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betaPIX (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 betaPIX and the Shank PDZ domain. The coiled-coil (CC) domain upstream of the PDZ binding motif allows multimerization of betaPIX, which is important for its physiological functions. We have solved the crystal structure of the betaPIX CC-Shank PDZ complex and determined the stoichiometry of complex formation. The betaPIX CC forms a 76-A-long parallel CC trimer. Despite the fact that the betaPIX CC exposes three PDZ binding motifs in the C-termini, the betaPIX 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 betaPIX 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 betaPIX trimer. The structure shows canonical class I PDZ binding with a beta-sheet interaction extending to position -6 of betaPIX. The betaB-betaC 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 betaPIX CC and Shank PDZ.