Flippases (rarely spelled flipases) are transmembrane lipid transporter

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s located in the

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

which belong to ABC transporter or P4-type ATPase families. They are responsible for aiding the movement of

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

molecules between the two leaflets that compose a cell's membrane (transverse diffusion, also known as a "flip-flop" transition). The possibility of active maintenance of an asymmetric distribution of molecules in the phospholipid bilayer was predicted in the early 1970s by Mark Bretscher. Although phospholipids diffuse rapidly in the plane of the membrane, their

polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the cli ...

head groups cannot pass easily through the

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

center of the bilayer, limiting their diffusion in this dimension. Some flippases - often instead called

scramblases Scramblase is a protein responsible for the translocation of phospholipids between the two monolayers of a lipid bilayer of a cell membrane. In humans, phospholipid scramblases (PLSCRs) constitute a family of five homologous proteins t ...

- are energy-independent and bidirectional, causing reversible equilibration of phospholipid between the two sides of the membrane, whereas others are energy-dependent and unidirectional, using energy from ATP hydrolysis to pump the phospholipid in a preferred direction. Flippases are described as transporters that move lipids from the exoplasmic to the cytosolic face, while floppases transport in the reverse direction. Many cells maintain asymmetric distributions of phospholipids between their cytoplasmic and exoplasmic membrane leaflets.Lodish, H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J. (2004). Molecular Cell Biology, 5th, New York: WH Freeman. The loss of asymmetry, in particular the appearance of the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

ic phospholipid

phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via ap ...

on the exoplasmic face, can serve as an early indicator of

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...

and as a signal for

efferocytosis In cell biology, efferocytosis (from ''efferre'', Latin for 'to take to the grave', 'to bury') is the process by which apoptotic cells are removed by phagocytic cells. It can be regarded as the 'burying of dead cells'. During efferocytosis, the cel ...

References

{{reflist, 30em Integral membrane proteins

See also

References