Structural Polarity of Kinetochore Microtubules

The polarity of kinetochore microtubules (MTs) has been studied in lysed PtK1 cells by polymerizing hook-shapdd sheets of neurotubulin onto the walls of preexisting cellular MTs in a fashion that reveals their structural polarity . Three different approaches are presented here : (a) we have screened the polarity of all MTs in a given spindle cross section taken from the region between the kinetochores and the poles, (b) we have determined the polarity of kinetochore MTs selectively in cold-treated spindles ; this approach takes advantage of the fact that kinetochore MTs are more stable to cold treatment than other spindle MTs; and (c) we have tracked bundles of kinetochore MTs from the vicinity of the pole to the outer layer of the kinetochore in cold-treated cells. In an anaphase cell, 90-95% of all MTs in an area between the kinetochores and the poles are of uniform polarity with their plus ends (i .e ., fast growing ends) distal to the pole . In cold-treated cells, all bundles of kinetochore MTs show the same polarity ; the plus ends of the MTs are located at the kinetochores . We therefore conclude that kinetochore MTs in both metaphase and anaphase cells have the same polarity as the aster MTs in each half-spindle . These results can be interpreted in two ways : (a) virtually all MTs are initiated at the spindle poles and some of them are "captured" by matured kinetochores using an as yet unknown mechanism to bind the plus ends of existing MTs; (b) the growth of kinetochore MTs is initiated at the kinetochore in such a way that the fast growing MT end is proximal to the