Polarity of 2-D and 3-D maps of tubulin sheets and motor-decorated sheets.

As there has been considerable disagreement about the polarity of two-dimensional and three-dimensional maps of tubulin and tubulin-motor complexes, we have re-investigated this issue by analyzing images of tubulin sheets and motor-decorated sheets found at the distal ends of microtubules nucleated from centrosomes and sperm tails. Our results are unambiguous and define the relationship of the structural features in the maps to the microtubule plus and minus ends. Possible reasons for previous mis-assignments of the polarity are discussed briefly.