G PROTEIN-MEDIATED NUTRITIONAL SIGNALING IN YEAST

细胞生长受响应细胞外刺激的信号转导通路调节。在酵母中，促进生长的信号被认为是营养的可用性。营养信号涉及Ras蛋白和腺苷酸环化酶的激活，其过程尚不清楚。本研究的结果表明，酵母中的营养信号也涉及G蛋白偶联受体对G蛋白α亚基的激活。一些观察结果表明，这一途径在对氮饥饿的反应中被激活。例如，缺乏受体或G蛋白的菌株在假菌丝生长方面存在缺陷，这是一种由氮饥饿诱导的发育开关。该项目的长期目标是描述G蛋白介导的信号通路对营养条件的反应所采用的机制。经典遗传学和蛋白质生物化学将用于阐明基因及其编码蛋白在这一过程中的作用。本项目的目的是表征和鉴定一种新描述的G蛋白偶联受体的配体，并确定该受体何时具有生理活性。第一个具体目的是研究Gpr1p受体与其配体结合的条件。这些研究将通过将Gpr1p受体偶联到信息素反应途径，并使用一个成熟的报告结构来检测该途径的激活来进行。第二个具体目标将使用Gpr1p报告系统来表征和识别Gpr1p配体。这种配体的鉴定将是激活真核生物营养信号通路的受体结合分子的第一个例子之一。最后的具体目的是表征Gpr1p受体表达和稳定性的调节。对这种新型信号系统的研究将有助于理解真核生物中的营养信号，这是一个许多重要问题尚未解决的领域。在阐明信号转导机制方面取得了重大进展，在生物体中可以很容易地进行复杂的遗传操作。对酵母生长控制的全面了解可能会为哺乳动物的生长控制提供新的见解，这也是通过G蛋白和Ras信号调节的。这些信息对于理解癌变过程至关重要，在癌变过程中，细胞逃避这种调节并表现出不受控制的生长。

Cell growth is regulated by signal transduction pathways that respond to extracellular stimuli. In yeast, the signal that promotes growth is thought to be the availability of nutrients. Nutritional signaling involves the activation of Ras proteins and adenylyl cyclase through a process that is not well understood. Results presented in this proposal demonstrate that nutritional signaling in yeast also involves the activation of a G protein alpha-subunit by a G protein-coupled receptor. Several observations suggest that this pathway is activated in response to nitrogen starvation. For example, strains lacking the receptor or the G protein are defective for pseudohyphal growth, a developmental switch that is induced by nitrogen starvation. The long-term objective of this project is a description of the mechanisms employed by the G protein-mediated pathway to signal in response to nutritional conditions. Classical genetics and protein biochemistry will be used to elucidate the roles of genes and their encoded proteins in this process. The aims of this project are to characterize and identify the ligand for a newly described G protein-coupled receptor and to determine when the receptor is physiologically active. The first specific aim will investigate the conditions under which the Gpr1p receptor is bound to its ligand. These studies will be performed by coupling the Gpr1p receptor to the pheromone response pathway and using a well-established reporter construct that detects activation of the pathway. The second specific aim will employ the Gpr1p reporter system to characterize and identify the Gpr1p ligand. Identification of such a ligand would be one of the first examples of a receptor-binding molecule that activates a nutritional signaling pathway in a eukaryotic organism. The final specific aim will characterize the regulation of Gpr1p receptor expression and stability. Investigation of this novel signaling system will contribute to an understanding of nutritional signaling in eukaryotes, an area in which many important questions have not yet been addressed. Significant progress in elucidating signal transduction mechanisms has been made in organisms in which sophisticated genetic manipulations can easily be performed. A complete understanding of growth control in yeast is likely to provide insight into growth control in mammals, which is also regulated by signaling through G proteins and Ras. Such information is essential for understanding the process of carcinogenesis, in which cells escape this regulation and exhibit uncontrolled growth.