This article is available online at http://www.jlr.orgUbiA prenyltransferase domain-containing protein-1 (UBIAD1) belongs to the UbiA superfamily of integral membrane prenyltransferases (1). These enzymes contain 8-10 transmembrane helices and catalyze transfer of isoprenyl groups to aromatic acceptors, producing a wide range of molecules such as ubiquinones, hemes, chlorophylls, vitamin E, and vitamin K. In animals, UBIAD1 catalyzes transfer of the 20-carbon geranylgeranyl moiety from geranylgeranyl pyrophosphate (GGpp) to menadione released from plant-derived phylloquinone, thereby generating the vitamin K 2 subtype, menaquinone-4 (MK-4) (2, 3).Mutations in the UBIAD1 gene are associated with Schnyder corneal dystrophy (SCD), an autosomal dominant human eye disease characterized by progressive opacification of the cornea, owing to abnormal accumulation of cholesterol and other lipids (4,5). Systemic dyslipidemia appears to be associated with some, but not all, SCD cases (6, 7). Missense mutations that alter 20 amino acid residues in UBIAD1 have been identified in 50 SCD families (8, 9). Several of these altered amino acids reside within the active site of UBIAD1 (10, 11). A link between UBIAD1 and cholesterol metabolism was first provided by coimmunoprecipitation studies that showed an association of UBIAD1 with the cholesterol biosynthetic enzyme, HMGCoA reductase (8). More recently, we showed that UBIAD1 inhibits sterol-accelerated endoplasmic reticulum (ER)-associated degradation (ERAD) of reductase (12), one of several feedback mechanisms that converge on the enzyme to maintain cholesterol homeostasis (13).Abstract UbiA prenyltransferase domain-containing protein-1 (UBIAD1) utilizes geranylgeranyl pyrophosphate (GGpp) to synthesize the vitamin K 2 subtype menaquinone-4. Previously, we found that sterols trigger binding of UBIAD1 to endoplasmic reticulum (ER)-localized HMG-CoA reductase, the rate-limiting enzyme in synthesis of cholesterol and nonsterol isoprenoids, including GGpp. This binding inhibits sterol-accelerated degradation of reductase, which contributes to feedback regulation of the enzyme. The addition to cells of geranylgeraniol (GGOH), which can become converted to GGpp, triggers release of UBIAD1 from reductase, allowing for its maximal degradation and permitting ER-to-Golgi transport of UBIAD1. Here, we further characterize geranylgeranyl-regulated transport of UBIAD1.