Dissection of the angle of single fluorophore attached to the nucleotide in corkscrewing microtubules

Membrane-bound myosin IC drives the chiral rotation of the gliding actin filament around its longitudinal axis

Myosin IC, a single-headed member of the myosin I family, specifically interacts with anionic phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) in the cell membrane via the pleckstrin homology domain located in the myosin IC tail. Myosin IC is widely expressed and physically links the cell membrane to the actin cytoskeleton; it plays various roles in membrane-associated physiological processes, including establishing cellular chirality, lipid transportation, and mechanosensing. In this study, we evaluated the motility of full-length myosin IC of Drosophila melanogaster via the three-dimensional tracking of quantum dots bound to actin filaments that glided over a membrane-bound myosin IC-coated surface. The results revealed that myosin IC drove a left-handed rotational motion in the gliding actin filament around its longitudinal axis, indicating that myosin IC generated a torque perpendicular to the gliding direction of the actin filament. The quantification of the rotational motion of actin filaments on fluid membranes containing different PI(4,5)P2 concentrations revealed that the rotational pitch was longer at lower PI(4,5)P2 concentrations. These results suggest that the torque generated by membrane-bound myosin IC molecules can be modulated based on the phospholipid composition of the cell membrane.

Torque generating properties of Tetrahymena ciliary three-headed outer-arm dynein

Eukaryotic cilia/flagella are cellular bio-machines that drive the movement of microorganisms. Molecular motor axonemal dyneins in the axoneme, which consist of an 9 + 2 arrangement of microtubules, play an essential role in ciliary beating. Some axonemal dyneins have been shown to generate torque coupled with the longitudinal motility of microtubules across an array of dyneins fixed to the coverglass surface, resulting in a corkscrew-like translocation of microtubules. In this study, we performed three-dimensional tracking of a microbead coated with axonemal outer-arm dyneins on a freely suspended microtubule. We found that microbeads coated with multiple outer-arm dyneins exhibited continuous right-handed helical trajectories around the microtubule. This unidirectional helical motion differs from that of other types of cytoplasmic dyneins, which exhibit bidirectional helical motility. We also found that, in an in vitro microtubule gliding assay, gliding microtubules driven by outer-arm dyneins tend to turn to the left, causing a curved path, suggesting that the outer-arm dynein itself is able to rotate on its own axis. Two types of torque generated by the axonemal dyneins, corresponding to the forces used to rotate the microtubule unidirectionally with respect to the long and short axes, may regulate ciliary beating with complex waveforms.

Diffracted X-ray Tracking Method for Measuring Intramolecular Dynamics of Membrane Proteins

Membrane proteins change their conformations in response to chemical and physical stimuli and transmit extracellular signals inside cells. Several approaches have been developed for solving the structures of proteins. However, few techniques can monitor real-time protein dynamics. The diffracted X-ray tracking method (DXT) is an X-ray-based single-molecule technique that monitors the internal motion of biomolecules in an aqueous solution. DXT analyzes trajectories of Laue spots generated from the attached gold nanocrystals with a two-dimensional axis by tilting (θ) and twisting (χ). Furthermore, high-intensity X-rays from synchrotron radiation facilities enable measurements with microsecond-timescale and picometer-spatial-scale intramolecular information. The technique has been applied to various membrane proteins due to its superior spatiotemporal resolution. In this review, we introduce basic principles of DXT, reviewing its recent and extended applications to membrane proteins and living cells, respectively.

