Interaction of the yeast gamma-tubulin complex-binding protein Spc72p withKar1p is essential for microtubule function during karyogamy

The spindle pole body component Kar1p has a function in nuclear fusion duri ng conjugation, a process known as karyogamy, The molecular role of Kar1p d uring this process is poorly understood. Here we show that the yeast gamma- tubulin complex-binding protein Spc72p interacts directly with the N-termin al domain of Kar1p, thereby targeting the gamma-tubulin complex to the half bridge, a substructure of the spindle pole body, where it organizes microt ubule:;. This binding of Spc72p to Kar1p has only a minor role during veget ative growth, whereas it becomes essential for karyogamy in mating cells, e xplaining the important role of Kar1p in this process. We also show that th e localization of Spc72p within the spindle pole body changes throughout th e cell cycle and even more strongly in response to mating pheromone. Taken together, these observations suggest that the relocalization of Spc72p with in the spindle pole body is the 'landmark' event in the pheromone-induced r eorganization of the cytoplasmic microtubules.