Structure of the utrophin actin-binding domain bound to F-actin reveals binding by an induced fit mechanism

Utrophin is a large ubiquitously expressed cytoskeletal protein, homologous to dystrophin, the protein disrupted in Duchenne muscular dystrophy. The a ssociation of both proteins with the actin cytoskeleton is functionally imp ortant and is mediated by a domain at their N termini, conserved in members of the spectrin superfamily, including alpha-actinin, beta-spectrin and fi mbrin. We present the structure of the actin-binding domain of utrophin in complex with F-actin, determined by cryo-electron microscopy and helical re construction, and a pseudo-atomic model of the complex, generated by dockin g the crystal structures of the utrophin domain and F-actin into the recons truction. In contrast to the model of actin binding proposed for fimbrin, t he utrophin actin-binding domain appears to associate with actin in an exte nded conformation. This conformation places residues that are highly conser ved in utrophin and other members of the spectrin superfamily at the utroph in interface with actin, confirming the likelihood of this binding orientat ion. This model emphasises the importance of protein flexibility in modelin g interactions and presents the fascinating possibility of a diversity of a ctin-binding mechanisms among related proteins. (C) 2000 Academic Press.