Hyokeun Park

Myosin VI Steps via a Hand-over-Hand Mechanism with Its Lever Arm Undergoing Fluctuations when Attached to Actin

Myosin VI is a reverse direction myosin motor that, as a dimer, moves processively on actin with an average center-of-mass movement of ∼30 nm for each step. We labeled myosin VI with a single fluorophore on either its motor domain or on the distal of two calmodulins (CaMs) located on its putative lever arm. Using a technique called FIONA (fluorescence imaging with one nanometer accuracy), step size was observed with a standard deviation of <1.5 nm, with 0.5-s temporal resolution, and observation times of minutes. Irrespective of probe position, the average step size of a labeled head was ∼60 nm, strongly supporting a hand-over-hand model of motility and ruling out models in which the unique myosin VI insert comes apart. However, the CaM probe displayed large spatial fluctuations (presence of ATP but not ADP or no nucleotide) around the mean position, whereas the motor domain probe did not. This supports a model of myosin VI motility in which the lever arm is either mechanically uncoupled from the motor domain or is undergoing reversible isomerization for part of its motile cycle on actin.

Influence of Synaptic Vesicle Position on Release Probability and Exocytotic Fusion Mode

A Synaptic Vesicle in Time Synaptic vesicles move extensively within presynaptic nerve terminals, and their positional features are thought to have an impact on the likelihood and mode of vesicle fusion and transmitter release. Park et al. (p. 1362, published online 16 February) now provide real-time, three-dimensional tracking of individual synaptic vesicles in living nerve terminals. Single synaptic vesicles were identified within hippocampal neurons right up to the moment of exocytosis. Tracking of individual synaptic vesicles reveals that kiss-and-run fusion is concentrated near the center of the synapse. Neurotransmission depends on movements of transmitter-laden synaptic vesicles, but accurate, nanometer-scale monitoring of vesicle dynamics in presynaptic terminals has remained elusive. Here, we report three-dimensional, real-time tracking of quantum dot-loaded single synaptic vesicles with an accuracy of 20 to 30 nanometers, less than a vesicle diameter. Determination of the time, position, and mode of fusion, aided by trypan blue quenching of Qdot fluorescence, revealed that vesicles starting close to their ultimate fusion sites tended to fuse earlier than those positioned farther away. The mode of fusion depended on the prior motion of vesicles, with long-dwelling vesicles preferring kiss-and-run rather than full-collapse fusion. Kiss-and-run fusion events were concentrated near the center of the synapse, whereas full-collapse fusion events were broadly spread.

The unique insert at the end of the myosin VI motor is the sole determinant of directionality

Myosin VI moves toward the pointed (minus) end of actin filaments, the reverse direction of other myosin classes. The myosin VI structure demonstrates that a unique insert at the end of the motor repositions its lever arm and is at least in part responsible for the reversal of directionality. However, it has been proposed that there must be additional modifications within the motor that contribute to its large step size and to the reversal of directionality. To ascertain the inherent directionality of the motor core, we attached the myosin V lever arm to myosin VI, with and without the unique insert. If the insert was maintained, the motor moved toward the minus end of actin filaments, but if removed, movement was redirected toward the plus end. Single-molecule studies revealed that further adaptations within the motor increase the magnitude and variability of the plus-end directed converter movements, and unexpectedly provide the source of the highly variable myosin VI step size. Thus, the unique insert is necessary and sufficient to reverse an inherently plus-end directed myosin.

How myosin VI coordinates its heads during processive movement

A processive molecular motor must coordinate the enzymatic state of its two catalytic domains in order to prevent premature detachment from its track. For myosin V, internal strain produced when both heads of are attached to an actin track prevents completion of the lever arm swing of the lead head and blocks ADP release. However, this mechanism cannot work for myosin VI, since its lever arm positions are reversed. Here, we demonstrate that myosin VI gating is achieved instead by blocking ATP binding to the lead head once it has released its ADP. The structural basis for this unique gating mechanism involves an insert near the nucleotide binding pocket that is found only in class VI myosin. Reverse strain greatly favors binding of ADP to the lead head, which makes it possible for myosin VI to function as a processive transporter as well as an actin‐based anchor. While this mechanism is unlike that of any other myosin superfamily member, it bears remarkable similarities to that of another processive motor from a different superfamily—kinesin I.

