Abstract
Tubulin dimers isolated from brain [sheep] contain 2 GTP binding sites, a nonexchangeable site and an exchangeable site. To localize the exchangeable site, a photoaffinity analog of GTP, 8-azidoguanosine triphosphate (8-N3GTP), was used which supoorts tubulin polymerization in the absence of activating light. Photolysis of tubulin polymerized in the presence of 0.01 to 0.1 mM [.beta.,.gamma.-32P]8-N3GTP resulted in covalent incorporation of radioactivity only onto the .beta. monomer. Photolysis with 8-N3GTP also prevented any further repolymerization of the tubulin whereas like treatment in the presence of GTP had no effect. Preincubation of tubulin with GTP prevented photoincorporation of [.beta.,.gamma.-32P]8-N3GTP whereas preincubation with ATP did not.