Related Experiment Videos
Substructural analysis of the microtubule and its polymorphic forms
Abstract:
Microtubules are composed of 13 protofilaments which are aligned parallel to the long axis of the tubule, as can be seen in thin sections of material fixed in the presence of tannic acid. Evidence is presented based upon Markham rotations of the isolated flagellar axoneme that the bridges (spoke, nexin, dynein) must connect to certain specified protofilaments. By means of geometric considerations it is concluded that the bridges in a Centrohelida and Raphidiophrys also connect certain specified protofilaments. Two polymorphic forms of microtubules were also examined using tannic acid in the fixative, a 320 A tubule produced in Echinosphaerium by treatment with low temperature, and a vinblastine-induced microtubule crystal. In both forms, the arrangement of the subunits is not in the form of linear protofilaments. The vinblastine crystal in transverse section is made up of hexagonally packed tubules approximately 320 A in diameter. The wall of each tubule in turn is composed of two parallel protofilaments which spiral upwards, giving the appearance of a barber pole. The center-to-center separation of adjacent protofilaments is 180 A. We conclude that the tubulin heterodimer lies parallel to the axis of the protofilaments, exposing the colchicine binding site.
Insights
Microtubules feature 13 protofilaments. Bridges connect specific protofilaments, and tubulin heterodimers lie parallel to protofilament axes, exposing the colchicine binding site.
Area of Science:
- Cell biology
- Structural biology
- Biochemistry
Background:
- Microtubules are essential cytoskeletal components formed by 13 protofilaments.
- The precise arrangement and connections of microtubule subunits and associated proteins are crucial for their function.
Purpose of the Study:
- To elucidate the specific protofilament connections of microtubule bridges.
- To investigate the subunit arrangement in polymorphic microtubule forms and determine tubulin heterodimer orientation.
Main Methods:
- Analysis of thin sections of microtubules fixed with tannic acid.
- Markham rotations of isolated flagellar axonemes.
- Geometric analysis of microtubule structures.
- Examination of low-temperature and vinblastine-induced microtubule polymorphs.
Main Results:
- Bridges (spoke, nexin, dynein) connect to specific protofilaments in flagellar axonemes, Centrohelida, and Raphidiophrys.
- Polymorphic microtubules, including low-temperature and vinblastine-induced forms, exhibit non-linear protofilament arrangements.
- Vinblastine crystals show hexagonally packed tubules with walls composed of two spiraling protofilaments (180 A separation).
Conclusions:
- Microtubule bridges have specific protofilament attachment sites.
- The tubulin heterodimer is oriented parallel to the protofilament axis, with the colchicine binding site exposed.