Carbamate formation on tubulin: CO2/bicarbonate buffers protect tubulin from inactivation by reductive methylation and carbamoylation and promote microtubule assembly at alkaline pH.

Carbamoylation and reductive methylation of tubulin have been shown previously to inhibit microtubule assembly, probably by attack on essential internal lysine residues [Mellado, W., Slebe, J., & Maccioni, R.B. (1982) Biochem. J. 203, 675-681; Szasz, J., Burns, R., & Sternlicht, H. (1982) J. Biol. Chem. 257, 3697-3704]. We show first that this inhibition is blocked by the presence of HCO3-/CO2 buffer at physiological concentrations during the carbamoylation or reductive methylation. Under conditions that block assembly, the amount of radiolabeled cyanate or formaldehyde incorporated by these reactions in the absence of HCO3-/CO2 was approximately four carbamoyl or five methyl groups in a ratio of approximately 1.7 alpha chain/beta chain. In the presence of HCO3-/CO2, the formaldehyde incorporation is decreased roughly 0.5 mol in each of the alpha and beta chains, and cyanate incorporation, roughly 1.0 mol/mol of alpha or beta monomer. These results are consistent with the hypothesis that CO2 competed with formaldehyde or cyanate for uncharged amino groups and led to the reversible formation of carbamates. The complete antagonism of the inhibition of microtubule assembly by reductive methylation by CO2, even though the number of methyl groups incorporated was reduced by only 0.5 mol/tubulin monomer, was consistent with the possibility that reductive methylation opened up additional residues for attack. Indeed, using an adaptation of the method of Gros et al. for measurement of carbamates [Gros, G., Forster, R.E., & Lin, L. (1976) J. Biol. Chem. 251, 4398-4407], we found that reductive methylation with 2 mM formaldehyde (assembly blocked) did not decrease carbamate formation (carbamate formation was inhibited at higher formaldehyde concentrations).(ABSTRACT TRUNCATED AT 250 WORDS)