Interactions can promote clustering of transmembrane receptors at specific subcellular sites. The domains play an important role in the spatial organization of voltage and ligand gated ion channels at synapses. The Shaker type K+ channels and all three classes of glutamate receptors are recognized by distinct PDZ domain proteins; specificity is conferred by ligand residues at the -2 to -4 position relative to the COOH terminus and may be regulated by phosphorylation because the -2 residue of the PDZ binding site is often a hydroxy-amino acid. X (S/T) X   -stop -large hydrophobic residue