Spindle pole body-anchored Kar3 drives the nucleus along microtubules from another nucleus in preparation for nuclear fusion during yeast karyogamy

Abstract

Nuclear migration during yeast karyogamy, termed nuclear congression, is required to initiate nuclear fusion. Congression involves a specific regulation of the microtubule minus end-directed kinesin-14 motor Kar3 and a rearrangement of the cytoplasmic microtubule attachment sites at the spindle pole bodies (SPBs). However, how these elements interact to produce the forces necessary for nuclear migration is less clear. We used electron tomography, molecular genetics, quantitative imaging, and first principles modeling to investigate how cytoplasmic microtubules are organized during nuclear congression. We found that Kar3, with the help of its light chain, Cik1, is anchored during mating to the SPB component Spc72 that also serves as a nucleator and anchor for microtubules via their minus ends. Moreover, we show that no direct microtubule-microtubule interactions are required for nuclear migration. Instead, SPB-anchored Kar3 exerts the necessary pulling forces laterally on microtubules emanating from the SPB of the mating partner nucleus. Therefore, a twofold symmetrical application of the core principle that drives nuclear migration in higher cells is used in yeast to drive nuclei toward each other before nuclear fusion.

Dynamics of Male and Female Chromatin during Karyogamy in Rice Zygotes

“…An intensively investigated karyogamic event is the fusion of two haploid nuclei during yeast mating, in which cytoskeleton-dependent nuclear migration, termed nuclear congression, and chaperon/endoplasmic reticulum (ER) protein-dependent nuclear fusion have been reported (Kurihara et al, 1994;Melloy et al, 2009;Tartakoff and Jaiswal, 2009;Gibeaux et al, 2013). Additionally, the cell cycle arrest and resumption induced by fertilization have been well studied using animal eggs (for review, see Kishimoto, 2003).…”

Molecular Determinants of Sporulation in Ashbya gossypii

“…Strikingly, A. gossypii kar3 mutants were severely reduced in sporulation. In S. cerevisiae, Kar3 is required for nuclear congression to promote nuclear fusion during karyogamy (Gibeaux et al 2013). This therefore suggests that a key role for Kar3 in sporulation may be to promote karyogamy in A. gossypii.…”

“…Here, we analyzed the function of A. gossypii orthologs of S. cerevisiae KAR genes, which in S. cerevisiae either have a function both in mitosis and karyogamy or are specifically required for nuclear fusion during mating (Kurihara et al 1994). Kar3 plays a central role in pulling yeast nuclei together to prepare for karyogamy (Gibeaux et al 2013). A. gossypii kar3 mutants did not reduce growth speed of fast growing mature Ashbya hyphae and, therefore, Kar3 is not required for nuclear migration or positioning in vegetative hyphae.…”

Cell biology of yeast zygotes, from genesis to budding

“…Early evidence showed that a microtubule cable extends from each SPB toward the other and that microtubule function is required for congression [147, 148]. Although cable shortening and lateral sliding had been thought to accomplish congression, investigations based on EM tomography show that congression results from the cables establishing lateral contact with the opposite SPB, where the minus-end directed motor, Kar3, exerts a pulling force [149]. It remains unclear whether the primary role of the cable is to bring the nuclei together or rather to cause the SPBs to contact each other.…”