Production and purification of CNBr peptides: The purified tubulins were cleaved by an optimized protocol involving 0.1M CNBr, which is roughly a 2,000-10,000-fold excess of CNBr relative to methionine, in 20% trifluoroacetic acid (TFA) solution. Cleavage proceeded overnight at room temperature in a light protected, closed vessel. The next day, reactions were quenched with water, dried in vacuo, re-dissolved in water and dried again 22 . Peptides were dissolved in 0.06 mL of 50 mM ammonium formate, pH 6, 20% acetonitrile (ACN), vortexed, sonicated, and purified by ion exchange chromatography on 0.01 mL microcolumns of Sephadex DEAE Fast Flow anion exchange resin, pre-equilibrated in the same buffer, and contained in Pierce Thermo micro spin columns. Samples were loaded by centrifuging at 140 x g for 30 sec, and the sample was run through a second time. Columns were washed (140 x g for 30 sec) with 0.06 mL of water, and samples eluted (140 x g for 30 sec) with 0.06 mL of 2.5% TFA: 20% ACN (the eluate was run through the column a second time as above). Eluted samples were dried in vacuo, re-suspended in 0.1 mL water, re-dried and then re-suspended in 0.01 mL 1:1 (ACN): 0.1% TFA for mass spectrometry analysis.