Structural features and regulatory properties of the brain glutamate decarboxylases

It is widely recognized that the two major forms of GAD present in adult ve rtebrate brains are each composed of two major sequence domains that differ in size and degree of similarity. The amino-terminal domain is smaller and shows little sequence identity between the two forms. This domain is thoug ht to mediate the subcellular targeting of the two GADs. Substantial parts of the amino-terminal domain appear to be exposed and flexible, as shown by proteolysis experiments and the locations of posttranslational modificatio ns. The carboxyl-terminal sequence domain contains the catalytic site and s hows substantial sequence similarity between the forms. The interaction of GAD with its cofactor, pyridoxal-5' phosphate (pyridoxal-P), plays a key ro le in the regulation of CAD activity. Although GAD(65) and GAD(67) interact differently with pyridoxal-P, their cofactor-binding sites contain the sam e set of nine putative cofactor-binding residues and have the same basic st ructural fold. Thus the cofactor-binding differences cannot be attributed t o fundamental structural differences between the GADs but must result from subtle modifications of the basic cofactor-binding fold. The presence of an other conserved motif suggests that the carboxylterminal domain is composed of two functional domains: the cofactor-binding domain and a small domain that closes when the substrate binds. Finally, GAD is a dimeric enzyme and conserved features of GADs superfamily of pyridoxal-P proteins indicate the dimer-forming interactions are mediated mainly by the carboxyl-terminal do main. Published by Elsevier Science Ltd.