Browsing by Author "Chen, Hsin"

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    Cytoskeletal Dynamics and the Temporal Control of Yeast Morphogenesis
    (2012) Chen, Hsin

    The cells of the budding yeast Saccharomyces cerevisiae undergo a robust morphological cycle, involving reorganization of the actin cytoskeleton, septin ring formation, and polarized growth. These events are crucial to the formation of a fully-equipped and properly-shaped bud, which gives rise to the daughter cell. The budding yeast, as a well-established genetic model system, has attracted numerous investigations aimed at uncovering the underlying principles of morphogenesis.

    Despite the important roles of the septin ring and collar in morphogenesis and cytokinesis, little is known about how they are assembled. We found that septins are recruited to the ring and collar following a tri-linear assembly/disassembly scheme.

    Polarization of actin cables enable directed secretion and growth. The formin Bni1p, an actin nucleator, is thought to polarize actin cables in response to the direct regulation by the master polarity regulator, Cdc42p. However, we found that all the known Bni1p-regulatory pathways are dispensable, including the direct regulation by Cdc42p, and we uncovered a novel pathway linking Bni1p to Cdc42p via the Cdc42p effector, Gic2p.

    Yeast morphogenesis is tightly coupled with the cell cycle. Contrary to the prevailing model, we found that G1-CDK activity, albeit required for bud emergence, is not needed to trigger polarization. This finding suggests that cells are in a default polarized state, which is negatively regulated by the G2-CDK.

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    Inhibitory GEF phosphorylation provides negative feedback in the yeast polarity circuit
    (Current Biology, 2014) Kuo, Chun-Chen; Savage, Natasha S; Chen, Hsin; Wu, Chi-Fang; Zyla, Trevin R; Lew, Daniel J
    Cell polarity is critical for the form and function of many cell types. During polarity establishment, cells define a cortical "front" that behaves differently from the rest of the cortex. The front accumulates high levels of the active form of a polarity-determining Rho-family GTPase (Cdc42, Rac, or Rop) that then orients cytoskeletal elements through various effectors to generate the polarized morphology appropriate to the particular cell type [1, 2]. GTPase accumulation is thought to involve positive feedback, such that active GTPase promotes further delivery and/or activation of more GTPase in its vicinity [3]. Recent studies suggest that once a front forms, the concentration of polarity factors at the front can increase and decrease periodically, first clustering the factors at the cortex and then dispersing them back to the cytoplasm [4-7]. Such oscillatory behavior implies the presence of negative feedback in the polarity circuit [8], but the mechanism of negative feedback was not known. Here we show that, in the budding yeast Saccharomyces cerevisiae, the catalytic activity of the Cdc42-directed GEF is inhibited by Cdc42-stimulated effector kinases, thus providing negative feedback. We further show that replacing the GEF with a phosphosite mutant GEF abolishes oscillations and leads to the accumulation of excess GTP-Cdc42 and other polarity factors at the front. These findings reveal a mechanism for negative feedback and suggest that the function of negative feedback via GEF inhibition is to buffer the level of Cdc42 at the polarity site. © 2014 Elsevier Ltd.