Structural Basis for Asymmetric Association of the βPIX Coiled Coil and Shank PDZ

beta PIX (p21-activated kinase interacting exchange factor) and Shank/ProSAP protein form a complex acting as a protein scaffold that integrates signaling pathways and regulates postsynaptic structure. Complex formation is mediated by the C-terminal PDZ binding motif of beta PIX and the Shank PDZ domain. The coiled-coil (CC) domain upstream of the PDZ binding motif allows multimerization of beta PIX, which is important for its physiological functions. We have solved the crystal structure of the beta PIX CC Shank PDZ complex and determined the stoichiometry of complex formation. The beta PIX CC forms a 76-angstrom-long parallel CC trimer. Despite the fact that the beta PIX CC exposes three PDZ binding motifs in the C-termini, the beta PIX trimer associates with a single Shank PDZ. One of the C-terminal ends of the CC forms an extensive beta-sheet interaction with the Shank PDZ, while the other two ends are not involved in ligand binding and form random coils. The two C-terminal ends of beta PIX have significantly lower affinity than the first PDZ binding motif due to the steric hindrance in the C-terminal tails, which results in binding of a single PDZ domain to the beta PIX trimer. The structure shows canonical class I PDZ binding with a beta-sheet interaction extending to position 6 of beta PIX. The beta B-beta C loop of Shank PDZ undergoes a conformational change upon ligand binding to form the beta-sheet interaction and to accommodate the bulky side chain of Trp -5. This structural study provides a clear picture of the molecular recognition of the PDZ ligand and the asymmetric association of beta PIX CC and Shank PDZ. (C) 2010 Elsevier Ltd. All rights reserved.