During asymmetric cell division, a number of organelles are inherited in an age-dependent manner. A prominent and highly conserved example of age-dependent segregation are the microtubule-organizing centers (MTOCs; mammalian centrosome and yeast spindle pole body - SPB) that mediate the position of the mitotic spindle within most dividing cells, except plants. In budding yeast for example, the daughter cell preferentially inherits the old SPB whereas the mother cell keeps the new SPB. Yet little is known about how cells distinguish between pre-existing and newly synthetized organelles. The MTOCs of many asymmetrically dividing cells nucleate more astral microtubules on one of the two spindle poles than the othe... More
During asymmetric cell division, a number of organelles are inherited in an age-dependent manner. A prominent and highly conserved example of age-dependent segregation are the microtubule-organizing centers (MTOCs; mammalian centrosome and yeast spindle pole body - SPB) that mediate the position of the mitotic spindle within most dividing cells, except plants. In budding yeast for example, the daughter cell preferentially inherits the old SPB whereas the mother cell keeps the new SPB. Yet little is known about how cells distinguish between pre-existing and newly synthetized organelles. The MTOCs of many asymmetrically dividing cells nucleate more astral microtubules on one of the two spindle poles than the other. This differential activity frequently correlates with MTOC age and is thought to drive orientation of the mitotic spindle. How this asymmetric MTOC activity is established is unclear. We investigated how maturation differences drive the distinct behaviors of the SPBs during spindle orientation. Using fluorophores with different maturation kinetics, we found that the SPB component Cnm67 exchanged at the SPBs, whereas Spc72 did not. However, these two proteins were rapidly as abundant on the new SPB as on the pre-existing one. Super-resolution microscopy established that this also applied to the γ-tubulin complex. Finally, astral microtubule organization correlated with the subcellular position rather than the age of the SPBs. Kar9-dependent orientation of the spindle and not intrinsic differences between pre-existing and new SPBs drove their differential activity in astral microtubule organization. Together, our data establish that spatial regulation controls the distinct behavior of pre-existing and new SPBs. It is also unclear, how cells differentiate pre-existing and newly synthesized material and use this information to orient their mitotic segregation. Here, we identify the SPB Inheritance Network (SPIN, composed of the kinases Swe1/Wee1 and Kin3/Nek2, and the acetyltransferase NuA4/Tip60) that functioned as a writer to encode age information onto SPBs. Most upstream, Swe1 phosphorylated Nud1/Centriolin on young SPBs before the new SPB was fully assembled. Inactivation of Swe1 upon cell cycle entry protected the new-born SPB from being marked. This age-code was subsequently relayed and maintained by Kin3 and NuA4 in successive divisions. Downstream of the SPIN, the Hippo-pathway regulator, Bfa1/Bub2, acted as a reader of this SPB code to direct the spindle positioning protein Kar9 towards the pre-existing SPB, facilitating its agedependent partition. Thus, the coordinated activity of SPIN and SPB assembly ensures the encoding of their age onto SPBs to enable their age-dependent segregation at mitosis.
