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

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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