Prof. David DeRosier
Professor of Biology
Research Interests
Actin and cell shape and motility:
Actin is best known as the railroad tracks along which the myosin motors pull themselves. Actin in its filamentous form is a helical arrangement of subunits upon which myosin steps as it walks its way along the filament. But actin lies at the heart of other kinds of cellular machines such as the hair cell of the inner ear. Hair cells transduce sound-generated motion into electrical impulses, which is how we hear. Protruding from each hair cell is an organ pipe array of rigid cellular protrusions called stereocilia. Fluid motion, driven by sound, moves the stereocilia, which by regulating ion channels, modulate the electrical potential across the hair cell's membrane. Thus by this path, the stereocilia convert sound to electrical changes.
The core of every stereocilium is an actin bundle. The stereocilium is held erect because its actin bundle sends rootlet into the body of the cell where it is anchored in an actin gel. Fimbrin, an actin-bundling protein, crosslinks the filaments providing integrity and rigidity to the bundle. Because the arrangement of filaments in bundles is not crystalline, a condition which allows study by conventional methods, we divided the structure into subcomplexes to make the problem tractable. We can better study the parts individually and then reassemble them into the bundle. This is the divide and conquer approach, which we have undertaken with our collaborators Prof. Paul Matsudaira (The Whitehead Institute, M.I.T.) and Steve Almo (Albert Einstein College of Medicine).
Fimbrin, a protein containing two actin binding domains, is a member of a superfamily of actin binding proteins known as the calponin homology superfamily. All members of this superfamily appear to possess homologous actin binding domains. The atomic structure of one of the fimbrin actin-binding domains is known from x-ray crystallographic studies. Similarly, the actin subunit is known to atomic resolution. Unfortunately, no one has been able to produce a crystal containing actin and fimbrin for study by x-ray crystallography. To get an atomic model of an actin-fimbrin interaction, we used electron microscopy and digital image processing to produce a three dimensional map of a complex of actin and the known actin binding domain of fimbrin. With a resolution of about 2 nanometers, the map allowed us to visualize single molecules and even the domains that comprise them. We then docked the atomic models of the subunits into our actin-fimbrin map to generate an atomic model of the complex.
Our next step was to find a suitable subassembly of the bundle so that we might visualize the whole fimbrin molecule in its role as a crosslinking protein. The actin-fimbrin raft is a two-dimensional array of actin filaments crosslinked by fimbrin molecules. The idea is that a raft corresponds to one row of filaments in a three dimensional bundle. We make the rafts by aligning filaments on a lipid sheet and crosslinking them with fimbrin. We are beginning the analysis of rafts by electron microscopy and image analysis.
Click here for files associated with the paper "An atomic model of fimbrin binding to F-actin and it's implications for filament crosslinking and regulation" by D. Hanein, N. Volkmann, S. Goldsmith, A-M. Michon, W. Lehman, R. Craig, D. DeRosier, S. Almo, and P. Matsudaira in Nature Structural Biology 5:787-792 (1998).
Bacterial propulsion:
The bacterial flagellum, which propels many types of bacteria, has a long corkscrew-shaped propeller attached to a rotary motor by a drive shaft and short flexible universal joint. The drive shaft passes through a bushing that holds the motor firm in the cell's envelope. The motor, which is powered by the proton gradient across the cell's membrane, can spin at 60,000 rpm. The tiny machine is made of thousands of protein molecules but requires only 40 genes. Using the electron microscope and image analysis, we were able to generate three-dimensional images of the flagellum. In our images we can visualize the arrangements and structures of the component protein parts. The subunits of the propeller, universal joint and drive shaft wind around in a continuous helical filament. The motor's subunits form rings rather than helices. Of the 40 proteins, only 5 appear to be responsible for generating torque and reversing the direction of the motor's rotation. We are identifying and determining the locations of these proteins in our images.
Research Description/Selected Publications: http://www.bio.brandeis.edu/faculty01/derosier.html
Mailing address:
David DeRosier
MS 029 Brandeis University
Waltham, MA 02454-9110
E-mail: derosier@brandeis.edu
Office: Rosenstiel 447
Phone: 781-736-2494 (Office)