolysis of adenosine triphosphate (ATP). A model for the rigor complex of F actin and the myosin head was obtained by combining the molecular structures of the individual proteins with the low-resolution electron density maps of the complex derived by cryo-electron microscopy and image analysis. The spatial relation between the ATP binding pocket on myosin and the major contact area on actin suggests a working hypothesis for the crossbridge cycle that is consistent with previous independent structural and biochemical studies.

Motor domains of kinesin and ncd interact with microtubule protofilaments with the same binding geometry.

Kinesin and ncd (non-claret disjunctional) are microtubule associated motor proteins which share several structural features: both motors are dimers; each monomer is composed of a stalk region, a cargo binding domain and a motor domain; the motor domains have approximately 41% sequence identity. Despite these similarities the two motors have strikingly different movement properties: kinesin is a plus-end directed molecular motor, while ncd is minus-end directed. Here we compare the structure and the microtubule-binding properties of these oppositely directed molecular motors. We determined the three-dimensional structure of tubulin sheets decorated with the motor domains of either kinesin or ncd to a resolution of < 20 A by negative stain electron microscopy and tilt series reconstruction. Comparisons with a control structure of tubulin alone revealed that in both cases the motor domain binds to the outer crest of a single protofilament making contacts with both alpha and beta tubulin. Despite their opposite directionality, the geometry of attachment of the motor domain to the protofilament in the presence of AMP-PNP is very similar for both motors. These data rule out models for directionality which have the motors binding in an opposite orientation to the microtubules. Binding of the ncd as well as the kinesin motor domain appears to induce conformational changes in tubulin. This observation suggests an active role of tubulin in motor movement and/or in the determination of directionality.

Three-dimensional structure of Brush Border Myosin-I at approximately 20 A resolution by electron microscopy and image analysis.

Brush Border Myosin-I (BBMI) is a single-headed, unconventional myosin found in the microvilli of intestinal epithelial cells where it forms lateral bridges between the core bundle of actin filaments and the plasma membrane of the microvillus. A three-dimensional (3D) reconstruction of BBMI was made from images of negatively stained, two-dimensional (2D) crystals grown on lipid monolayers formed from mixtures of phosphatidylserine and phosphatidylcholine. The resolution of the 3D map extends to approximately 20 A and allows identification of all of the major structural domains of BBMI. The BBMI molecule is composed of three domains: a globular motor domain, a light-chain-binding domain and a lipid-binding domain. In our map, the putative motor domain is connected to an extended density, which we believe to be the light-chain-binding domain. This long, narrow region has three distinct bends, which may delineate the bound calmodulin light chains. Following the last calmodulin there is density which extends for a short distance across the lipid surface and is presumably the carboxy-terminal lipid-binding domain.

Polarity of 2-D and 3-D maps of tubulin sheets and motor-decorated sheets.

As there has been considerable disagreement about the polarity of two-dimensional and three-dimensional maps of tubulin and tubulin-motor complexes, we have re-investigated this issue by analyzing images of tubulin sheets and motor-decorated sheets found at the distal ends of microtubules nucleated from centrosomes and sperm tails. Our results are unambiguous and define the relationship of the structural features in the maps to the microtubule plus and minus ends. Possible reasons for previous mis-assignments of the polarity are discussed briefly.

Conformational changes due to calcium-induced calmodulin dissociation in brush border myosin I-decorated F-actin revealed by cryoelectron microscopy and image analysis.

Brush border myosin I (BBMI) is a single-headed molecular motor. Its catalytic domain exhibits extensive sequence homology to the catalytic domain of myosin II, while its tail lacks the coiled-coil nature of myosin II. The BBMI tail domain contains at least three IQ motifs and binds calmodulin. Addition of calcium removes one of these calmodulin light chains, with effects on ATPase activity and motility in in vitro assays. Using the techniques of cryoelectron microscopy and helical image analysis we have calculated three-dimensional (3D) maps of BBMI-decorated actin filaments prepared in the presence and absence of calcium. The 3D maps describe a BBMI catalytic domain that is strikingly similar to the catalytic domain of myosin II subfragment 1 (S1), with the exception of a short amino-terminal region of the heavy chain, which is absent from BBMI. The tail domains of BBMI and S1 are highly divergent in structure, continuing on from their respective motor domains with very different geometries. Addition of calcium to BBMI, and the concomitant loss of a calmodulin light chain, results in an extensive reorganization of mass in the tail domain.

Fine tuning a molecular motor: the location of alternative domains in the Drosophila myosin head.

