Berkeley Lab Researchers Achieve Record 3.5 Angstroms
Resolution and Visualize Action of a Major Microtubule-Regulating Protein
(August 5, 2015) Microtubules,
hollow fibers of tubulin protein only a few nanometers in diameter, form the
cytoskeletons of living cells and play a crucial role in cell division
(mitosis) through their ability to undergo rapid growth and shrinkage, a
property called “dynamic instability.” Through a combination of high-resolution
cryo-electron microscopy (cryo-EM) and a unique methodology for image analysis,
a team of researchers with Berkeley Lab and the University of California (UC)
Berkeley has produced an atomic view of microtubules that enabled them to
identify the crucial role played by a family of end-binding (EB) proteins in
regulating microtubule dynamic instability.
During mitosis, microtubules disassemble and reform into
spindles that are used by the dividing cell to move chromosomes. For chromosome
migration to occur, the microtubules attached to them must disassemble,
carrying the chromosomes in the process. The dynamic instability that makes it
possible for microtubules to transition from a rigid polymerized or “assembled”
nucleotide state to a flexible depolymerized or “disassembled” nucleotide state
is driven by guanosine triphosphate (GTP) hydrolysis in the microtubule
lattice.
“Our study shows how EB proteins can either facilitate
microtubule assembly by binding to sub-units of the microtubule, essentially
holding them together, or else cause a microtubule to disassemble by promoting
GTP hydrolysis that destabilizes the microtubule lattice,” says Eva Nogales, a
biophysicist with Berkeley Lab’s Life Sciences Division who led this research.
Nogales, who is also a professor of biophysics and
structural biology at UC Berkeley and investigator with the Howard Hughes
Medical Institute, is a leading authority on the structure and dynamics of
microtubules. In this latest study, she and her group used cryo-EM, in which
protein samples are flash-frozen at liquid nitrogen temperatures to preserve
their natural structure, to determine microtubule structures in different
nucleotide states with and without EB3. With cryo-EM and their image analysis
methodology, they achieved a resolution of 3.5 Angstroms, a record for
microtubules. For perspective, the diameter of a hydrogen atom is about 1.0
Angstroms.
“We can now study the atomic details of microtubule
polymerization and depolymerization to develop a complete description of
microtubule dynamics,” Nogales says.
Beyond their importance to our understanding of basic cell
biology, microtubules are a major target for anticancer drugs, such as Taxol,
which can prevent the transition from growing to shrinking nucleotide states or
vice versa.