Kar9 controls the nucleocytoplasmic distribution of yeast EB1

Abstract

The precise temporal and spatial concentration of microtubule-associated proteins (MAPs) within the cell is fundamental to ensure chromosome segregation and correct spindle positioning. MAPs form an intricate web of interactions among each other and compete for binding sites on microtubules. Therefore, when assessing cellular phenotypes upon MAP up-or downregulation, it is important to consider the protein interaction network between individual MAPs. Here, we show that changes in the amounts of the spindle positioning factor Kar9 specifically affect the distribution of yeast EB1 on spindle microtubules, without influencing other microtubule-associated interacting partners of Kar9, i.e. yeast XMAP215 and CLIP-170. Alterations in the distribution of yeast EB1 explain chromosome segregation defects upon knockout, overexpression or stabilization of Kar9 and provide an example for non-linear effects on MAP behavior after perturbation of their equilibrium.

Systematic analysis of microtubule plus-end networks defines EB-cargo complexes critical for mitosis in budding yeast

“…Thus, Kar9 and Bim1 are mutually dependent for their cytoplasmic localization and lack of Kar9 impacts the distribution of Bim1 between the cytoplasm and the nucleus. This finding supports a recently published model which states that nuclear-cytoplasmic shuttling of Kar9 controls the cytoplasmic localization of Bim1 (Schweiggert et al, 2016). Cik1 depletion had no detectable effect on the amount of Bim1 on the spindle, but instead caused an increased recruitment of Bim1 on astral microtubules ( Figure 3D, E, F ).…”

“…The increased association of Bik1 with cytoplasmic microtubules is required for nuclear migration in the absence of Bim1. This highlights the notion that nucleo-cytoplasmic distribution relies both on nuclear import and export signals in individual proteins, but also on the distribution of binding partners that can shift the equilibrium between protein pools in different compartments (Schweiggert et al, 2016).…”

A mechanistic model for nuclear migration in hyphae during mitosis

“…Interactions between astral microtubules and the cortex generate three types of forces on the microtubule-tips, viz., pushing force, pulling force and sweeping force [8,13,[56][57][58][59][60][61][62][63]. Pushing force repeals the tips away from the cortex, whereas pulling force attracts the tips toward the cortex and sweeping force pushes (slides) the tips toward the septin ring along the cortex [8,13,[56][57][58][59][60][61][62][63].…”

“…Interactions between astral microtubules and the cortex generate three types of forces on the microtubule-tips, viz., pushing force, pulling force and sweeping force [8,13,[56][57][58][59][60][61][62][63]. Pushing force repeals the tips away from the cortex, whereas pulling force attracts the tips toward the cortex and sweeping force pushes (slides) the tips toward the septin ring along the cortex [8,13,[56][57][58][59][60][61][62][63]. Pushing force is generated as cortex opposes polymerization of microtubule, pulling force is generated as the cortically anchored dyneins bind the microtubule and walk along the filament toward -ve and sweeping force is generated as cortical bim1-kar9-myo2 complex is formed on the microtubule-tip sliding toward the septin ring [8,13,[56][57][58][59][60][61][62][63].…”

“…Pushing force repeals the tips away from the cortex, whereas pulling force attracts the tips toward the cortex and sweeping force pushes (slides) the tips toward the septin ring along the cortex [8,13,[56][57][58][59][60][61][62][63]. Pushing force is generated as cortex opposes polymerization of microtubule, pulling force is generated as the cortically anchored dyneins bind the microtubule and walk along the filament toward -ve and sweeping force is generated as cortical bim1-kar9-myo2 complex is formed on the microtubule-tip sliding toward the septin ring [8,13,[56][57][58][59][60][61][62][63]. Among the three forces, sweeping force is mother-bud specific [8,13,[60][61][62][63] and the other two forces are active in both the mother and daughter cortices [56,58,59].…”