Spindle pole body history intrinsically links pole identity with asymmetric fate in budding yeast

Background: Budding yeast is a unique model for exploring differential fate in a cell dividing asymmetrically. In yeast, spindle orientation begins with the old spindle pole body (SPB) (from the preceding cell cycle) contacting the bud by its existing astral microtubules (aMTs) while the new pole delays astral microtubule organization. This appears to prime the inheritance of the old pole by the bud. The basis for this asymmetry and the discrimination of the poles by virtue of their history remain a mystery. Results: Here, we report that asymmetric aMT organization stems from an outstanding structural asymmetry linked to the SPB cycle. We show that the γ-tubulin nucleation complex (γTC) favors the old spindle pole, an asymmetry inherent to the outer plaque (the cytoplasmic face of the SPB). Indeed, Spc72 (the receptor for the γTC) is acquired by the new SPB outer plaque partway through spindle assembly. The significance of this asymmetry was explored in cells expressing an Spc72^1-276-Cnm67 fusion that forced symmetric nucleation at the SPB outer plaques. This manipulation triggered simultaneous aMT organization by both spindle poles from the outset and led to symmetric contacts between poles and the bud, effectively disrupting the program for spindle polarity. Temporally symmetric aMT organization perturbed Kar9 polarization by randomizing the choice of the pole to be guided toward the bud. Accordingly, the pattern of SPB inheritance was also randomized. Conclusions: Spc72 differential recruitment imparting asymmetric aMT organization represents the most upstream determinant linking SPB historical identity and fate.