Determining the molecular mechanisms underlying the size-scaling of biosynthesis

这个项目的首要目标是了解细胞生长是如何触发细胞分裂的。人们早就知道，为了启动细胞分裂，通常需要细胞生长，但这一过程的方式仍不清楚。核心问题是，与使其分裂的小细胞相比，大细胞在生物化学上有什么不同？为了解决这个问题，这个项目是基于研究人员最近在理解生长如何触发萌芽酵母分裂方面的突破。与预期相反，研究人员发现，生长并没有增加细胞分裂激活剂的活性。相反，细胞生长通过稀释一种抑制细胞分裂的蛋白质来触发分裂。该项目利用这一概念上的突破，将细胞生长如何触发细胞分裂的问题重新表述为细胞生长如何导致不同数量的特定蛋白质产生的问题。换句话说，是什么机制确保激活细胞分裂的蛋白质与细胞的大小成比例，从而使它们的浓度不会随着细胞的变大而改变？此外，大细胞和小细胞如何产生相同数量的细胞分裂抑制物，从而使它们的浓度随着细胞变大而降低，从而触发分裂？通过回答这些问题，该项目将确定细胞用来将细胞生长与细胞分裂联系起来的分子机制，这是细胞和发育生物学中的一个基本问题。该项目的更广泛影响包括研究的内在本质，因为所有分裂细胞都需要调节细胞大小。其他活动包括培训本科生和研究生以及博士后研究人员。还将开展广泛促进量化培训的活动，同时向高中生推广。细胞的生长、分裂和生物合成必须精心安排，以维持有效的细胞功能。要做到这一点，细胞大小和生物合成是密不可分的。总RNA和蛋白质合成速率与大小成比例增加，因此基因产物的拷贝数随细胞体积的增加而增加。这确保了在细胞生长过程中保持恒定的大分子浓度，以维持细胞的有效功能。然而，如果细胞变得过大，生物合成的这种比例就会崩溃，细胞生理和有效生长就会受到严重影响。为了防止这种情况，萌芽酵母细胞通过将细胞生长与细胞分裂结合起来，将其大小保持在细胞功能的最佳范围内。研究人员发现，细胞周期激活剂Cln3的合成与细胞大小成比例增加，而发芽酵母细胞周期抑制因子Whi5的合成与细胞大小无关，因此每个细胞周期都会产生恒定数量的Whi5分子。因此，在较大的细胞中，WHI5的浓度更稀薄，这会在较大的细胞中触发更早的细胞周期进入。因此，与细胞大小成正比的细胞周期激活剂和与细胞大小无关的细胞周期抑制因子的不同大小依赖关系将生长与分裂结合在一起，以将细胞维持在生理上最佳的大小范围内。细胞周期激活剂和抑制物合成中的不同大小依赖关系提出了两个关键问题，该项目将解决这两个问题。首先，是什么确保了大多数蛋白质和信使核糖核酸的合成与细胞大小成比例增加？其次，是什么让像Whi5这样的蛋白质打破一般规则，独立于细胞大小表达？这个项目将使用模式生物发芽酵母来解决这两个问题，使用蛋白质组学、遗传学和定量活细胞成像的组合。该项目的成功完成将解决细胞生物学中的一个基本问题，即细胞大小、细胞分裂和生物合成如何密切和机械地联系起来促进细胞功能。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The overarching goal of this project is to understand how cell growth triggers cell division. It has long been known that there is often a requirement for cell growth in order to initiate cell division, but the way this works has remained unknown. The central question is what is biochemically different about a large cell compared to a small cell that makes it divide? To address this question, this project is based on a recent breakthrough by the investigator in understanding how growth triggers division in budding yeast. Contrary to expectations, the investigator found that growth did not increase the activity of a cell division activator. Instead, cell growth triggers division by diluting a protein that inhibits cell division. This project leverages this conceptual breakthrough to rephrase the question of how cell growth triggers cell division to the question of how cell growth results in different amounts of specific proteins being made. In other words, what are the mechanisms that ensure that proteins activating cell division are made in proportion to the size of the cell so that their concentrations do not change as cells get larger? Moreover, how do big and small cells make the same number of cell division inhibitors so that their concentrations decrease as cells grow larger to trigger division? By answering these questions, this project will determine the molecular mechanisms cells use to link cell growth to cell division, which is a fundamental question in cell and developmental biology. The Broader Impact of the project includes the intrinsic nature of the research as all dividing cells need to regulate cell size. Additional activities include the training of undergraduate and graduate students along with post-doctoral researchers. Activities to broadly promote quantitative training along with outreach to high school students will also be carried out. Cell growth, division, and biosynthesis must be carefully orchestrated to maintain efficient cellular function. To achieve this, cell size and biosynthesis are inextricably linked. Total RNA and protein synthesis rates increase in proportion to size so that gene product copy numbers scale with cell volume. This ensures that macromolecule concentrations are kept constant as a cell grows to maintain efficient cellular function. However, if cells become excessively large, this scaling of biosynthesis collapses and cellular physiology and efficient growth become severely compromised. To prevent this, budding yeast cells maintain their size within an optimal range for cellular function by coupling cell growth to cell division. The researchers found that the synthesis of the cell cycle activator Cln3 increased in proportion to cell size, while the synthesis of the budding yeast cell cycle inhibitor Whi5 was independent of cell size such that a constant number of Whi5 molecules are made in each cell cycle. Thus, Whi5 concentration is more dilute in larger cells which triggers an earlier cell cycle entry in larger cells. Thus, the differential size-dependencies of cell cycle activators, which scale in proportion to cell size, and cell cycle inhibitors, which are independent of cell size, couple growth to division to maintain cells within a physiologically optimal size range. The differential size-dependencies in the synthesis of cell cycle activators and inhibitors raise two key questions will be addressed in this project. First, what ensures most protein and mRNA synthesis increases in proportion to cell size? Second, what allows proteins, such as Whi5, to break the general rule and be expressed independently of cell size? This project will employ the model organism budding yeast to address these twin questions using a combination of proteomics, genetics, and quantitative live cell imaging. Successful completion of the project will thereby resolve a fundamental question in cell biology of how cell size, cell division, and biosynthesis are intimately and mechanistically linked to promote cellular function.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.