Solution structure and backbone dynamics of the second PDZ domain of postsynaptic density-95

The second PDZ domain of postsynaptic density-95 (PSD-95 PDZ2) plays a crit ical role in coupling N-methyl-D-aspartate receptors to neuronal nitric oxi de synthase (nNOS). In this work, the solution structure of PSD-95 PDZ2 was determined to high resolution by NMR spectroscopy. The structure of PSD-95 PDZ2 was compared in detail with that of alpha 1-syntrophin PDZ domain, as the PDZ domains share similar target interaction properties. The interacti on of the PSD-95 PDZ2 with a carboxyl-terminal peptide derived from a cytop lasmic protein CAPON was studied by NMR titration experiments. Complex form ation between PSD-95 PDZ2 and the nNOS PDZ was modelled on the basis of the crystal structure of the alpha 1-syntrophin PDZ/nNOS PDZ dimer. We found t hat the prolonged loop connecting the beta B and beta C strands of PSD-95 P DZ2 is likely to play a role in both the binding of the carboxyl-terminal p eptide and the nNOS beta-finger. Finally, the backbone dynamics of the PSD- 95 PDZ2 in the absence of bound peptide were studied using a model-free app roach. The "GLGF"-loop and the loop connecting alpha B and beta F of the pr otein display some degree of flexibility in solution. The rest of the prote in is rigid and lacks detectable slow time-scale (microseconds to milliseco nds) motions. Ln particular, the loop connecting beta B and beta C loop ado pts a well-defined, rigid structure in solution. It appears that the loop a dopts a pre-aligned conformation for the PDZ domain to interact with its ta rgets. (C) 2000 Academic Press.