The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways

The P4-ATPases also help form transport vesicles that bud from Golgi and endosomal membranes, thereby impacting the sorting and localization of many different proteins in the secretory and endocytic pathways. of lysosomes and endosomes. In fact, a remarkable number of distinct protein trafficking pathways, mediated by Rabbit Polyclonal to KLF specific types of vesicles, transport proteins between the TGN, plasma membrane and endocytic/lysosomal compartments [2]. The small GTP-binding protein Arf and its ArfGEF and ArfGAP regulators, along with clathrin and its various adaptor proteins are critically important and highly conserved components of the protein trafficking machinery in this system. The Golgi also plays an important role in establishing the appropriate composition and organization of lipids in the plasma membrane and internal organelles. For example, sphingolipids and glycosphingolipids are synthesized in the lumenal leaflet of late Golgi elements and glycerophospholipids are translocated to the cytosolic leaflet by phospholipid flippases to establish an asymmetric membrane structure [3]. The type IV P-type ATPases (P4-ATPases) are phospholipid flippases that not only establish membrane phospholipid asymmetry, but are also tightly coupled to vesicle-mediated protein transport in the Golgi and endosomal systems. P4-ATPases were first implicated in vesicular transport through studies inSaccharomyces cerevisiae[46], but more recent studies inArabidopsis thaliani,Caenorhabditis elegansand mammalian tissue culture cells indicate that this function is conserved [710]. Precisely how and why phospholipid flippases are coupled to vesicle budding events is uncertain and remains an active area of investigation. Most flippases in the P4-ATPase family are comprised of a catalytic -subunit (the P4-ATPase) and a noncatalytic -subunit in the Cdc50 family of integral membrane proteins [3]. The budding yeast flippase Drs2, for example, associates with Cdc50 and the complex must be formed before the newly synthesized flippase is allowed to leave the ER [11]. This arrangement is well conserved though evolution and several metazoan P4-ATPases are known to have a functional requirement for association with a Cdc50 homolog [12,13]. In this review, we will trace the lines of evidence supporting the contention that P4-ATPases are phospholipid flippases. The transverse flip of specific phospholipid species from GSK726701A the exofacial to the cytosolic leaflet is an unusual activity for a P-type ATPase as members of this protein family are more famous for their roles in pumping ions or heavy metals across membranes [14]. How the P4-ATPases evolved such a different transport substrate is unclear. Only a single human disease, familial intrahepatic cholestasis, is currently known to result from a P4-ATPase GSK726701A deficiency [15]. However, studies in mice are beginning to illuminate additional physiological roles for mammalian P4-ATPases that will be discussed. We will also describe specific protein transport pathways linked to P4-ATPase activity and a model for how flippases may help establish the membrane curvature required to bud vesicles from Golgi and endosomal membranes. Other emerging topics from studies in budding yeast that will be addressed are regulatory mechanisms controlling flippase activity with connections to sterol, sphingolipid and GSK726701A phosphoinositide metabolism. We will conclude with a discussion of future directions for the phospholipid flippase field. == 2. The P4-ATPase and Cdc50 family of proteins == The first P4-ATPase sequence to appear in the literature was Drs2 from budding yeast, although GSK726701A it was initially thought to be a Ca++ATPase because these pumps were the closest homologs known at the time.