Switch II mutants sequester tubulin in vivo. (A) Cells transfected with E491A or G489A mutant MCAK have near-normal total tubulin content. For E491A and G489A, n = 52 cells; for wt, n = 38 cells. Error bars indicate SEM of the tubulin ratio. (B) Images of representative cells containing wt EGFP-MCAK, G489A mutant (G), or E491A mutant (E). Although quantification shows near normal tubulin content, immunofluorescence reveals that most of the MT polymer is gone, which suggests that the MTs have been disassembled via tubulin sequestration. Bar, 10 μm.