Reactions of tubulin-associated guanine nucleotides.

Only exchangeably bound nucleotide (E-site) is involved in the reaction of the transplhosphorylase activity in microtubular protein. Contrary to earlier reports, we find that the nonexchangeable nucleotide (N-site) is not a substrate. This conclusion is based upon comparison of: (a) rates of hydrolysis of endogenous tubulin-associated GTP and added [32p]GTP: (b) hydrolysis rates for added [32p]GTP and [3h]GTP; (c) the 32P/3H ratio in bound and free GTP after reaction with [3h, 32p]gTP. During the course of the above studies we have made the unusual observation of a time dependent augmentation in the expected amount of GTP relative to GDP at the E-site; there is either a net conversion of E-site GDP to E-site GTP, or a means for providing additional E-site GTP from another source.