Scientists reveal structural secrets of nature’s little locomotive

A team led by scientists at The Scripps Research Institute (TSRI) has determined the basic structural organization of a molecular motor that hauls cargoes and performs other critical functions within cells.    

Biologists have long wanted to know how this molecular motor—called the “dynein-dynactin complex"—works. But the complex’s large size, myriad subunits and high flexibility have until now restricted structural studies to small pieces of the whole.

In the new research, however, TSRI biologist Gabriel C. Lander and his laboratory, in collaboration with Trina A. Schroer and her group at Johns Hopkins University, created a picture of the whole dynein-dynactin structure.

"This work gives us critical insights into the regulation of the dynein motor and establishes a structural framework for understanding why defects in this system have been linked to diseases such as Huntington’s, Parkinson’s, and Alzheimer’s,” said Lander.

The findings are reported in a Nature Structural & Molecular Biology advance online publication on March 9, 2015.

Credit: Lander lab, The Scripps Research Institute.      

How herpesvirus invades nervous system

Northwestern Medicine scientists have identified a component of the herpesvirus that “hijacks” machinery inside human cells, allowing the virus to rapidly and successfully invade the nervous system upon initial exposure.

Led by Gregory Smith, associate professor in immunology and microbiology at Northwestern University Feinberg School of Medicine, researchers found that viral protein 1-2, or VP1/2, allows the herpesvirus to interact with cellular motors, known as dynein. Once the protein has overtaken this motor, the virus can speed along intercellular highways, or microtubules, to move unobstructed from the tips of nerves in skin to the nuclei of neurons within the nervous system.

This is the first time researchers have shown a viral protein directly engaging and subverting the cellular motor; most other viruses passively hitch a ride into the nervous system.

“This protein not only grabs the wheel, it steps on the gas,” says Smith. “Overtaking the cellular motor to invade the nervous system is a complicated accomplishment that most viruses are incapable of achieving. Yet the herpesvirus uses one protein, no others required, to transport its genetic information over long distances without stopping.”

Herpesvirus is widespread in humans and affects more than 90 percent of adults in the United States. It is associated with several types of recurring diseases, including cold sores, genital herpes, chicken pox, and shingles. The virus can live dormant in humans for a lifetime, and most infected people do not know they are disease carriers. The virus can occasionally turn deadly, resulting in encephalitis in some.

Until now, scientists knew that herpesviruses travel quickly to reach neurons located deep inside the body, but the mechanism by which they advance remained a mystery.

Smith’s team conducted a variety of experiments with VP1/2 to demonstrate its important role in transporting the virus, including artificial activation and genetic mutation of the protein. The team studied the herpesvirus in animals, and also in human and animal cells in culture under high-resolution microscopy. In one experiment, scientists mutated the virus with a slower form of the protein dyed red, and raced it against a healthy virus dyed green. They observed that the healthy virus outran the mutated version down nerves to the neuron body to insert DNA and establish infection.

“Remarkably, this viral protein can be artificially activated, and in these conditions it zips around within cells in the absence of any virus. It is striking to watch,” Smith says.

He says that understanding how the viruses move within people, especially from the skin to the nervous system, can help better prevent the virus from spreading.

Additionally, Smith says, “By learning how the virus infects our nervous system, we can mimic this process to treat unrelated neurologic diseases. Even now, laboratories are working on how to use herpesviruses to deliver genes into the nervous system and kill cancer cells.”

Smith’s team will next work to better understand how the protein functions. He notes that many researchers use viruses to learn how neurons are connected to the brain.

“Some of our mutants will advance brain mapping studies by resolving these connections more clearly than was previously possible,” he says.

Revealing the Inner Workings of a Molecular Motor

Read the full article Revealing the Inner Workings of a Molecular Motor at NeuroscienceNews.com.

In research published in the Journal of Cell Biology, scientists from the RIKEN Brain Science Institute in Japan have made important steps toward understanding how dynein—a “molecular motor”—walks along tube-like structures in the cell to move cellular cargo from the outer structures toward the cell body of neurons.

The research in in Journal of Cell Biology. (full access paywall)

Research: “A flipped ion pair at the dynein–microtubule interface is critical for dynein motility and ATPase activation” by Seiichi Uchimura, Takashi Fujii, Hiroko Takazaki, Rie Ayukawa, Yosuke Nishikawa, Itsushi Minoura, You Hachikubo, Genji Kurisu, Kazuo Sutoh, Takahide Kon, Keiichi Namba, and Etsuko Muto in Journal of Cell Biology. doi:10.1083/jcb.201407039 (http://dx.doi.org/10.1083/jcb.201407039)

Image: Schematic representation of the dynein-microtubule complex showing the structural elements likely to be involved in allosteric communication between the microtubule and the ATPase site in dynein. Image adapted from the RIKEN press release.

Intracellular cargo is transported by multiple motor proteins. Because of the force balance of motors with mixed polarities, cargo moves bidirectionally to achieve biological functions. Here, we propose a microtubule gliding assay for a tug-of-war study of kinesin and dynein. A boundary of the two motor groups is created by photolithographically patterning gold to selectively attach kinesin to the glass and dynein to the gold surface using a self-assembled monolayer. The relationship between the ratio of two antagonistic motor numbers and the velocity is derived from a force-velocity relationship for each motor to calculate the detachment force and motor backward velocity. Although the tug-of-war involves >100 motors, values are calculated for a single molecule and reflect the collective dynein and non-collective kinesin functions when they work as a team. This assay would be useful for detailed in vitro analysis of intracellular motility, e.g., mitosis, where a large number of motors with mixed polarities are involved.
—  Ikuta J, et al. 2014

(TitleTug-of-war of microtubule filaments at the boundary of a kinesin- and dynein-patterned surface)

[ Authors ]
Anton Souslov, Paul M. Goldbart
[ Abstract ]
Along a microtubule, certain active motors propel themselves in one direction whereas others propel themselves in the opposite direction. For example, the cargo transporting motor proteins dynein and kinesin propel themselves towards the so-called plus- and minus-ends of the microtubule, respectively, and in so doing are able to pass one another, but not without interacting. We address the emergent collective behavior of systems composed of many motors, some propelling towards the plus-end and others propelling towards the minus-end. To do this, we used an analogy between this strongly interacting, far-from-equilibrium, classical stochastic many-motor system and a certain quantum-mechanical many-body system evolving in imaginary time. We apply well-known methods from quantum many-body theory, including self-consistent mean-field theory and bosonization, to shed light on phenomena exhibited by the many-motor system such as structure formation and the dynamics of collective modes at low-frequencies and long-wavelengths. In particular, via the bosonized description we find analogs of chiral Luttinger liquids, as well as a qualitative transition in the nature of the low-frequency modes—from propagating to purely dissipative—controlled by density and interaction strength.