P4-ATPases as Phospholipid Flippases—Structure, Function, and Enigmas

Abstract

P4-ATPases comprise a family of P-type ATPases that actively transport or flip phospholipids across cell membranes. This generates and maintains membrane lipid asymmetry, a property essential for a wide variety of cellular processes such as vesicle budding and trafficking, cell signaling, blood coagulation, apoptosis, bile and cholesterol homeostasis, and neuronal cell survival. Some P4-ATPases transport phosphatidylserine and phosphatidylethanolamine across the plasma membrane or intracellular membranes whereas other P4-ATPases are specific for phosphatidylcholine. The importance of P4-ATPases is highlighted by the finding that genetic defects in two P4-ATPases ATP8A2 and ATP8B1 are associated with severe human disorders. Recent studies have provided insight into how P4-ATPases translocate phospholipids across membranes. P4-ATPases form a phosphorylated intermediate at the aspartate of the P-type ATPase signature sequence, and dephosphorylation is activated by the lipid substrate being flipped from the exoplasmic to the cytoplasmic leaflet similar to the activation of dephosphorylation of Na+/K+-ATPase by exoplasmic K+. How the phospholipid is translocated can be understood in terms of a peripheral hydrophobic gate pathway between transmembrane helices M1, M3, M4, and M6. This pathway, which partially overlaps with the suggested pathway for migration of Ca2+ in the opposite direction in the Ca2+-ATPase, is wider than the latter, thereby accommodating the phospholipid head group. The head group is propelled along against its concentration gradient with the hydrocarbon chains projecting out into the lipid phase by movement of an isoleucine located at the position corresponding to an ion binding glutamate in the Ca2+- and Na+/K+-ATPases. Hence, the P4-ATPase mechanism is quite similar to the mechanism of these ion pumps, where the glutamate translocates the ions by moving like a pump rod. The accessory subunit CDC50 may be located in close association with the exoplasmic entrance of the suggested pathway, and possibly promotes the binding of the lipid substrate. This review focuses on properties of mammalian and yeast P4-ATPases for which most mechanistic insight is available. However, the structure, function and enigmas associated with mammalian and yeast P4-ATPases most likely extend to P4-ATPases of plants and other organisms.

Structural basis of MsbA-mediated lipopolysaccharide transport

“…Our model also differs from the “credit card model” proposed for P4-ATPase flippases 6 and the TMEM16 scramblase 49 , in which the hydrophobic acyl chains stay in the membrane during flipping. Further studies of different types of flippases in complex with their lipid substrates are essential to establish whether a universal flipping mechanism exists.…”

Characterization of the scrambling domain of the TMEM16 family

“…However, how phospholipid scramblases and flippases translocate phospholipids, which consist of hydrophobic tails and a hydrophilic head, through hydrophobic membrane bilayers has been a particular challenge in cell biology (20)(21)(22). We previously identified mouse and human TMEM16F as a phospholipid scramblase (14,15).…”

Lipids and Ions Traverse the Membrane by the Same Physical Pathway in the nhTMEM16 Scramblase

“…For example, transport of lipid-linked oligosaccharides by the ABC lipid flippase PglK involves interaction of the oligosaccharide moiety with positively charged side chains in a cavity in the protein while the lipidic polyprenyl tail remain exposed to the lipid bilayer during transport (Perez et al, 2015). P4-ATPases are presumed to operate by a similar mechanism (Andersen et al, 2016).…”