Microtubules (MTs) are hollow cylindrical polymers composed of αβ-tubulin heterodimers that align head-to-tail in the MT wall, forming linear protofilaments that interact laterally. We introduce a probe of the interprotofilament interactions within MTs and show that this technique gives insight into the mechanisms by which MT-associated proteins (MAPs) and taxol stabilize MTs. In addition, we present further measurements of the mechanical properties of MT walls, MT-MT interactions, and the entry of polymers into the MT lumen. These results are obtained from a synchrotron small angle x-ray diffraction (SAXRD) study of MTs under osmotic stress. Above a critical osmotic pressure, Pcr, we observe rectangular bundles of MTs whose cross sections have buckled to a noncircular shape; further increases in pressure continue to distort MTs elastically. The Pcr of -600 Pa provides, for the first time, a measure of the bending modulus of the interprotofilament bond within an MT. The presence of neuronal MAPs greatly increases Pcr, whereas surprisingly, the cancer chemotherapeutic drug taxol, which suppresses MT dynamics and inhibits MT depolymerization, does not affect the interprotofilament interactions. This SAXRD-osmotic stress technique, which has enabled measurements of the mechanical properties of MTs, should find broad application for studying interactions between MTs and of MTs with MAPs and MT-associated drugs.
Bibliographical noteFunding Information:
U. Raviv acknowledges the support of the International Human Frontier Science Program Organization. This work was supported by National Science Foundation grants DMR 0503347, CTS 0404444, and CTS 0103516, National Institutes of Health grants GM-59288 and NS-13560, and Department of Energy contract No. W-7405-ENG-36 with the University of California. The Materials Research Science and Engineering Center at the University of California, Santa Barbara, is supported by National Science Foundation DMR-0080034. The Stanford Synchrotron Radiation Laboratory is supported by the Department of Energy.