Structural basis for NHERF1 PDZ domain binding.

The Na(+)/H(+) exchange regulatory factor-1 (NHERF1) is a scaffolding protein that possesses two tandem PDZ domains and a carboxy-terminal ezrin-binding domain (EBD). The parathyroid hormone receptor (PTHR), type II sodium-dependent phosphate cotransporter (Npt2a), and β2-adrenergic receptor (β2-AR), through their respective carboxy-terminal PDZ-recognition motifs, individually interact with NHERF1 forming a complex with one of the PDZ domains. In the basal state, NHERF1 adopts a self-inhibited conformation, in which its carboxy-terminal PDZ ligand interacts with PDZ2. We applied molecular dynamics (MD) simulations to uncover the structural and biochemical basis for the binding selectivity of NHERF1 PDZ domains. PDZ1 uniquely forms several contacts not present in PDZ2 that further stabilize PDZ1 interactions with target ligands. The binding free energy (ΔG) of PDZ1 and PDZ2 with the carboxy-terminal, five-amino acid residues that form the PDZ-recognition motif of PTHR, Npt2a, and β2-AR was calculated and compared with the calculated ΔG for the self-association of NHERF1. The results suggest that the interaction of the PTHR, β2-adrenergic, and Npt2a involves competition between NHERF1 PDZ domains and the target proteins. The binding of PDZ2 with PTHR may also compete with the self-inhibited conformation of NHERF1, thereby contributing to the stabilization of an active NHERF1 conformation.