Elucidating role of UBIAD1 in sterol-accelerated ERAD of HMG CoA reductase

描述（由申请人提供）：酶HMG辅酶a还原酶催化HMG辅酶a转化为甲羟戊酸，甲羟戊酸是合成胆固醇和必需非固醇类异戊二烯的限速步骤。还原酶通过包含八个跨膜螺旋的n端结构域锚定在内质网（ER）膜上；C端结构域投射到细胞质中并发挥催化活性。加速内质网相关降解（ERAD）是还原酶反馈控制的几种机制之一，是由甾醇诱导的还原酶与内质网膜蛋白insg -1和insg -2结合而产生的。insg相关的泛素连接酶使还原酶泛素化，标志着该酶可以跨内质网膜逆转录，并位错进入细胞质以进行蛋白酶体降解。非固醇类异戊二烯香叶醇通过一种完全未知的机制调节了这些磺化后的步骤。在初步研究中，我们发现甾醇还能触发还原酶与含有UbiA戊烯基转移酶结构域的蛋白-1 （UBIAD1）结合，从而催化细菌和植物维生素K衍生物的香叶基酰化，生成甲基萘醌-4 （MK-4）。UBIAD1的突变导致一种罕见的常染色体显性眼病，称为施耐德角膜营养不良（SCD），其特征是角膜中胆固醇的进行性积累。醇诱导的还原酶与UBIAD1的结合被香叶醇抑制，RNA干扰研究表明，还原酶与UBIAD1的结合导致了还原酶的insg募集和ERAD。因此，我们假设1)UBIAD1介导了还原酶- insg结合所需的甾醇感应反应；2)香叶醇介导的UBIAD1位移是启动还原酶ERAD的复化后步骤所必需的。为了评估这些假设，我们提出了三个具体目标：1)描述UBIAD1介导甾醇加速还原酶ERAD的机制；2)探索甾醇调节UBIAD1功能的机制；3)确定UBIAD1在全动物还原酶ERAD和胆固醇稳态调节中的作用。总的来说，这些研究将提供关于从内质网膜去除多聚蛋白以进行蛋白酶体降解的关键信息。此外，这些研究具有重要的临床意义。还原酶是他汀类药物的靶标

DESCRIPTION (provided by applicant): The enzyme HMG CoA reductase catalyzes conversion of HMG CoA to mevalonate, a rate-limiting step in synthesis of cholesterol and essential nonsterol isoprenoids. Reductase is anchored to endoplasmic reticulum (ER) membranes through an N-terminal domain that contains eight membrane-spanning helices; the C- terminal domain projects into the cytosol and exerts catalytic activity. Accelerated ER-associated degradation (ERAD), one of several mechanisms for feedback control of reductase, results from sterol-induced binding of reductase to ER membrane proteins called Insig-1 and Insig-2. Ubiquitination of reductase by Insig-associated ubiquitin ligases marks the enzyme for retrotranslocation across ER membranes and dislocation into the cytosol for proteasomal degradation. These postubiquitination steps are modulated by the nonsterol isoprenoid geranylgeraniol through a completely unknown mechanism. In preliminary studies, we find that sterols also trigger binding of reductase to UbiA prenyltransferase domain containing protein-1 (UBIAD1), which catalyzes geranylgeranylation of bacterial and plant vitamin K derivatives to produce menaquinone-4 (MK-4). Mutations in UBIAD1 cause a rare autosomal dominant eye disease called Schnyder corneal dystrophy (SCD), which is characterized by progressive accumulation of cholesterol in the cornea. Sterol-induced binding of reductase to UBIAD1 is inhibited by geranylgeraniol and RNA interference studies reveal that reductase-UBIAD1 binding leads to Insig recruitment and ERAD of reductase. Thus, we hypothesize that 1) UBIAD1 mediates the sterol-sensing reaction required for reductase-Insig binding; and 2) geranylgeraniol-mediated displacement of UBIAD1 is required for initiation of postubiquitination steps in reductase ERAD. To appraise these hypotheses, we propose three Specific Aims: 1) delineate mechanisms by which UBIAD1 mediates sterol-accelerated ERAD of reductase; 2) explore mechanism through which sterols modulate function of UBIAD1; and 3) determine the role of UBIAD1 in ERAD of reductase and regulation of cholesterol homeostasis in whole animals. Collectively, these studies will provide key information regarding the removal of polytopic proteins from ER membranes for proteasomal degradation. In addition, these studies have significant clinical implications. Reductase is the target of statins, widely prescribed drugs that lower plasma LDL-cholesterol and reduce the incidence of cardiovascular disease. Statins trigger responses that cause accumulation of reductase, which blunts their clinical effects. Part of this increase results from slowed ERAD of reductase. Thus, elucidating mechanisms for reductase ERAD holds promise for development of new therapies that increase the effectiveness of statins and ultimately reduce the incidence of heart attacks. Moreover, insight into mechanisms for reductase ERAD may lead to therapeutic interventions that retard or prevent corneal accumulation of cholesterol associated with SCD.