Autopalmitoylation of tubulin

Pure rat brain tubulin is readily palmitoylated in vitro using [3H]palmitoyl CoA but no added enzymes. A maximum of approximately six palmitic acids are added per dimer in 2-3 h at 36-37 degrees C under native conditions. Both alpha and beta tubulin are labeled, and 63-73% of the label was hydroxylamine-labile, presumed thioesters. Labeling increases with increasing pH and temperature, and with low concentrations of guanidine HCl or KCl (but not with urea) to a maximum of approximately 13 palmitates/dimer. High SDS and guanidine HCl concentrations are inhibitory. At no time could all 20 cysteine residues of the dimer be palmitoylated. Polymerization to microtubules, or use of tubulin S, markedly decreases the accessibility of the palmitoylation sites. Palmitoylation increases the electrophoretic mobility of a portion of alpha tubulin toward the beta band. Palmitoylated tubulin binds a colchicine analogue normally, but during three warm/cold polymerization/depolymerization cycles there is a progressive loss of palmitoylated tubulin, indicating decreased polymerization competence. We postulate that local electrostatic factors are major regulators of reactivity of tubulin cysteine residues toward palmitoyl CoA, and that the negative charges surrounding a number of the cysteines are sensitive to negative charges on palmitoyl CoA.