STRUCTURAL COMPARISONS OF CALPONIN HOMOLOGY DOMAINS - IMPLICATIONS FOR ACTIN-BINDING

Background: The actin-binding site of several cytoskeletal proteins is comprised of two calponin homology (CH) domains in a tandem arrangeme nt. As a single copy, the CH domain is also found in regulatory protei ns in muscle and in signal-transduction proteins. The three-dimensiona l structures of three CH domains are known, but they have not yet clar ified the molecular details of the interaction between actin filaments and proteins harbouring CH domains. Results: We have compared the cry stal structure of a CH domain from beta-spectrin, which has been refin ed to 1.1 Angstrom resolution, with the two CH domains that constitute the actin-binding region of fimbrin. This analysis has allowed the co nstruction 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 det ermine domain architecture, and has led to discussion on the functiona l differences that seem to exist between subfamilies of CH domains, as regards binding to F-actin. Conclusions: Our analysis supports bioche mical data that implicate a surface centered at the last helix of the N-terminal CH domain as the most probable actin-binding site in cytosk eletal 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 funct ions.