Single-molecule fluorescence to study molecular motors

Molecular motors, which use energy from ATP hydrolysis to take nanometer-scale steps with run-lengths on the order of micrometers, have important roles in areas such as transport and mitosis in living organisms. New techniques have recently been developed to measure these small movements at the single-molecule level. In particular, fluorescence imaging has contributed to the accurate measurement of this tiny movement. We introduce three single-molecule fluorescence imaging techniques which can find the position of a fluorophore with accuracy in the range of a few nanometers. These techniques are named after Hollywood animation characters: Fluorescence Imaging with One Nanometer Accuracy (FIONA), Single-molecule High-REsolution Colocalization (SHREC), and Defocused Orientation and Position Imaging (DOPI). We explain new understanding of molecular motors obtained from measurements using these techniques.

The myosin X motor is optimized for movement on actin bundles.

Myosin X has features not found in other myosins. Its structure must underlie its unique ability to generate filopodia, which are essential for neuritogenesis, wound healing, cancer metastasis and some pathogenic infections. By determining high-resolution structures of key components of this motor, and characterizing the in vitro behaviour of the native dimer, we identify the features that explain the myosin X dimer behaviour. Single-molecule studies demonstrate that a native myosin X dimer moves on actin bundles with higher velocities and takes larger steps than on single actin filaments. The largest steps on actin bundles are larger than previously reported for artificially dimerized myosin X constructs or any other myosin. Our model and kinetic data explain why these large steps and high velocities can only occur on bundled filaments. Thus, myosin X functions as an antiparallel dimer in cells with a unique geometry optimized for movement on actin bundles.

Photoelectron spectroscopy of s-triazine anion clusters: Polarization-induced electron binding in aza-aromatic molecule

Photoelectron spectroscopy was carried out for the mass-selected cluster anions of s-triazine molecule, Tzn− (n=1–6). The mass spectrum and vibrationally resolved photoelectron spectrum of Tz− showed that unlike pyridine and pyrazine, Tz binds an electron and thus becomes the first molecule in the azabenzene series with a positive electron affinity (0.03 eV). This indicates that the local charge polarization in the aromatic ring by the three nitrogen atoms is large enough to facilitate electron binding to a homologue of benzene. A Jahn–Teller distortion was proposed to explain the vibrational progressions of the photoelectron spectrum of Tz−. A series of Ar-solvated clusters of Tz−, Tz−⋅Arm (m=1–7), have been also studied. Their photoelectron spectra showed a drop in the incremental electron binding energy when going from m=4 to 5, indicating the closure of a solvation shell with four Ar atoms. In the mass abundance spectrum of Tzn−, a distinctly high intensity for Tz2− indicated its exceptional stability...

Fluorescence Imaging with One-Nanometer Accuracy (FIONA).

INTRODUCTIONFluorescence imaging with one-nanometer accuracy (FIONA) is a technique for localizing a single dye, or a single group of dyes, to within ~1-nm accuracy. This high degree of precision is achieved using total internal reflection fluorescence microscopy, deoxygenation agents, and a high quantum yield, low-noise detector. There are several variations of FIONA, including some capable of better than 10-nm resolution. One such variant is single-molecule high-resolution imaging with photobleaching (SHRIMP), which requires only one type of dye, e.g., two green fluorescent proteins (GFPs), or two rhodamines. However, SHRIMP can only achieve high resolution on static systems. Single-molecule high-resolution colocalization (SHREC), on the other hand, is a FIONA variant that is capable of high resolution with dynamic systems. Defocused orientation and positional imaging (DOPI) enables the three-dimensional orientation to be determined, and either by itself or in combination with FIONA can localize the dye-bound molecules to within a few nanometers. Finally, bright-field imaging with one-nanometer accuracy (bFIONA) achieves the temporal and spectral localization of FIONA but with bright-field microscopy, thus avoiding the use of fluorescence.

Constructing Sample Chambers for Fluorescence Imaging with One-Nanometer Accuracy (FIONA).

INTRODUCTIONFIONA, short for fluorescence imaging with one-nanometer accuracy, is a simple method for achieving localization of single (or single groups of) fluorophores with nanometer accuracy in the xy plane. This protocol provides details on constructing an inexpensive sample chamber for use in single-molecule FIONA experiments and two methods for cleaning slides and coverslips.

Real-time imaging of single synaptic vesicles in live neurons

Recent advances in fluorescence microscopy have provided researchers with powerful new tools to visualize cellular processes occurring in real time, giving researchers an unprecedented opportunity to address many biological questions that were previously inaccessible. With respect to neurobiology, these real-time imaging techniques have deepened our understanding of molecular and cellular processes, including the movement and dynamics of single proteins and organelles in living cells. In this review, we summarize recent advances in the field of real-time imaging of single synaptic vesicles in live neurons.