Phosphatidylserine

Phosphatidylserine (PS) is a negatively charged phospholipid found in all cell membranes. In healthy cells it is almost entirely in the inner layer of the lipid bilayer, facing the cytoplasm, and its appearance on the outer surface acts as a signal.

Like other phospholipids, PS has two fatty acid tails and a head group, here containing the amino acid serine. Enzymes called flippases use ATP to pump it into the inner leaflet of the plasma membrane, keeping the two layers different. It makes up about 5 to 10 percent of membrane lipid, and it is especially abundant in the brain, where it supports signaling.

During apoptosis, scramblase enzymes are activated and the flippases are shut down, so PS appears on the outer surface of the dying cell. Macrophages and other cells carry receptors that recognize exposed PS and engulf the cell, clearing it before it can leak harmful contents. Laboratories detect apoptotic cells by labeling them with the protein annexin V, which binds PS.

Activated platelets expose PS, which provides a surface for the assembly of clotting enzymes. Some viruses and parasites display PS on their envelopes to mimic dying cells, helping them to enter host cells or avoid an immune response. This is relevant to the biology of enveloped viruses, and PS is one of the lipid classes described in biochemical macromolecules.

• Apoptosis — where it signals cell death
• Lipid Bilayer — the structure it sits in
• Macrophage — the cell that reads its signal