Large-scale opening of utrophin’s tandem calponin homology (CH) domains upon actin binding by an induced-fit mechanism
Open Access
- 18 July 2011
- journal article
- research article
- Published by Proceedings of the National Academy of Sciences in Proceedings of the National Academy of Sciences of the United States of America
- Vol. 108 (31), 12729-12733
- https://doi.org/10.1073/pnas.1106453108
Abstract
We have used site-directed spin labeling and pulsed electron paramagnetic resonance to resolve a controversy concerning the structure of the utrophin–actin complex, with implications for the pathophysiology of muscular dystrophy. Utrophin is a homolog of dystrophin, the defective protein in Duchenne and Becker muscular dystrophies, and therapeutic utrophin derivatives are currently being developed. Both proteins have a pair of N-terminal calponin homology (CH) domains that are important for actin binding. Although there is a crystal structure of the utrophin actin-binding domain, electron microscopy of the actin-bound complexes has produced two very different structural models, in which the CH domains are in open or closed conformations. We engineered a pair of labeling sites in the CH domains of utrophin and used dipolar electron–electron resonance to determine the distribution of interdomain distances with high resolution. We found that the two domains are flexibly connected in solution, indicating a dynamic equilibrium between two distinct open structures. Upon actin binding, the two domains become dramatically separated and ordered, indicating a transition to a single open and extended conformation. There is no trace of this open conformation of utrophin in the absence of actin, providing strong support for an induced-fit model of actin binding.Keywords
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