Microtubule assembly and phage morphogenesis: new results and classical paradigms.

The classical analyses of phage morphogenesis provide experimental paradigms for dissecting other assembly pathways. A new set of results, derived from examination of the quantitative controls of tubulin levels and microtubule assembly in Saccharomyces cerevisiae, evokes the 'balance of components' hypothesis. These results show that balanced levels of the tubulin proteins are crucial for microtubule assembly. Imbalances leading to excess beta tubulin have far more deleterious consequences than those leading to excess alpha tubulin, including dramatic cellular toxicity, quantitative depolymerization of cellular microtubules, and self-aggregation of the excess beta tubulin. These and other results suggest that beta tubulin may possess a unique ability to interact with a component of microtubule nucleating sites, and provide a rationale for the universal polarity of nucleated microtubules.