Three-dimensional tracking of the ciliate Tetrahymena reveals the mechanism of ciliary stroke-driven helical swimming

Helical swimming in free-space is a common behavior among microorganisms, such as ciliates that are covered with thousands hair-like motile cilia, and is thought to be essential for cells to orient directly to an external stimulus. However, a direct quantification of their three-dimensional (3D) helical trajectories has not been reported, in part due to difficulty in tracking 3D swimming behavior of ciliates, especially Tetrahymena with a small, transparent cell body. Here, we conducted 3D tracking of fluorescent microbeads within a cell to directly visualize the helical swimming exhibited by Tetrahymena. Our technique showed that Tetrahymena swims along a right-handed helical path with right-handed rolling of its cell body. Using the Tetrahymena cell permeabilized with detergent treatment, we also observed that influx of Ca²⁺ into cilia changed the 3D-trajectory patterns of Tetrahymena swimming, indicating that the beating pattern of cilia is the determining factor in its swimming behavior.

Angle change of the A-domain in a single SERCA1a molecule detected by defocused orientation imaging

The sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) transports Ca²⁺ ions across the membrane coupled with ATP hydrolysis. Crystal structures of ligand-stabilized molecules indicate that the movement of actuator (A) domain plays a crucial role in Ca²⁺ translocation. However, the actual structural movements during the transitions between intermediates remain uncertain, in particular, the structure of E2PCa₂ has not been solved. Here, the angle of the A-domain was measured by defocused orientation imaging using isotropic total internal reflection fluorescence microscopy. A single SERCA1a molecule, labeled with fluorophore ReAsH on the A-domain in fixed orientation, was embedded in a nanodisc, and stabilized on Ni–NTA glass. Activation with ATP and Ca²⁺ caused angle changes of the fluorophore and therefore the A-domain, motions lost by inhibitor, thapsigargin. Our high-speed set-up captured the motion during EP isomerization, and suggests that the A-domain rapidly rotates back and forth from an E1PCa₂ position to a position close to the E2P state. This is the first report of the detection in the movement of the A-domain as an angle change. Our method provides a powerful tool to investigate the conformational change of a membrane protein in real-time.

Characterization of the motility of monomeric kinesin-5/Cin8

Cin8, the Saccharomyces cerevisiae kinesin-5, has an essential role in mitosis. In in vitro motility assays, tetrameric and dimeric Cin8 constructs showed bidirectional motility in response to ionic strength or Cin8 motor density. However, whether property-switching directionality is present in a monomeric form of Cin8 is unknown. Here we engineered monomeric Cin8 constructs with and without the Cin8-specific ∼99 residues in the loop 8 domain and examined the directionality of these constructs using an in vitro polarity-marked microtubule gliding assay within the range of the motor density or ionic strength. We found that both monomeric constructs showed only plus end-directed activity over the ranges measured, which suggested that minus end-directed motility driven by Cin8 is necessary for at least dimeric forms. Using an in vitro microtubule corkscrewing assay, we also found that monomeric Cin8 corkscrewed microtubules around their longitudinal axes with a constant left-handed pitch. Overall, our results imply that plus-end-directed and left-handed motor activity comprise the intrinsic properties of the Cin8 motor domain as with other monomeric N-kinesins.

Insights into the mechanism of ATP-driven rotary motors from direct torque measurement

Motor proteins are molecular machines that convert chemical energy into mechanical work. In addition to existing studies performed on the linear motors found in eukaryotic cells, researchers in biophysics have also focused on rotary motors such as F₁-ATPase. Detailed studies on the rotary F₁-ATPase motor have correlated all chemical states to specific mechanical events at the single-molecule level. Recent studies showed that there exists another ATP-driven protein motor in life: the rotary machinery that rotates archaeal flagella (archaella). Rotation speed, stepwise movement, and variable directionality of the motor of Halobacterium salinarum were described in previous studies. Here we review recent experimental work discerning the molecular mechanism underlying how the archaellar motor protein FlaI drives rotation by generation of motor torque. In combination, those studies found that rotation slows as the viscous drag of markers increases, but torque remains constant at 160 pN·nm independent of rotation speed. Unexpectedly, the estimated work done in a single rotation is twice the expected energy that would come from hydrolysis of six ATP molecules in the FlaI hexamer. To reconcile the apparent contradiction, a new and general model for the mechanism of ATP-driven rotary motors is discussed.