Long filaments of a protein known as actin run through every cell in the body, serving as a kind of railroad along which another protein called myosin transports vital materials to locations throughout the cell. Until now, scientists believed that the fifteen known variants of myosin all moved in only one direction on the actin filaments, towards the plus end of the polarized filaments. If a cell needed materials carried in two directions, it simply provided parallel actin filaments running in both directions - but the myosin itself could only go forward along the actin.
Now, in a finding likely to surprise many cell biologists, University of Pennsylvania Medical Center scientists have discovered that one of the myosin variants - myosin VI - moves backwards on actin, toward the minus end of the filaments. A report on the new study appears in the September 30 issue of Nature and is featured on the cover.
The observation suggests how, in the relatively few structures in the body in which the actin filaments are known to run in only one direction - notably the inner-ear projections responsible for sensing sound - the cells are able to assemble and maintain themselves. In fact, mice in whom myosin VI is defective are deaf, suggesting the critical importance of the variant motor protein to the so-called hair cells of the inner ear upon which hearing depends. Myosin VI may be similarly crucial in other parts of the body where actin filaments are aligned in only one direction.
"In many ways, the myosin protein moving along an actin filament is like a sure-footed gymnast able to walk only one direction on a balance beam," says H. Lee Sweeney, PhD, chairman of the department of physiology and senior author on the study. "What we've shown here is that, surprisingly, one form of myosin is able to walk backwards on the beam."
Sweeney and his team began their efforts to identify myosin VI's unique capabilities by assuming the existence of a backwards-moving myosin, based on the probable need for such a motor protein in structures containing only single-orientation actin filaments. They then asked what the simplest strategy might be for creating a backwards-moving myosin, deciding on an approach that conserved the basic motor region shared by the myosins but that altered the attachment of the part of the molecule that undergoes large movements, referred to as the lever arm, so that it rotated in the opposite direction.
Then they compared the sequences of the fifteen known myosins to look for significant differences in the region corresponding to the molecule's lever arm. Myosin VI promptly emerged as a candidate for consideration because of a 53-amino-acid insertion in the relevant part of the molecule.
Graduate student Amber L. Wells, BS, lead author on the study, devised a motility assay that differentially labeled the plus and minus ends of actin filaments with a fluorescent dye, allowing the researchers to use a fluorescence microscope to see which direction the myosin molecules moved on the filaments when exposed to an energy source, ATP.
"When we looked, we saw that all the other myosin molecules moved in the direction scientists had thought all myosins must move in," Sweeney says. "Myosin VI, however, moved in the opposite direction." Class VI myosins were first discovered in 1992 in fruit flies. Since then, they have been found in life forms ranging from worms to humans on the scale of organizational complexity.
In addition to Sweeney and Wells, the other Penn-based authors on the study are Li-Qiong Chen, MD, and Daniel Safer, PhD. The other coauthors are Abel W. Lin, BS, Shane M. Cain, BS, and Ronald A. Milligan, PhD, at The Scripps Research Institute, Tama Hasson, PhD, at the University of California, San Diego, and Bridget O. Carragher, PhD, at the University of Illinois at Urbana-Champaign. Funding for the work was provided by the National Institutes of Health.
The University of Pennsylvania Medical Center's sponsored research and training ranks second in the United States based on grant support from the National Institutes of Health, the primary funder of biomedical research and training in the nation -- $201 million in federal fiscal year 1998. In addition, the institution continued to maintain the largest absolute growth in funding for research and training among all 125 medical schools in the country since 1991. News releases from the University of Pennsylvania Medical Center are available to reporters by direct e-mail, fax, or U.S. mail, upon request. They are also posted electronically to the medical center's home page (http://www.med.upenn.edu).
Journal
Nature