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First published online August 24, 2006
doi: 10.1242/10.1242/jcs.03193

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Microtubule dynamics in the budding yeast mating pathway

Department of Biology, University of North Carolina, 622 Fordham Hall, Chapel Hill, NC 27599, USA

View larger version (83K): [in a new window]   Fig. 1. Microtubule (MT) and plus-end-tracking protein (+TIP) distribution in the budding yeast mating pathway. (A) MT morphologies through the first zygotic division. Upper panel shows fluorescent images of GFP fused to tubulin (GFP-Tub1p) in the mating pathway. Lower panel are corresponding DIC images. (B) +TIP localization in pheromone-treated cells. Schematic of mating cell is shown on the left. Formation of the shmoo after cell polarization allows MT plus ends to interact with the shmoo tip. Bik1p-3xGFP localizes to MT plus ends in the shmoo tip, free MT plus ends, and to the SPB (right, bright-field image is displayed in lower right corner). Bars, 2 µm.

View larger version (26K): [in a new window]   Fig. 2. Schematic of +TIP function during nuclear orientation to the shmoo tip. (A) In unbudded cells, Kar9p (red) and Bik1p (purple) are transported along the MT (green) by the motor protein Kip2p (brown) to the plus end where Bim1p (blue) binds. Kar3p (cyan) and Cik1p (yellow) form heterodimers that localize near the plus ends. (B) After cell polarization, the MT plus end interacts with actin cables (red) via Myo2p (gray). Myo2p links to Bim1p through Kar9p and moves the nucleus towards the shmoo tip. (C) Once at the shmoo tip, the plus end interacts persistently with the shmoo tip. This interaction could be mediated by proteins at the shmoo tip cortex that act as attachment factors.

View larger version (28K): [in a new window]   Fig. 3. Models for MT interaction with the shmoo tip. (A) As hypothesized by Maddox et al. (Maddox et al., 2003), an unknown binding factor may interact with Kar3p to keep this motor near the cortex. Bim1p may link the plus end to the shmoo tip through Kar9p. During polymerization, Bim1p and Kar9p maintain the interactions between the plus end and the shmoo tip. A switch to depolymerization will cause Bim1p to leave the plus end while Kar3p maintains the attachment of the MT to the tip. (B) Plus end cycling hypothesis. In this model, the MT plus end can detach from the shmoo tip at some frequency governed by the affinity of +TIPs for either the cortex or the ends of actin cables. After detachment, the MT shrinks back to the SPB while a newly nucleated MT grows and interacts with the actin network. The newly nucleated MT is transported by Myo2p along the actin network towards the shmoo tip, generating movement of the nucleus towards the mating projection. Once at the shmoo tip, the MT attaches and begins to polymerize, moving the nucleus away from the shmoo tip. Therefore, the combination of MT transport and MT dynamics at the shmoo tip generates nuclear oscillations.

View larger version (70K): [in a new window]   Fig. 4. Karyogamy in budding yeast. (A) Models for nuclear congression in budding yeast. (Left) Sliding cross-bridge model for nuclear congression. In this model, Kar3p-Cik1p slides MTs past one another and then depolymerization occurs at the SPBs. (Right) Plus end model for nuclear congression. MTs are linked at the plus end by Kar3p-Cik1p and then depolymerize, drawing both nuclei together for karyogamy. (B) MT dynamics in the wild type and karyogamy mutants. (Left) Wild-type MT dynamics during nuclear congression. (Middle) MT dynamics when nuclear orientation is defective. MTs grow and shrink the cytoplasm and rely on stochastic interactions to cross-link and perform nuclear congression. (Right) MT dynamics when cross-linking is defective. MTs undergo dynamic instability in the cytoplasm but never interact. A failure to undergo karyogamy does not block the zygotic bud from forming, resulting in the assembly of two mitotic spindles within the same cell.