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The Mitotic Spindle02:27

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Updated: Mar 14, 2026

Combining Mitotic Cell Synchronization and High Resolution Confocal Microscopy to Study the Role of Multifunctional Cell Cycle Proteins During Mitosis
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Molecular basis of Kar9-Bim1 complex function during mating and spindle positioning.

Cristina Manatschal1, Ana-Maria Farcas2, Miriam Steiner Degen1

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Kar9 utilizes distinct microtubule-binding sites to regulate nuclear fusion and spindle positioning in yeast. These interactions allow Kar9 to control cell proliferation and differentiation processes.

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Genetics

Background:

  • The Kar9 pathway is crucial for nuclear fusion during yeast mating and spindle alignment during metaphase.
  • Understanding how Kar9 orchestrates these distinct cellular processes remains an open question.

Purpose of the Study:

  • To investigate the molecular mechanisms by which Kar9 mediates its diverse functions.
  • To elucidate the role of Kar9's interaction with Bim1 (an EB1 orthologue) in regulating microtubule dynamics and cellular events.

Main Methods:

  • Biochemical assays to characterize Kar9-Bim1 interactions.
  • Analysis of Kar9 binding sites, including novel motifs.
  • Functional studies in budding yeast to assess Kar9's role in nuclear movement, karyogamy, and spindle positioning.

Main Results:

  • Three distinct sites in Kar9's C-terminal domain mediate interaction with Bim1.
  • Site2 and Site3 are essential for Kar9 recruitment to microtubule tips, nuclear movement, and karyogamy.
  • Site2 uniquely functions in spindle positioning during metaphase, while Site1 has an inhibitory role during mating.
  • The Kar9-Bim1 complex also exhibits microtubule-independent functions during mating.

Conclusions:

  • Multiple, partially redundant EB1-binding sites allow Kar9 to modulate its biochemical properties.
  • This modulation enables Kar9 to promote distinct cellular processes, including proliferation and differentiation.
  • Kar9's interaction with Bim1 is a key regulatory hub for diverse cellular functions.