title: Determination of microtubule polarity by cryo-electron microscopy.
abstract: BACKGROUND
  Microtubules are tubular polymers of tubulin dimers, which are arranged head-to-tail in protofilaments that run lengthwise along the microtubules, giving them an overall structural polarity. Many of the functions of microtubules depend on this polarity, including directed intracellular transport and chromosome segregation during mitosis. The determination of microtubule polarity for lengthwise views of microtubules observed by electron microscopy has not previously been possible. Here, we present methods for directly determining the polarity of individual microtubules imaged by cryo-electron microscopy.
  
  
  RESULTS
  When observed in vitreous ice by cryo-electron microscopy, microtubules with skewed protofilaments show arrowhead moiré patterns. We have used centrosome nucleated microtubules to relate the directionality of the moiré patterns to microtubule polarity. We show that the arrowheads point towards the plus end of microtubules with protofilaments having a right-handed skew, and towards the minus end of microtubules with protofilaments having a left-handed skew. We describe two methods for determining the handedness of the protofilament skew. The first method uses two or more tilted views. The second method involves analysis of the diffraction patterns of the microtubule images.
  
  
  CONCLUSIONS
  It is now possible to determine directly the polarity of in vitro assembled microtubules from cryo-electron micrographs. This will be helpful in a number of types of studies, including studies of the three-dimensional structure of microtubules interacting with motor proteins, as knowledge of the polarity of the microtubule is essential to understand motor directionality.
authors.affiliation: Structural Biology Programme, European Molecular Biology Laboratory, Meyerhofstrasse 1, Postfach 10.2209, D-69117 Heidelberg, Germany. chretien@embl-heidelberg.de
authors.forename: D
authors.initials: D
authors.lastname: Chrétien
authors.forename: J M
authors.initials: JM
authors.lastname: Kenney
authors.forename: S D
authors.initials: SD
authors.lastname: Fuller
authors.forename: R H
authors.initials: RH
authors.lastname: Wade
mesh_terms: D003080:Cold Temperature; D008854:Microscopy, Electron / Q000379:methods*; D008870:Microtubules / Q000737:chemistry*