Modulation of Microtubule Interprotofilament Interactions by Modified Taxanes

Abstract

Microtubules assembled with paclitaxel and docetaxel differ in their numbers of protofilaments, reflecting modification of the lateral association between αβ-tubulin molecules in the microtubule wall. These modifications of microtubule structure, through a not-yet-characterized mechanism, are most likely related to the changes in tubulin-tubulin interactions responsible for microtubule stabilization by these antitumor compounds. We have used a set of modified taxanes to study the structural mechanism of microtubule stabilization by these ligands. Using small-angle x-ray scattering, we have determined how modifications in the shape and size of the taxane substituents result in changes in the interprotofilament angles and in their number. The observed effects have been explained using NMR-aided docking and molecular dynamic simulations of taxane binding at the microtubule pore and luminal sites. Modeling results indicate that modification of the size of substituents at positions C7 and C10 of the taxane core influence the conformation of three key elements in microtubule lateral interactions (the M-loop, the S3 β-strand, and the H3 helix) that modulate the contacts between adjacent protofilaments. In addition, modifications of the substituents at position C2 slightly rearrange the ligand in the binding site, modifying the interaction of the C7 substituent with the M-loop.

The Unique Binding Mode of Laulimalide to Two Tubulin Protofilaments

“…The laulimalide-tubulin crystal structure also reveals structural differences in the b-tubulin M-loop, which becomes more helical and increases hydrogen bonding (15). Structural changes to the M-loop have also been reported for drugs such as taxanes (33,34). The specific interactions and structural reorganization reported here may explain the stabilizing effect laulimalide has on microtubules, therefore explaining the mode of action of the drug.…”

Molecular control of stress transmission in the microtubule cytoskeleton

“…Small angle X-ray scattering (SAXS) has been used to measure structural features including the regulation of protofilament number by the MT-associated protein tau [96], as well as the effects of small-molecule inhibitors of dynamic instability (i.e. the taxane compound paclitaxel) on interprotofilament interactions [97]. Both methods typically require averaging over many static filaments, precluding measurement of heterogeneities, and making it nearly impossible to measure dynamic, time-resolved changes in MT structure.…”

Recent progress with microtubule stabilizers: new compounds, binding modes and cellular activities

“…Recently, a set of modified taxanes was used to study the structural mechanisms of differential microtubule stabilization. 130 Small angle X-ray scattering was employed to evaluate how modifications in the size and shape of taxane substituents affected changes in protofilament number and angles. These studies found that modifications at C7 and C10 of the taxane backbone influence elements involved in protofilament lateral interactions: the M-loop, the S3 β strand, and the H3 helix.…”

“…These studies found that modifications at C7 and C10 of the taxane backbone influence elements involved in protofilament lateral interactions: the M-loop, the S3 β strand, and the H3 helix. 130 Modifications at C2 caused a rearrangement of the ligand in the taxane binding site, which changed the interaction of C7 with the M-loop. 130 The finding that differences at C2 affect regions known to be involved in lateral protofilament interactions explains differences in the structures of microtubules formed in the presence of paclitaxel or docetaxel and may lead to lack of cross-resistance clinically.…”

“…141 Initial modelling studies suggested peloruside A bound to α-tubulin in a location equivalent to the paclitaxel site on β -tubulin. 130, 142 However, other studies using hydrogen-deuterium exchange mass spectrometric techniques in combination with data-directed computational strategies were used to propose that peloruside A binds to β -tubulin on the exterior surface of the microtubule. 143, 144 Like epothilone A and docetaxel, peloruside A was found to stabilize microtubules through strengthening of the longitudinal interactions at the interdimer surface but, in addition, it was also able to stabilize interactions at the intradimer interface.…”