Size Control and Commitment to Division in S. cerevisiae

描述（由申请人提供）：分裂细胞必须协调其分裂速度与细胞生长和蛋白质合成的速度。在许多微生物中，尤其是酵母中，这种协调是通过一种大小控制机制来实现的：当细胞生长到一定的“临界大小”时，它会触发细胞周期的承诺。因此，分工取决于增长。我们的中心兴趣在于理解“临界尺寸”是如何被细胞测量或决定的；换句话说，从机制上讲，是什么让细胞生长到一定的大小，从而使细胞进入细胞周期？在葡萄球菌中，人们早就认识到G1周期蛋白Cln3在这种协调中起作用。在临界细胞大小时，Cln3-Cdc28激酶激活转录因子SBF和MBF，进而诱导超过200个基因的转录，这些基因对细胞周期进入s期至关重要。最近，我们发现了两个额外的大小控制机制的初步证据，一个涉及翻译，另一个涉及储存碳水化合物的积累。在这里，我们将研究这三种尺寸控制机制。首先，我们将研究Cln3激活SBF转录因子的机制，以及为什么这种激活是大小依赖的。最近一项令人兴奋的结果表明，Cln3的数量正根据基因组中SBF结合位点的数量进行滴定，这构成了尺寸测量装置。其次，我们将研究翻译质量和数量的大小变化。这些变化可能受到大小控制基因wh3和wh4以及神秘的“翻译因子”Tif51的影响。第三，对于在营养不良条件下生长的细胞，出现了另一种大小控制，这种大小控制测量内部储存碳水化合物（糖原和海藻糖）的水平。我们将对这一机制进行描述和探讨。这三条通路相互联系，相互影响。这三种途径在哺乳动物细胞中可能有类似的（如果不是同源的）途径。

DESCRIPTION (provided by applicant): Dividing cells must co-ordinate their rate of division with their rate of cell growth and protein synthesis. In many microbes, and in yeast in particular, this co-ordination is achieved by a size control mechanism: when the cell grows to a certain "critical size", it triggers commitment to the cell cycle. Thus division depends on growth. Our central interest is in understanding how "critical size" is measured or determined by the cell; in other words, mechanistically, what is it about growth to a certain size that allows cells to commit to the cell cycle? In S. cerevisiae, it has long been recognized that the G1 cyclin Cln3 plays a role in this co-ordination. At critical cell size, a Cln3-Cdc28 kinase activates the transcription factors SBF and MBF, which in turn induce the transcription of over 200 genes important for cell cycle progress into S-phase. More recently, we have found preliminary evidence for two additional mechanisms of size control, one involving translation, and one involving accumulation of storage carbohydrates. Here, we will study these three mechanisms of size control. First, we will study the mechanism by which Cln3 activates the SBF transcription factor, and why this activation is size-dependent. An exciting recent result suggests that the amount of Cln3 is being titrated against the number of SBF binding sites in the genome, and this constitutes the size measurement device. Second, we will study size-dependent changes in the quality and quantity of translation. These changes may be influenced by the size control genes WHI3 and WHI4, and by the mysterious "translation factor" Tif51. Third, for cells growing under poor nutrient conditions, another kind of size control appears, and this size control measures the levels of internal storage carbohydrates (glycogen and trehalose). We will characterize and explore this mechanism. These three pathways have interconnections and crosstalk. All three of these pathways may have analogous if not homologous pathways in mammalian cells.