Sorting and shedding of GPI proteins

输出到细胞外的蛋白质必须从其他蛋白质中分选出来，并且必须在内质网（ER）内适当折叠，然后才能将其细胞内运输到细胞表面。细胞已经精心设计了一种称为“未折叠蛋白质反应”的复杂机制，以消除无法采用其正确结构的蛋白质。这种质量控制机制的一个显著特征是通过ER相关降解途径靶向和降解错误折叠的蛋白质。病理条件可导致“未折叠蛋白质反应”的持续激活，并且细胞无法消除错误折叠的蛋白质被认为会导致神经退行性疾病和II型糖尿病。本实验室使用的是啤酒酵母S。酿酒酵母作为模式生物来研究与称为GPI蛋白的特定类别的膜蛋白的分选和质量控制机制相关的基本方面，GPI蛋白与富含胆固醇和鞘脂的专门的膜微结构域（所谓的“脂质筏”）相关。 在过去的几年中，我们提供的证据表明，GPI蛋白命名为Yps1p，这是一个家庭的真菌酶存在于细胞表面的一部分，似乎是保持隔离从其他膜蛋白在其细胞内运输到细胞表面。我们还发现，Yps1p在未折叠蛋白反应期间细胞表面GPI蛋白的消除中起作用。重要的是，我们还提供了证据表明，在脂质不平衡的条件下，在ER内没有错误折叠的蛋白质，GPI蛋白质从细胞表面消除了一种新的质量控制机制，涉及磷脂酶。这种新机制与“未折叠蛋白质反应”有关，但尚不清楚如何联系。 在目前的提案中，我们设计了实验来建立这种新的质量控制机制和“未折叠蛋白质反应”是如何连接的。我们还建议进一步记录Yps1p如何识别和消除GPI蛋白在“未折叠的蛋白质反应”。拟议的研究应该加深我们对GPI蛋白如何从其他膜蛋白中分选出来的理解，并且应该有助于阐明在不存在错误折叠蛋白的情况下，在脂质失衡的条件下触发“未折叠蛋白反应”的原因。

Proteins that are exported out of the cell must be sorted out from other proteins and must properly fold within the endoplasmic reticulum (ER) before their intracellular transport to the cell surface can take place. The cell has elaborated a sophisticated mechanism called the 'unfolded protein response' to eliminate proteins that cannot adopt their proper structure. A salient feature of this quality control mechanism is the targeting and degradation of misfolded proteins by the ER-associated degradation pathway. Pathological conditions can lead to the sustained activation of the 'unfolded protein response' and, inability of the cell to eliminate misfolded proteins is believed to cause neurodegenerative diseases and type II diabetes. Our laboratory is using the brewer yeast S. cerevisiae as a model organism to study fundamental aspects related to the sorting and quality control mechanism of a particular class of membrane proteins called GPI proteins, which associate to specialized membrane microdomains rich in cholesterol and sphingolipid (so called 'lipid rafts'). In the past few years we provided evidence that the GPI protein named Yps1p, which is part of a family of fungal enzymes present at the cell surface, appears to be maintained segregated apart form other membrane proteins during its intracellular transport to the cell surface. We also showed that Yps1p plays a role in the elimination of GPI proteins at the cell surface during the unfolded protein response. Importantly, we also provided evidence that in conditions of lipid imbalance, in the absence of misfolded proteins within the ER, GPI proteins are eliminated from the cell surface by a novel quality control mechanism implicating phospholipases. This novel mechanism is linked to the 'unfolded protein response', but it is not clear how. In the current proposal, we have designed experiments to establish how this novel quality control mechanism and the 'unfolded protein response' are connected. We also propose to further document how Yps1p recognizes and eliminates GPI proteins during the 'unfolded protein response'. The proposed studies should deepen our understanding on how GPI proteins are sorted from other membrane proteins and should help elucidating what triggers the 'unfolded protein response' in conditions of lipid imbalance, in the absence of misfolded proteins.