CLEAVAGE OF TUBULIN BY VANADATE ION

Vanadate is known to cleave proteins in a near-uv-dependent manner. We have found that vanadate will cleave alpha- and beta-tubulin upon pho toirradiation (419 nm emission maxima) under conditions when tetravana date, pentavanadate, and decavanadate are in solution. The reaction is independent of GTPMg or GDPMg, and cleavage occurs at two or more sit es per chain. Cleavage was studied at pH 6.0 (2(N-morpholino)ethanesul fonic acid (Mes) and phosphate), pH 6.9 (piperazine-N,N'-bis(2-ethanes ulfonic acid) (Pipes)), pH 7.0 (phosphate), and pH 8.0 (2-hydroxyethyl )piperazine-N'-bis(2-ethanesulfonic acid) (Hepes) and phosphate). The concentration of vanadate oligomer species, as determined by V-51 NMR, was correlated with the extent of cutting. In organic buffers, low pH and high vanadate concentration favored oligomer formation, especiall y tetra and decavanadate. In phosphate buffer at pH 7 and 8, decamer i s more prevalent, and at pH 6, phosphate buffer appears to favor a dif ferent oligomer form, V', appearing at -582 ppm. Cleavage is best corr elated with the presence of cyclic tetravanadate at pH 6.9 in Pipes bu ffer and the V' species at pH 6.0 in phosphate buffer. Cleavage effici ency is also affected by interactions of photoactivated vanadate speci es with organic buffer components. In phosphate buffer no photochemica l degradation of vanadate species occurs. Analysis using sodium dodecy l sulfate (SDS) gel electrophoresis and western blotting showed that v anadate produced cleavage patterns and nonenzymatic cleavage patterns resulting from boiling tubulin in SDS sample buffer (J. J. Correia, L. D. Lipscomb, and S. Lobert, 1993, Arch. Biochem. Biophys. 300, 105-11 4) are not the same. Attempts to identify the locations of the vanadat e cleavage sites on the protein through N-terminal sequencing was unsu ccessful, apparently due to the presence of blocked amino groups. We c onclude that tetravanadate cleaves tubulin upon photoirradiation, that organic buffers can interact with vanadate oligomers upon photoirradi ation, and that in phosphate buffer photocleavage is enhanced by an ab sence of photochemical degradation and a preference for forming photoa ctive vanadate oligomers. These results have general application to ph otoirradiation studies of any protein in the presence of vanadate. (C) 1994 Academic Press,Inc.