SUPRAMOLECULAR ASSEMBLIES |
Microtubule protofilament number is modulated in a step-wise fashion by the charge density of an enveloping layer
Uri Raviv 1*, Toan Nguyen 2, Rouzbeh Ghafouri 2, Daniel J Needleman 3, Youli Li 4, Herbert P Miller 4, Leslie Wilson 4, Robijn F Bruinsma 2 and Cyrus R Safinya 4
1 The Hebrew University of Jerusalem
2 UCLA
3 Harvard University
4 UCSB
* To whom correspondence should be addressed. E-mail: raviv{at}chem.ch.huji.ac.il.
Submitted on May 9, 2006
Revised on July 11, 2006
Accepted on 5 September 2006
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Abstract |
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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 those variations is poorly understood. Using synchrotron X-ray scattering and transmission electron microscopy we studied how charged membranes that under certain conditions can envelope preassembled MTs regulate their PF number. We show that the mean PF number, < N > is modulated primarily by the charge density of the membranes. < N > decreases in a step-wise 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 kBT/nm, associated with the observed changes in PF number.
Key Words:
lipid – protein nanotube, membrane, small angle x-ray scattering, transmission electron microscopy, tubulin
