Microtubule polarity and spindle function

Abstract

The idea that microtubule (MT) polarity is important for spindle function was first proposed more than a decade ago. However, it was not until 1974 that this idea could be put to a direct test. The identification of methods for MT polymerization in vitro made it possible to use flagella and cilia as seeds for purified brain tubulin; it was demonstrated that under given conditions one end of the MT grows at a faster rate than the other. The two different ends were designated 'plus' and 'minus' end. Subsequently efforts were made to determine the polarity of spindle MTs with the help of these rate measurements. Mitotic centers as well as chromosomes were isolated and the rate of MT growth from these initiation sites was compared to the rates of MT growth from the two ends of flagella or cilia. The results from these experiments revealed that MTs were growing at a fast rate from both structures, suggesting that the plus ends of aster and kinetochore MTs are distal to these sites.