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Microtubule-Associated Protein Assembled on Polymerized Microtubules

Larry H. Bernstein, MD, FCAP, Curator

LPBI

Magic-Angle-Spinning NMR Enables First-Ever Determination of Atomic-Resolution Structure of a Microtubule-Associated Protein Assembled on Polymerized Microtubules

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A latticework of tiny tubes called microtubules gives your cells their shape and also acts like a railroad track that essential proteins travel on. But if there is a glitch in the connection between train and track, diseases can occur. In the November 24, 2015 issue of PNAS, Tatyana Polenova, Ph.D., Professor of Chemistry and Biochemistry, and her team at the University of Delaware (UD), together with John C. Williams, Ph.D., Associate Professor at the Beckman Research Institute of City of Hope in Duarte, California, reveal for the first time — atom by atom — the structure of a protein bound to a microtubule. The protein of focus, CAP-Gly, short for “cytoskeleton-associated protein-glycine-rich domains,” is a component of dynactin, which binds with the motor protein dynein to move cargoes of essential proteins along the microtubule tracks. Mutations in CAP-Gly have been linked to such neurological diseases and disorders as Perry syndrome and distal spinal bulbar muscular dystrophy. The research team used magic-angle-spinning nuclear magnetic resonance spectrometry (NMR) in the Department of Chemistry and Biochemistry at UD to unveil the structure of the CAP-Gly protein assembled on polymerized microtubules. The CAP-Gly protein has 1,329 atoms, and each tubulin dimer, which is a building block for microtubules, has nearly 14,000 atoms. “This is the first time anyone has been able to get an atomic-resolution structure of any microtubule-associated protein assembled on polymerized microtubules,” Dr. Polenova says. “With magic-angle-spinning NMR, we can look into the structure of this and other assemblies of microtubules and their associated proteins and gain critical insights into their function and dynamics, as well as begin to gather clues as to how mutations cause disease.” In magic-angle-spinning NMR, a sample is placed in the NMR’s small, tube-like rotor, which is then spun inside the NMR magnet at an angle of 54.74 degrees — called the “magic angle” because it suppresses the atoms from interacting magnetically. The result is a high-resolution protein fingerprint, a graph of hundreds of peaks representing the frequencies of two or more interacting atoms. These data are then used to calculate the 3-D structures.

The 3-D structures of CAP-Gly, which show the spatial arrangement of atoms in the protein molecule, are different between the free state of the protein and its bound state to the microtubule. These structures reveal how the protein interacts with microtubules, predominantly through its loop regions, which adopt specific conformations upon binding.

However, static structures of CAP-Gly do not tell the whole story about the protein.

“Just as we are always moving our arms and legs about, proteins are very dynamic. They do not stand still,” Dr. Polenova says.

“These motions are essential to their biological function, and NMR spectroscopy is the only technique that can record such movements, with atomic resolution, on a variety of time scales, from picoseconds to arbitrarily long time scales — seconds, days, weeks — to help us understand the protein’s function.”

“We know from our prior studies that CAP-Gly is dynamic on timescales from nano- to milliseconds, and this mobility is essential for the protein’s ability to interact with microtubules and with multiple other binding partners.”

The research, which has been ongoing since 2008 when the first data sets were collected, required the development of new protocols for preparing the samples, new NMR experiments to gather various information on structure and dynamics, and new protocols for data analysis.

In the future, Dr. Polenova and her team envision using NMR in combination with cryo-electron microscopy, in which samples are studied at extremely low temperatures, typically below -200 degrees Fahrenheit, to look at even more complex systems in a highly preserved form.

Dr. Polenova’s research team at UD included Dr. Si Yan, who received her doctorate from the University in 2014, current doctoral student Changmiao Guo, NMR spectroscopist Guangjin Hou, and postdoctoral researchers Dr. Huilan Zhang and Dr. Xingyu Lu. Dr. Williams, at Beckman Research Institute, was also a co-author of the study.

Atomic-resolution structure of the CAP-Gly domain of dynactin on polymeric microtubules determined by magic angle spinning NMR spectroscopy

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Significance

Microtubules and their associated proteins are central to most cellular functions. They have been extensively studied at multiple levels of resolution; however, significant knowledge gaps remain. Structures of microtubule-associated proteins bound to microtubules are not known at atomic resolution. We used magic angle spinning NMR to solve a structure of dynactin’s cytoskeleton-associated protein glycine-rich (CAP-Gly) domain bound to microtubules and to determine the intermolecular interface, the first example, to our knowledge, of the atomic-resolution structure of a microtubule-associated protein on polymeric microtubules. The results reveal remarkable structural plasticity of CAP-Gly, which enables CAP-Gly’s binding to microtubules and other binding partners. This approach offers atomic-resolution information of microtubule-binding proteins on microtubules and opens up the possibility to study critical parameters such as protonation states, strain, and dynamics on multiple time scales.

Microtubules and their associated proteins perform a broad array of essential physiological functions, including mitosis, polarization and differentiation, cell migration, and vesicle and organelle transport. As such, they have been extensively studied at multiple levels of resolution (e.g., from structural biology to cell biology). Despite these efforts, there remain significant gaps in our knowledge concerning how microtubule-binding proteins bind to microtubules, how dynamics connect different conformational states, and how these interactions and dynamics affect cellular processes. Structures of microtubule-associated proteins assembled on polymeric microtubules are not known at atomic resolution. Here, we report a structure of the cytoskeleton-associated protein glycine-rich (CAP-Gly) domain of dynactin motor on polymeric microtubules, solved by magic angle spinning NMR spectroscopy. We present the intermolecular interface of CAP-Gly with microtubules, derived by recording direct dipolar contacts between CAP-Gly and tubulin using double rotational echo double resonance (dREDOR)-filtered experiments. Our results indicate that the structure adopted by CAP-Gly varies, particularly around its loop regions, permitting its interaction with multiple binding partners and with the microtubules. To our knowledge, this study reports the first atomic-resolution structure of a microtubule-associated protein on polymeric microtubules. Our approach lays the foundation for atomic-resolution structural analysis of other microtubule-associated motors.