title: Microtubule protofilament number is modulated in a stepwise fashion by the charge density of an enveloping layer.
abstract: 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.
authors.affiliation: Materials Department, University of California, Santa Barbara, California 93106, USA. raviv@chem.ch.huji.ac.il
authors.forename: Uri
authors.initials: U
authors.lastname: Raviv
authors.forename: Toan
authors.initials: T
authors.lastname: Nguyen
authors.forename: Rouzbeh
authors.initials: R
authors.lastname: Ghafouri
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Needleman
authors.forename: Youli
authors.initials: Y
authors.lastname: Li
authors.forename: Herbert P
authors.initials: HP
authors.lastname: Miller
authors.forename: Leslie
authors.initials: L
authors.lastname: Wilson
authors.forename: Robijn F
authors.initials: RF
authors.lastname: Bruinsma
authors.forename: Cyrus R
authors.initials: CR
authors.lastname: Safinya
mesh_terms: D000818:Animals; D001703:Biophysics / Q000379:methods*; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D007477:Ions; D008055:Lipids / Q000737:chemistry; D046529:Microscopy, Electron, Transmission; D008870:Microtubules / Q000737:chemistry* / Q000378:metabolism; D043942:Nanotubes / Q000737:chemistry; D017239:Paclitaxel / Q000737:chemistry; D012542:Scattering, Radiation; D055672:Static Electricity; D017356:Synchrotrons; D014404:Tubulin / Q000737:chemistry*; D014965:X-Rays
chemical_list: D007477:Ions; D008055:Lipids; D014404:Tubulin; D017239:Paclitaxel