Microtubule protofilament number is modulated in a stepwise fashion by the charge density of an enveloping layer.

Microtubules are able to adjust their protofilament (PF) number and, as a consequence, their dynamics and function, to the assembly conditions and presence of cofactors. However, the principle behind such variations is poorly understood. Using synchrotron x-ray scattering and transmission electron microscopy, we studied how charged membranes, which under certain conditions can envelop pre-assembled MTs, regulate the PF number of those MTs. We show that the mean PF number, <N>, is modulated primarily by the charge density of the membranes. <N> decreases in a stepwise fashion with increasing membrane charge density. <N> does not depend on the membrane-protein stoichiometry or the solution ionic strength. We studied the effect of taxol and found that <N> increases logarithmically with taxol/tubulin stoichiometry. We present a theoretical model, which by balancing the electrostatic and elastic interactions in the system accounts for the trends in our findings and reveals an effective MT bending stiffness of order 10-100 k(B)T/nm, associated with the observed changes in PF number.