Structural comparisons of calponin homology domains: implications for actin binding

The actin-binding site of several cytoskeletal proteins is comprised of two calponin homology (CH) domains in a tandem arrangement. As a single copy, the CH domain is also found in regulatory proteins in muscle and in signal-transduction proteins. The three-dimensional structures of three CH domains are known, but they have not yet clarified the molecular details of the interaction between actin filaments and proteins harbouring CH domains.

We have compared the crystal structure of a CH domain from beta-spectrin, which has been refined to 1.1 A resolution, with the two CH domains that constitute the actin-binding region of fimbrin. This analysis has allowed the construction of a structure-based sequence alignment of CH domains that can be used in further comparisons of members of the CH domain family. The study has also improved our understanding of the factors that determine domain architecture, and has led to discussion on the functional differences that seem to exist between subfamilies of CH domains, as regards binding to F-actin.

Our analysis supports biochemical data that implicate a surface centered at the last helix of the N-terminal CH domain as the most probable actin-binding site in cytoskeletal proteins. It is not clear whether the C-terminal domains of the tandem arrangement or the single CH domains have this function alone. This may imply that although the CH domains are homologous and have a conserved structure, they may have evolved to perform different functions.