Myosin isoform sequence variation is likely critical for generating differences in contraction velocity and force production exhibited by the various skeletal muscles in an animal. To examine how myosin heavy chain (MHC) isoform diversity could affect physiological function, we studied the locations of structural differences in the motor domains of muscle MHCs from Drosophila melanogaster. Drosophila has only one muscle Mhc gene. Isoform variation is achieved by alternative splicing of a limited number of exons, clearly delineating the domains of MHC that are critical for muscle-specific functions. There are four alternative regions that contribute to the motor domain of Drosophila myosin. We used the X-ray structure of chicken skeletal S1 as a framework to examine the locations of these four regions. One lies near the ATP-binding pocket in a position where amino acid changes might be expected to modulate entry or exit of the nucleotide. Interestingly, the other three are clustered at the distal end of the molecule, surrounding the reactive cysteine SH1 and the pivot point about which the light chain-containing region swings. These observations underscore the importance of this region, distant from the site of ATP entry and the actin binding interface, as a part of the molecule where modulation of function can be achieved.

Three-dimensional structure of ncd-decorated microtubules obtained by a back-projection method.

We have used cryo-electron microscopy and image analysis to obtain the three-dimensional (3D) structure of 11, 12, 14 and 15 protofilament microtubules decorated with the motor domain of ncd. To obtain the 3D maps, we developed a back-projection method that does not require a helical arrangement of the tubulin heterodimers. This method allows the calculation of 3D maps even when lattice discontinuities (seams) are present. The maps show that the microtubules we studied conform to a B-type lattice with one or more seams. In the presence of 5'-adenylim-idodiphosphate (AMP-PNP), the motor domain of ncd binds to the microtubule protofilament crest interacting with only one protofilament. Viewing the structures along the microtubule axis shows that the ncd motor domain and the tubulin are titled in opposite directions. We determined that a clockwise tilt of the tubulin subunits corresponds to a view from the minus end towards the plus end of the microtubule.

Motor protein decoration of microtubules grown in high salt conditions reveals the presence of mixed lattices.

We have used back-projection methods to obtain three-dimensional maps of motor-protein decorated nine and ten protofilament microtubules polymerized in the presence of high salt and preserved in vitreous ice. The resulting three-dimensional maps show that the vast majority of these microtubules have multiple seams, rather than being helical as would be expected according to the lattice accommodation model. These results indicate that microtubules should be analyzed by back-projection before using helical reconstruction approaches, and that nine and ten protofilament microtubules polymerized in high salt conditions are not suitable for helical analysis.

Self-organizing neural networks bridge the biomolecular resolution gap.

Topology-representing neural networks are employed to generate pseudo-atomic structures of large-scale protein assemblies by combining high-resolution data with volumetric data at lower resolution. As an application example, actin monomers and structural subdomains are located in a three-dimensional (3D) image reconstruction from electron micrographs. To test the reliability of the method, the resolution of the atomic model of an actin polymer is lowered to a level typically encountered in electron microscopic reconstructions. The atomic model is restored with a precision nine times the nominal resolution of the corresponding low-resolution density. The presented self-organizing computing method may be used as an information-processing tool for the synthesis of structural data from a variety of biophysical sources.

Family history as a predictor of blood pressure in a longitudinal study of Australian children.

BACKGROUND: Sex both of parent and of child might influence associations between parental hypertension and blood pressure in offspring. OBJECTIVE: To examine these associations. DESIGN: A cohort of Australians was surveyed 3-yearly from age 9 to 18 years. SETTING: A community-based sample. PARTICIPANTS: When they were aged 18 years, 630 of 1565 participants who had been selected randomly at the age of 9 years were re-surveyed. MAIN OUTCOME MEASURES: Systolic and diastolic blood pressures. RESULTS: Paternal hypertension was reported by 18% of men and 15% of women and maternal hypertension by 15% of men and 14% of women. By the time they were aged 9 years, systolic blood pressure was significantly higher in sons [117.8 mmHg, 95% confidence interval (CI) 116A-119.2 versus 114.7 mmHg, CI 113.4-116.0] and daughters (118.2 mmHg, CI 116.9-119.5 versus 114.9 mmHg, CI 112.8-117.0) of hypertensive fathers than it was in sons and daughters of normotensive fathers. When they were aged 18 years, paternal hypertension predicted blood pressures in men and women independently of their weight at birth, fitness, alcohol consumption and weight for height for age. Systolic blood pressures increased more rapidly (by 0.6 mmHg/year) in men with hypertensive fathers. CONCLUSIONS: Systolic blood pressure in young adults differs in relation to parental hypertension according to the sex of the affected parent and the sex of the offspring. This could reflect unmeasured environmental variables or the action of sex-related genetic or intrauterine factors.