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Purpose: To explore possible ways in which yew tree tubulin is naturally resistant to paclitaxel. While the yew produces a potent cytotoxin, paclitaxel, it is immune to paclitaxel's cytotoxic action., Methods: Tubulin sequence data for plant species were obtained from Alberta 1000 Plants Initiative. Sequences were assembled with Trinity de novo assembly program and tubulin identified. Homology modeling using MODELLER software was done to generate structures for yew tubulin. Molecular dynamics simulations and molecular mechanics Poisson-Boltzmann calculations were performed with the Amber package to determine binding affinity of paclitaxel to yew tubulin. ClustalW2 program and PHYLIP package were used to perform phylogenetic analysis on plant tubulin sequences., Results: We specifically analyzed several important regions in tubulin structure: the high-affinity paclitaxel binding site, as well as the intermediate binding site and microtubule nanopores. Our analysis indicates that the high-affinity binding site contains several substitutions compared to human tubulin, all of which reduce the binding energy of paclitaxel., Conclusions: The yew has achieved a significant reduction of paclitaxel's affinity for its tubulin by utilizing several specific residue changes in the binding pocket for paclitaxel.

Using comparative modeling, we have generated structural models of 475 alpha and beta tubulins. Using these models, we observed a global, structural similarity between the tubulin isotypes. However, a number of subtle differences in the isotypes physical properties, including net electric charges, solvent accessible surface areas, and electric dipole moments were also apparent. In order to examine the roles that these properties may play in microtubule (MT) assembly and stability, we have created a model to evaluate the dipole-dipole interaction energies of varying MT lattice conformations, using human tubulin isotypes as particularly important examples. We conclude that the dipole moments of each tubulin isotype may influence their functional characteristics within the cell, resulting in differences for MT assembly kinetics and stability.

This paper discusses the results of homology modeling and resulting calculation of key structural and physical properties for close to 300 tubulin sequences, including alpha, beta, gamma, delta and epsilon -tubulins. The basis for our calculations was the structure of the tubulin dimer published several years ago by Nogales et al. (1998), later refined to 3.5 resolution by Lowe et al. (2001). While, it appears that the alpha, beta and gamma-tubulins segregate into distinct structural families, we have found several differences in the physical properties within each group. Each of the alpha, beta and gamma- tubulin groups exhibit major differences in their net electric charge, dipole moments and dipole vector orientations. These properties could influence functional characteristics such as microtubule stability and assembly kinetics, due to their effects on the strength of protein-protein interactions. In addition to the general structural trends between tubulin isoforms, we have observed that the carboxy-termini of alpha and beta-tubulin exists in at least two stable configurations, either projecting away from the tubulin (or microtubule) surface, or collapsed onto the surface. In the latter case, the carboxy-termini form a lattice distinctly different from that of the well-known A and B lattices formed by the tubulin subunits. However, this C-terminal lattice is indistinguishable from the lattice formed when the microtubule-associated protein tau binds to the microtubule surface. Finally, we have discussed how tubulin sequence diversity arose in evolution giving rise to its particular phylogeny and how it may be used in cell- and tissue-specific expression including embryonal development.

This paper discusses the results of homology modeling and resulting calculation of key structural and physical properties for close to 300 tubulin sequences, including α, β, γ, δ and ε -tubulins. The basis for our calculations was the structure of the tubulin dimer published several years ago by Nogales et al. (1998), later refined to 3.5 A resolution by Lowe et al. (2001). While, it appears that the alpha;, β and γ-tubulins segregate into distinct structural families, we have found several differences in the physical properties within each group. Each of the α, β and γ- tubulin groups exhibit major differences in their net electric charge, dipole moments and dipole vector orientations. These properties could influence functional characteristics such as microtubule stability and assembly kinetics, due to their effects on the strength of protein-protein interactions. In addition to the general structural trends between tubulin isoforms, we have observed that the carboxy-termini of α and β-tubulin exists in at least two stable configurations, either projecting away from the tubulin (or microtubule) surface, or collapsed onto the surface. In the latter case, the carboxy-termini form a lattice distinctly different from that of the well-known A and B lattices formed by the tubulin subunits. However, this C-terminal lattice is indistinguishable from the lattice formed when the microtubule-associated protein tau binds to the microtubule surface. Finally, we have discussed how tubulin sequence diversity arose in evolution giving rise to its particular phylogeny and how it may be used in cell- and tissue-specific expression including embryonal development. [ABSTRACT FROM AUTHOR]