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The Type 4 Subfamily of P-Type Atpases, Putative Aminophospholipid Translocases with a Role in Human Disease

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The Type 4 Subfamily of P-Type Atpases, Putative Aminophospholipid Translocases with a Role in Human Disease View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Biochimica et Biophysica Acta 1741 (2005) 11 – 24 http://www.elsevier.com/locate/bba Review The type 4 subfamily of P-type ATPases, putative aminophospholipid translocases with a role in human disease C.C. Paulusma*, R.P.J. Oude Elferink Department of Experimental Hepatology, Academic Medical Center/AMC Liver Center, Meibergdreef 69-71, 1105 BK Amsterdam, The Netherlands Received 30 March 2005; received in revised form 21 April 2005; accepted 25 April 2005 Available online 16 May 2005 Abstract The maintenance of phospholipid asymmetry in membrane bilayers is a paradigm in cell biology. However, the mechanisms and proteins involved in phospholipid translocation are still poorly understood. Members of the type 4 subfamily of P-type ATPases have been implicated in the translocation of phospholipids from the outer to the inner leaflet of membrane bilayers. In humans, several inherited disorders have been identified which are associated with loci harboring type 4 P-type ATPase genes. Up to now, one inherited disorder, Byler disease or progressive familial intrahepatic cholestasis type 1 (PFIC1), has been directly linked to mutations in a type 4 P-type ATPase gene. How the absence of an aminophospholipid translocase activity relates to this severe disease is, however, still unclear. Studies in the yeast Saccharomyces cerevisiae have recently identified important roles for type 4 P-type ATPases in intracellular membrane- and protein- trafficking events. These processes require an (amino)phospholipid translocase activity to initiate budding or fusion of membrane vesicles from or with other membranes. The studies in yeast have greatly contributed to our cell biological insight in membrane dynamics and intracellular-trafficking events; if this knowledge can be translated to mammalian cells and organs, it will help to elucidate the molecular mechanisms which underlie severe inherited human diseases such as Byler disease. D 2005 Elsevier B.V. All rights reserved. Keywords: Type 4 P-type ATPase; Aminophospholipid translocase; Phospholipid asymmetry; Byler disease; Membrane- and protein-trafficking; Yeast Saccharomyces cerevisiae 1. The P-type ATPase superfamily by Jens C. Skou who was awarded the Nobel Prize in Chemistry in 1997 for his work on the abundant Na+/K+- The P-type ATPase superfamily is an evolutionary ATPase [7,8]. Following this finding, a whole range of conserved, large family of proteins of which members are different P-type ATPases have been identified with varying widely expressed in both prokaryotes and eukaryotes, protein topologies and substrate specificities. In general, P- including bacteria, yeast, parasites, nematodes, insects, type ATPases are important in the generation of cation plants, and animals [1–5] (See also the on-line FP-type gradients which make these proteins indispensable in the ATPase database_ by Axelsen, K.B., http://www.patbase. regulation of cell volume, excitability of nerve cells, muscle kvl.dk). P-type ATPases are integral membrane proteins contraction, intracellular pH, acid secretion in the stomach, which, in most cases, mediate the ATP-dependent transport uptake of substrates, and signaling pathways. of small cations across biological membranes, including Based on phylogenetic analysis, this large family is those of intracellular organelles [5,6]. The first P-type divided into 5 major subfamilies, each unique in their class of ATPase activity (Na+/K+-ATPase) was discovered in 1957 substrates and in subfamily-specific sequence motifs [1–3]. In general, the type 1 ATPases transport heavy metals such as copper, cadmium, and zinc. The type 2 subfamily * Corresponding author. Tel.: +31 20 5668850; fax: +31 20 5669190. transports non-heavy metal cations and includes proteins + + + + 2+ E-mail address: [email protected] (C.C. Paulusma). like the Na /K -, H /K -, and Ca -ATPases. The type 3 0925-4439/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.bbadis.2005.04.006 12 C.C. Paulusma, R.P.J. Oude Elferink / Biochimica et Biophysica Acta 1741 (2005) 11–24 family includes ATPases only expressed in plants and fungi [26–28]. Phospholipids tend to equilibrate between the which are involved in the transport of magnesium- and two leaflets of bilayers, albeit at a slow rate (the order of proton-ions. Type 4 subfamily members are exclusively magnitude of the movement across the bilayer tends to be expressed in eukaryotic cells and transport phospholipids hours in artificial membranes (reviewed in [29])). In (see below) [9–12]. Finally, of the recently