3-D Imaging Of Motor Proteins Provides New Insights To Molecular Mechanics Of Cell Motility, Muscle Contraction

The Burnham team, led by Dorit Hanein, Ph.D., was the first toreveal the 3D representation of myosin V "walking" along actinfilament, a key protein involved in motility and muscle contraction.Using electron-cryo microscopy to take 3D snapshots of myosin V andactin interacting, researchers were able to see myosin V moving alongthe actin substrate in a "natural state." Previous 2D models have beenbased on staining or other treatment of the myosin that might alter thecomplex's natural mechanism of action.

Myosins are a large family of motor proteins that interact withactin filaments for motor movement and muscle contraction. Myosin V isthe workhorse of the myosin protein family. It exists to ferry a cargoof proteins needed in a specific place at a specific time. Fueled byhydrolysis -- the process of converting the molecule adenosinetriphosphate (ATP) into energy -- myosin V travels in one directionusing actin as a track to deliver its payload of cell vesicles andorganelles. Myosin V is also involved in transporting proteins thatsignal and communicate with other cells.

Myosin V has a two-chained "tail" that diverges to form two "heads"that bind to specific grooves on actin and walk hand over hand alongthe track, similar to the way a child moves along the monkey bars in aplayground. Myosin V differs from the other myosin family proteins inthat it is able to sustain this processive motion, enduring manyhydrolysis cycles. The other myosins grab on tightly to actin andrelease after one hydrolysis cycle.

"This study required a different way of thinking about imageanalysis. This is the first time we were able to structurally visualizethe weak binding states of actin and myosin, not interpolated fromcrystal structures, and not interpolated from biophysical methods,"said Dr. Hanein. "We were able to see structural changes in the myosinlever arm as well as in the actin interface as it propagates throughthe hydrolysis cycle."

Structural information from previous studies providedinformation about parts of this process, but until the presentcollaboration, visualizing Myosin V in its weakly bound state to actinhad not been possible. The Hanein group captured snapshots of Myosin Vat several points during a hydrolysis cycle. Their use of electroncryo-microscopy made it possible to visualize flexible structuraldomains, which tether the Myosin V, helping to keep the protein on itsactin track through the weak binding phase of the processive movement.

The detailed molecular knowledge of how myosin interactsthrough the hydrolysis cycle with actin provides an exciting newresearch template onto which scientists can design new sets ofexperiments to further refine the myosin-actin binding region and tocorrelate it with loss or gain of function. The precisecharacterization of this myosin-actin interface is critical, evident bythe way a single amino acid change in myosin leads to familialhypertrophic cardiomyopathy (FHC), an undetectable condition resultingin death by sudden cardiac arrest in otherwise healthy young adults.

Contributors to this work include: Niels Volkmann, Ph.D.,assistant professor and first author on this publication, Dorit Hanein,Ph.D., associate professor, Hong-Jun Liu and Larnele Hazelwood from theBurnham Institute for Medical Research; and Kathleen M. Trybus, Ph.D.,Susan Lowey, Ph.D., and Elena B. Kremenstova, Ph.D., from theDepartment of Molecular Physiology and Biophysics at the University ofVermont.

Functional, biochemical assays were conducted by collaboratorsfrom the University of Vermont, directed by Kathleen Trybus, Ph.D.

This research was supported by grants from the National Institutes of Health.

The Burnham Institute for Medical Research, founded in 1976, isan independent not-for-profit biomedical research institution dedicatedto advancing the frontiers of scientific knowledge and providing thefoundation for tomorrow's medical therapies. The Institute is home tothree major research centers: the Cancer Center, the Del E. WebbNeuroscience and Aging Center, and the Infectious and InflammatoryDisease Center. Since 1981, the Institute's Cancer Center has been amember of the National Cancer Institute's prestigious cancer centersprogram. Discoveries by Burnham scientists have contributed to thedevelopment of new drugs for Alzheimer's Disease, heart disease andseveral forms of cancer. Today, the Burnham Institute employs over 700,including more than 550 scientists. The majority of the Institute'sfunding is derived from federal sources, but private philanthropicsupport is essential to continuing bold and innovative research. Foradditional information about the Institute and ways to support theresearch efforts of the Institute, visit: http://www.burnham.org.