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离出来，并且必须在内质网(ER)内正确折叠，然后才能进行细胞内运输到细胞表面。该细胞精心设计了一种名为“未折叠蛋白质反应”的复杂机制，以消除不能采用正确结构的蛋白质。这种质量控制机制的一个显著特征是通过内质网相关的降解途径靶向和降解错误折叠的蛋白质。病理条件会导致“未折叠蛋白反应”的持续激活，细胞无法清除错误折叠的蛋白被认为是导致神经退行性疾病和II型糖尿病的原因。我们的实验室正在使用酿酒酵母作为模式生物来研究一类特殊的膜蛋白的分类和质量控制机制的基本方面，这种蛋白被称为GPI蛋白，它与富含胆固醇和神经鞘脂脂的特殊膜微域(即所谓的脂筏)有关。在过去的几年里，我们提供的证据表明，名为Yps1p的GPI蛋白是存在于细胞表面的真菌酶家族的一部分，在其细胞内转运到细胞表面的过程中，似乎与其他膜蛋白保持分离。我们还表明，在未折叠的蛋白反应中，Yps1p在消除细胞表面的GPI蛋白方面发挥了作用。重要的是，我们还提供了证据表明，在脂质失衡的条件下，在内质网内没有错误折叠的蛋白质的情况下，GPI蛋白通过一种涉及磷脂酶的新的质量控制机制从细胞表面消除。这一新机制与“未折叠蛋白质反应”有关，但目前还不清楚是如何进行的。在目前的方案中，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”是如何联系在一起的。我们还建议进一步证明Yps1p在“未折叠蛋白反应”中是如何识别和消除GPI蛋白的。拟议的研究将加深我们对GPI蛋白如何从其他膜蛋白中分离出来的理解，并应有助于阐明在没有错误折叠的蛋白质的情况下，在脂质失衡的条件下，是什么触发了“未折叠蛋白质反应”。

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.