identified type living cells, this process is accelerated by fusion and 5 subfamily (exclusively expressed in eukaryotic cells), pinching of vesicles to and from membranes. The only one member has been identified yet which is asymmetry therefore needs to be actively maintained. implicated in cellular calcium homeostasis and endoplas- Three classes of proteins are involved in regulating mic reticulum function in yeast [13–15]. All P-type phospholipid distribution in membrane bilayers, i.e., ATPases share a common topology; usually 10 trans- floppases, flippases, and scramblases [19,20,25,30–35]. membrane domains, NH2- and COOH-termini protruding Phospholipid asymmetry is maintained by floppases and into the cytoplasm, and a large intracellular loop harboring flippases, ATP-dependent proteins that translocate specific P-type ATPase-specific sequences and an ATP-binding site. phospholipid species from the inner to the outer leaflet of The core of all P-type ATPases contains a conserved stretch the bilayer and vice versa, respectively. Scramblases are of seven amino acids with the P-type signature sequence bidirectional activities with little specificity that, upon FDKTGT[L,I,V,M][T,I,S]_. This motif includes an aspartic activation, dissipate phospholipid asymmetry. Scramblase acid residue (FD_) which is essential in the reaction cycle of activities have been identified in plasma membranes and the protein. The mechanism via which substrates are in membranes of the endoplasmic reticulum and the pumped is based on the E1E2 model, in which ATP- Golgi apparatus [22,34,36–39]. In many cells, these mediated phosphorylation, substrate binding, and subse- activities are dependent on elevation of cytosolic calcium quent dephosphorylation of the invariant aspartic acid are concentrations, e.g., in cells involved in blood coagu- coupled to a conformational change of the transporter lation [34]. However, in apoptotic cells, it seems that [6,16]. The phosphotransfer reaction drives the transport another mechanism (rather than calcium) is involved in cycle of these proteins, hence the name P-type ATPase. activation of the scramblase [40]. Although phosphotransfer remains the driving force for In most eukaryotic cells, phosphatidylcholine (PC) and these transporters, the E1E2 model is outdated, and sphingolipids (i.e., glycolipids and sphingomyelin (SM)) recently, a revised model has been proposed [17,18]. are abundant in the outer leaflet of the plasma membrane Briefly, in the E1E2 model, only two conformational states and in the luminal leaflet of endo- and exocytic vesicles. of the protein are recognized; the revised model, however, Proteins of the ATP-binding cassette (ABC) transporter recognizes at least four conformational changes, all of superfamily have been implicated in the outward transport, which are supported by a large body of biochemical and i.e., Fflopping_ of choline-containing phospholipids [41]. structural data (reviewed in [17,18]). Mice in which the gene encoding the liver-specific Mdr2 Type 4 P-type ATPases are clearly deviant from the protein (Abcb4) has been knocked out do not secrete PC other subfamilies in that they are thought to translocate into the canalicular lumen [42]. The human ortholog, phospholipids (rather than cations) from the outer- and MDR3 (ABCB4) protein, likewise specifically translocates inner leaflet of membrane bilayers [9,11,19–23]. Studies PC (as well as its fluorescent analogs) from the inner to the in yeast have recently demonstrated an association outer leaflet of the bilayer when expressed in mouse between type 4 ATPase-mediated phospholipid translocase fibroblasts or pig epithelial (LLC-PK1) cells [42–44]. activity and vesicular (protein) trafficking [11,20,24,25]. MDR1 (ABCB1) protein, a broad-range xenobiotic trans- Furthermore, members of this subfamily have been porter, also translocates choline-containing lipids, including implicated in human disease. The molecular mechanisms short-chain analogs of PC, glucosylceramide, and platelet- underlying these disease phenotypes are, however, still activating factor [44,45]. In addition, MRP1 (ABCC1) unclear. The scope of this review is to highlight the role protein has been implicated in the outward transport of of type 4 P-type ATPases in human disease and to relate fluorescent analogs of phosphatidylserine (PS) and SM this to recent findings in yeast in which an important [46,47]. function for type 4 ATPases in intracellular trafficking has The aminophospholipids PS and phosphatidylethano- been demonstrated. lamine (PE) localize to the cytosolic leaflet of bilayers and their distribution is maintained by flippases or amino- phospholipid translocases (APLT) in a process termed 2. Aminophospholipid translocases and membrane Fflipping_. Obviously, maintenance of phospholipid asym- asymmetry metry must be a physiologically
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