Tandem calponin-homology (CH) domains constitute a major class of actin-binding domains that include dystrophin and utrophin, the two key proteins involved in muscular dystrophy. Despite their importance, how their structure controls their function is not understood. Here, we study the contribution of individual CH domains to the actin-binding function and thermodynamic stability of utrophin’s tandem CH domain. Traditional actin co-sedimentation assays indicate that the isolated C-terminal CH2 domain binds weakly to F-actin when compared with the full-length tandem CH domain. In contrast, isolated CH1 binds to F-actin with a similar efficiency as that of the full-length tandem CH domain. Thus, the obvious question that arises is why tandem CH domains require CH2, when their actin-binding efficiency is originating primarily from CH1. To answer, we probed the thermodynamic stabilities of individual CH domains. Isolated CH1 domain is unstable and is prone to serious aggregation. Isolated CH2 is very stable, even appears to be more stable than the full-length tandem CH domain. In addition, the CH2 domain, which is more stable, is less functional. These results indicate that the main function of CH2 is to stabilize CH1. Consistently, the proposed structure of utrophin’s tandem CH domain based on earlier X-ray studies indicates a close proximity between the C-terminal helix of CH2 and the N-terminal helix of CH1, and this helix in CH2 is more dynamic in the full-length protein when compared with that in the absence of CH1, suggesting the mechanism by which CH2 stabilizes CH1. These observations indicate that the two CH domains contribute differentially to the folding and function of tandem CH domains, although both domains essentially have the same native structure in the tandem CH domain. The N-terminal domain determines the function, whereas the C-terminal domain determines the stability.
This work was funded by the AHA Grant 11SDG4880046.
Tissue-Specificity of Dystrophin–Actin Interactions: Isoform-Specific Thermodynamic Stability and Actin-Binding Function of Tandem Calponin-Homology Domains
