The utrophin actin-binding domain binds F-actin in two different modes: implications for the spectrin superfamily of proteins.
Utrophin, like its homologue dystrophin, forms a link between the actin cytoskeleton and the extracellular matrix. We have used a new method of image analysis to reconstruct actin filaments decorated with the actin-binding domain of utrophin, which contains two calponin homology domains. We find two different modes of binding, with either one or two calponin-homology (CH) domains bound per actin subunit, and these modes are also distinguishable by their very different effects on F-actin rigidity. Both modes involve an extended conformation of the CH domains, as predicted by a previous crystal structure. The separation of these two modes has been largely dependent upon the use of our new approach to reconstruction of helical filaments. When existing information about tropomyosin, myosin, actin-depolymerizing factor, and nebulin is considered, these results suggest that many actin-binding proteins may have multiple binding sites on F-actin. The cell may use the modular CH domains found in the spectrin superfamily of actin-binding proteins to bind actin in manifold ways, allowing for complexity to arise from the interactions of a relatively few simple modules with actin.
Galkin
Vitold E
VE
Department of Biochemistry and Molecular Genetics, University of Virginia Health Sciences Center, Charlottesville, VA 22908, USA.
Orlova
Albina
A
VanLoock
Margaret S
MS
Rybakova
Inna N
IN
Ervasti
James M
JM
Egelman
Edward H
EH
D000199:Actins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D001665:Binding Sites; D018360:Crystallography, X-Ray; D003598:Cytoskeletal Proteins / Q000737:chemistry* / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D008854:Microscopy, Electron; D008954:Models, Biological; D008958:Models, Molecular; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D013049:Spectrin / Q000378:metabolism*; D049411:Utrophin
D000199:Actins; D003598:Cytoskeletal Proteins; D008565:Membrane Proteins; D049411:Utrophin; D013049:Spectrin