3D Protein Imaging Yields Molecular Insights

This research was a collaboration among biochemists and structural biologists and was published in the September 2005 issue of the journal Molecular Cell. Scientists from the Burnham Institute for Medical Research (La Jolla, CA, USA) and the University of Vermont (Burlington, VT, USA), led by Dorit Hanein, Ph.D., were the first to show the 3D representation of myosin V "walking” along an actin filament, a major protein involved in motility and muscle contraction. Utilizing electron-cryo microscopy to take 3D snapshots of myosin V and actin interacting, scientists were able to see myosin V moving along the actin substrate in a "natural state.” Earlier 2D models have been based on staining or other treatment of the myosin that might change the compound's natural mechanism of action.

Myosins are a large family of motor proteins that interact with actin filaments for motor movement and muscle contraction. Myosin V is the major worker of the myosin protein family. It exists to transport a group of proteins needed in a specific place at a specific time. Fueled by hydrolysis (the process of converting the molecule adenosine troposphere [ATP] into energy), myosin V moves in one direction using actin as a pathway to deliver its cargo of cell vesicles and organelles. Myosin V is also involved in transporting proteins that signal and communicate with other cells.

Myosin V has a two-chained "tail” that deviates to form two "heads” that attach to certain grooves on actin and move hand over hand along the track, similar to the way a child moves along the monkey bars in a playground. Myosin V is different from the other myosin family proteins in that it is able to maintain this processive motion, enduring many hydrolysis cycles. The other myosins hold on tightly to actin and release after one hydrolysis cycle.

"This study required a different way of thinking about image analysis. This is the first time we were able to structurally visualize the weak binding states of actin and myosin, not interpolated from crystal structures, and not interpolated from biophysical methods,” stated Dr. Hanein. "We were able to see structural changes in the myosin lever arm as well as in the actin interface as it propagates through the hydrolysis cycle.”

Structural data from earlier research provided insights into parts of this process, but until the present collaboration, visualizing myosin V in its weakly bound state to actin had not been possible. The investigators captured images of myosin V at several points during a hydrolysis cycle. Their use of electron cryo-microscopy made it possible to see flexible structural domains, which attach the myosin V, helping to keep the protein on its actin track through the weak binding phase of the processive movement.

The detailed molecular characteristics of how myosin interacts through the hydrolysis cycle with actin provides a novel new research tool with which scientists can design new sets of studies to additionally refine the myosin-actin binding area and to correlate it with loss or gain of function. The exact characterization of this myosin-actin interface is crucial, evident by the way a single amino acid alteration in myosin leads to familial hypertrophic cardiomyopathy (FHC), an undetectable disorder resulting in death by sudden cardiac arrest in otherwise healthy young adults.



Related Links:
Burnham Institute for Medical Research
University of Vermont