Characterization of the Covalent Binding of N-Phenyl-N -(2-chloroethyl)ureas to -Tubulin: Importance of Glu198 in Microtubule Stability

N-Phenyl-N -(2-chloroethyl)ureas (CEUs) are antimicrotubule agents interacting covalently with -tubulin near the colchicine-binding site (C-BS). Glutamyl 198 residue in -tubulin (Glu198), which is adjacent to the C-BS behind the two potent nucleophilic residues, Cys239 and Cys354, has been shown to covalently react with 1-(2-chloroethyl)-3-(4iodophenyl)urea (ICEU). By use of mass spectrometry, we have now identified residues in -tubulin that have become modified irreversibly by 1-(2-chloroethyl)-3-[3-(5-hydroxypentyl)phenyl]urea (HPCEU), 1-[4-(3-hydroxy-4-methoxystyryl)phenyl]-3-(2-chloroethyl)urea (4ZCombCEU), and N,N -ethylenebis(iodoacetamide) (EBI). The binding of HPCEU and 4ZCombCEU to -tubulin resulted in the acylation of Glu198, a protein modification of uncommon occurrence in living cells. Prototypical CEUs then were used as molecular probes to assess, in mouse B16F0 and human MDAMB-231 cells, the role of Glu198 in microtubule stability. For that purpose, we studied the effect of Glu198 modification by ICEU, HPCEU, and 4ZCombCEU on the acetylation of Lys40 on -tubulin, a key indicator of microtubule stability. We show that modification of Glu198 by prototypical CEUs correlates with a decrease in Lys40 acetylation, as observed also with other microtubule depolymerizing agents. Therefore, CEU affects the stability and the dynamics of microtubule, likewise a E198G mutation, which is unusual for xenobiotics. We demonstrate for the first time that EBI forms an intramolecular cross-link between Cys239 and Cys354 of -tubulin in living cells. This work establishes a novel basis for the development of future chemotherapeutic agents and provides a framework for the design of molecules useful for studying the role of Asp and Glu residues in the structure/ function and the biological activity of several cellular proteins under physiological conditions.