Identification and characterization of an intermediate taxol binding site within microtubule nanopores and a mechanism for tubulin isotype binding selectivity.
Tubulin, the primary subunit of microtubules, is remarkable for the variety of small molecules to which it binds. Many of these are very useful or promising agents in cancer chemotherapy. One of the most useful of these is paclitaxel. The tubulin molecule is itself an alpha/beta heterodimer, both alpha- and beta-tubulin monomers existing as multiple isotypes. Despite the success of paclitaxel as an anticancer drug, resistance often occurs in cancer cells and has been associated with variations in tubulin isotype expression, most notably with the increased expression of betaIII-tubulin. Paclitaxel is thought to reach its binding site on beta-tubulin by diffusion through nanopores in the microtubule wall. It has been suggested that a transitional step in this process may be the binding of paclitaxel to an intermediate site within a nanopore, from which it moves directly to its binding site in the microtubule interior facing the lumen. To test this hypothesis, we have computationally docked paclitaxel within a microtubule nanopore and simulated its passage to the intermediate binding site. Targeted molecular dynamics was then used to test the hypothesis that paclitaxel utilizes the H6/H7 loop as a hinge to move directly from this intermediate binding site to its final position in the luminal binding site. We observed that this motion appears to be stabilized by the formation of a hydrogen bond involving serine 275 in beta-tubulin isotypes I, IIa, IIb, IVa, IVb, V, VII, and VIII. Interestingly, this residue is replaced by alanine in the betaIII and VI isotypes. This observation raises the possibility that the observed isotype difference in paclitaxel binding may be a kinetic effect arising from the isotype difference at this residue. We are now able to suggest derivatives of paclitaxel that may reverse the isotype-specificity or lead to an alternate stabilizing hydrogen-bond interaction with tubulin, thus increasing the rate of passage to the luminal binding site and hopefully offering a therapeutic advantage in paclitaxel resistant cases.
Freedman
Holly
H
Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, Alberta, Canada.
Huzil
J Torin
JT
Luchko
Tyler
T
Ludueña
Richard F
RF
Tuszynski
Jack A
JA
D000970:Antineoplastic Agents / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D008870:Microtubules / Q000378:metabolism*; D008958:Models, Molecular; D017239:Paclitaxel / Q000737:chemistry / Q000378:metabolism*
D000970:Antineoplastic Agents; D017239:Paclitaxel