Visualization of the structural polarity of microtubules.

Microtubules (MTs) are intrinsically polar fibres. Many models for the mechanism of MT-mediated transport and mitosis postulate an important role for MT polarity because a symmetric fibre could not generate force in one direction along its surface. The lack of a convenient method to determine the polar orientation of cellular MTs has hindered experimental testing of these ideas. We report here a convenient technique for visualizing MT polarity based on conditions for tubulin assembly in vitro whereby MT elongation occurs with the associated formation of hooked, protofilament appendages on the MT wall (see Fig. 1). There is polarity information in such junctions between MT walls; our method reveals the polarity of the cellular Mt by the handedness of the hooked appendage as seen in cross-section (Fig. 1).