Summary: © 2000 Macmillan Magazines Ltd
NATURE CELL BIOLOGY | VOL 2 | JUNE 2000 | www.nature.com/ncb 365
Structure of the -tubulin ring complex:
a template for microtubule nucleation
Michelle Moritz*, Michael B. Braunfeld*, Vincent Guénebaut*, John Heuser and David A. Agard*
*Department of Biochemistry and Biophysics, Howard Hughes Medical Institute, University of California,
513 Parnassus Avenue, San Francisco, California 94143, USA
Department of Cell Biology, Washington University School of Medicine, 660 South Euclid Avenue, St Louis, Missouri 63110, USA
The -tubulin ring complex (TuRC) is a protein complex of relative molecular mass ~2.2×××× 106
microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the
structure of isolated Drosophila TuRCs and the ends of microtubules nucleated by TuRCs and by centrosomes. We
show that the TuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap.
A similar capped ring is also visible at one end of microtubules grown from isolated TuRCs and from centrosomes.
Antibodies against -tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a
template-mediated mechanism for microtubule nucleation by the TuRC.
he microtubule cytoskeleton is of fundamental importance to
animal cells in interphase for vesicle transport and cell polar-