Geranylgeranyl-Mediated Regulation of Prenyltransferase UBIAD1: A Sensing Mechanism Controlling HMG CoA Reductase

施奈德角膜营养不良（SCD）是一种罕见的常染色体显性遗传眼病，其特征是 由于胆固醇的不正常积累而导致的角膜进行性混浊。相关的突变 在编码UBIAD 1（含有UbiA异戊烯基转移酶结构域的 蛋白-1），利用非甾醇类异戊二烯GGpp（香叶基香叶基焦磷酸）合成维生素 K2我们的初步研究表明，甾醇触发UBIAD 1与ER（内质网）的结合， HMG CoA还原酶。还原酶产生甲羟戊酸，一种重要的中间体， 胆固醇和非甾醇类异戊二烯的合成，如泛醌、维生素K2、多萜醇和法呢基 以及与许多细胞蛋白质相连的香叶基香叶基。类固醇调节的泛素化是 还原酶的ERAD（ER相关降解）是必需的。GGpp抑制UBIAD 1与还原酶的结合， 其允许还原酶的最大ERAD和UBIAD 1的ER至高尔基体转运。消除UBIAD 1 减轻还原酶ERAD对GGpp的需求，表明反应被异戊二烯基转移酶抑制。 SCD相关的UBIAD 1突变体抵抗GGpp诱导的还原酶释放，并保持隔离， 在ER中它们阻断甾醇加速还原酶ERAD。 基于这些观察，我们现在提出研究来阐明GGpp 调节UBIAD 1的细胞内运输，并确定这种调节如何影响维生素K2的合成 和胆固醇为了实现这一目标，我们将追求以下具体目标：1）阐明机制 GGpp通过其控制UBIAD 1的细胞内转运; 2）确定ER对UBIAD 1的细胞内转运的意义。 UBIAD 1的高尔基体转运;和3）检查UBIAD 1作为GGpp的膜传感器的作用。总的来说， 这些研究的结果将解释UBIAD 1的细胞内转运如何控制还原酶ERAD， 以允许在甾醇充足的细胞中连续合成必需的非甾醇类异戊二烯。深入了解机制 对于还原酶ERAD可能导致治疗干预，延缓或防止角膜积聚 与施奈德角膜营养不良相关的胆固醇，突出了我们提出的临床意义， 问题研究 C/PPG 2015 -RP 2 - 30-行摘要

Schnyder corneal dystrophy (SCD) is a rare autosomal dominant eye disease characterized by progressive opacification of the cornea, owing to abnormal accumulation of cholesterol. Mutations associated with SCD have been identified in the gene encoding UBIAD1 (UbiA prenyltransferase domain-containing protein-1), which utilizes the nonsterol isoprenoid GGpp (geranylgeranyl pyrophosphate) to synthesize vitamin K2. Our preliminary studies reveal that sterols trigger binding of UBIAD1 to the ER (endoplasmic reticulum)- localized enzyme HMG CoA reductase. The reductase produces mevalonate, an important intermediate in synthesis of cholesterol and nonsterol isoprenoids such as ubiquinone, vitamin K2, dolichol, and the farnesyl and geranylgeranyl groups that are attached to many cellular proteins. Sterol-regulated ubiquitination is obligatory for ERAD (ER-associated degradation) of reductase. GGpp inhibits binding of UBIAD1 to reductase, which allows for maximal ERAD of reductase and ER-to-Golgi transport of UBIAD1. Eliminating UBIAD1 relieves the GGpp requirement for reductase ERAD, indicating the reaction is inhibited by the prenyltransferase. SCD-associated mutants of UBIAD1 resist GGpp-induced release from reductase and remain sequestered in the ER where they block sterol-accelerated reductase ERAD. Building on these observations, we now propose studies to elucidate mechanisms through which GGpp regulates intracellular trafficking of UBIAD1 and determine how this regulation impacts synthesis of vitamin K2 and cholesterol. To achieve this goal, we will pursue the following Specific Aims: 1) Elucidate mechanism through which GGpp governs intracellular transport of UBIAD1; 2) Determine significance of the ER-to- Golgi transport of UBIAD1; and 3) Examine role for UBIAD1 as a membrane sensor of GGpp. Collectively, the results of these studies will explain how intracellular transport of UBIAD1 controls reductase ERAD so as to permit continuous synthesis of essential nonsterol isoprenoids in sterol-replete cells. Insight into mechanisms for reductase ERAD may lead to therapeutic interventions that retard or prevent corneal accumulation of cholesterol associated with Schnyder corneal dystrophy, highlighting the clinical significance of our proposed studies. C/PPG 2015 – RP2 – 30-line Summary