Phosphatidylserine (PS) is a glycerophospholipid present in the membranes of all eukaryotic cells. Like the majority of glycerophospholipids, PS has a glycerol backbone esterified on the sn
-1 and sn
-2 carbons of the glycerol moiety with 2 fatty acyl chains of variable length and saturation, and a phosphate group on sn
-3. The distinguishing feature of PS is the attachment of a serine to the phosphate; the resulting combined head-group gives PS a net negative charge . In mammalian cells, PS is produced by the exchange of the head-group of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) for serine by the enzymes PS-synthase 1 (PSS1) and PSS2, respectively. Both enzymes are found in the mitochondrial-associated membranes (MAM), specialized regions of the endoplasmic reticulum (ER) that are tightly apposed to and co-isolate with mitochondria [1
]. From the MAM, PS travels to other membranes within the cell.
Figure 1. (A)
Diagram of the structure of a prototypical phosphatidylserine (PS), a glycerophospholipid, with saturated and unsaturated fatty acyl chains. Note that at physiological pH the head-group bears one net negative charge. (B)
Representation of the interaction (more ...)
While PS is present in all cells, it is a comparatively minor constituent of their membranes, comprising 3–10% of the total lipids [1
]. However, this low relative abundance of PS belies its importance within the cell. The best-studied roles of PS involve signalling, not within the intracellular environment, but in an extracellular context such as during apoptosis [3
] and during blood clotting. Like most lipids, PS is not evenly distributed throughout all cellular membranes, nor is it always equally distributed between leaflets of a membrane bilayer [4
]. In healthy cells plasmalemmal PS is exclusively on the inner (cytoplasmic-facing) leaflet due to the action of ATP-dependent aminophospholipid flippases [5
]. When cells undergo apoptosis (regulated cell death) PS appears on the outside-facing (extracellular) leaflet, signalling phagocytic cells to engulf the dying cell. PS is also exposed exofacially in activated blood platelets, which prompts the binding and activation of a number of clotting factors, including factors V, VIII, X and prothrombin [6
There is little doubt that, in addition to these extracellular functions, PS plays important roles within the intracellular environment. Indeed, a number of important intracellular proteins require PS for their proper localization and/or activation. Such proteins include the E3 ubiquitin-protein ligase NEDD4, a number of protein kinase C isoforms, several phospholipase C and D isoforms, PTEN ,an important phosphatidylinositol (3,4,5)-tris
phosphate phosphatase, dysferlin, a protein important in muscle repair, as well as a number of synaptotagmin isoforms that are important for vesicular trafficking and fusion [7
]. Additionally, it is known on the whole that PS is important, as mice with a complete loss of ability to synthesize PS are not viable [8
], and though yeast are able to survive without PS synthesis, their growth is greatly impaired [9
]. However, despite the obvious importance of intracellular PS, its distribution, dynamics and function have not been thoroughly investigated. This is attributable, at least in part, to methodological limitations.
Early attempts to study PS relied upon cellular disruption and organellar purification, followed by chemical determination of the total PS content. Clearly, more subtle and sensitive techniques are required to fully understand the biology of PS. The ability to detect PS in intact cells and tissues, and to monitor its dynamics by non-invasive means is essential to decipher the physiology of PS. This review will discuss various methods that have been used to detect PS localization, highlighting a recent addition to the PS-sensing arsenal that allows for the detection of intracellular PS in live cells, and some novel findings derived from its use.