The role of PA in the cell can be divided into three categories: • PA is the precursor for the biosynthesis of many other lipids. • The physical properties of PA influence membrane curvature. • PA acts as a signaling lipid, recruiting cytosolic proteins to appropriate membranes (e.g.,
sphingosine kinase 1). • PA plays very important role in
phototransduction in
Drosophila. • PA is a lipid ligand that gates ion channels. See also
lipid-gated ion channels. The first three roles are not mutually exclusive. For example, PA may be involved in vesicle formation by promoting membrane curvature and by recruiting the proteins to carry out the much more energetically unfavourable task of neck formation and pinching.
Roles in biosynthesis PA is a vital cell lipid that acts as a biosynthetic precursor for the formation (directly or indirectly) of all acylglycerol lipids in the cell. In mammalian and
yeast cells, two different pathways are known for the de novo synthesis of PA, the glycerol 3-phosphate pathway or the dihydroxyacetone phosphate pathway. In bacteria, only the former pathway is present, and mutations that block this pathway are lethal, demonstrating the importance of PA. In mammalian and yeast cells, where the enzymes in these pathways are redundant, mutation of any one enzyme is not lethal. However, it is worth noting that
in vitro, the various acyltransferases exhibit different substrate specificities with respect to the acyl-CoAs that are incorporated into PA. Different acyltransferases also have different intracellular distributions, such as the
endoplasmic reticulum (ER), the
mitochondria or peroxisomes, and local concentrations of activated fatty acids. This suggests that the various acyltransferases present in mammalian and yeast cells may be responsible for producing different pools of PA. and fusion, and these roles may relate to the biophysical properties of PA. At sites of membrane budding or fusion, the membrane becomes or is highly curved. A major event in the budding of vesicles, such as transport carriers from the
Golgi, is the creation and subsequent narrowing of the membrane neck. Studies have suggested that this process may be lipid-driven, and have postulated a central role for DAG due to its, likewise, unique molecular shape. The presence of two acyl chains but no headgroup results in a large negative curvature in membranes. The LPAAT BARS-50 has also been implicated in budding from the Golgi. The curvature induced by these lipids was shown to be dependent not only on the structure of lysoPA versus PA but also on dynamic properties, such as the hydration of head groups and inter- and intramolecular interactions. For instance, Ca2+ may interact with two PAs to form a neutral but highly curved complex. The neutralisation of the otherwise repulsive charges of the headgroups and the absence of any
steric hindrance enables strong intermolecular interactions between the acyl chains, resulting in PA-rich microdomains. Thus
in vitro, physiological changes in pH, temperature, and cation concentrations have strong effects on the membrane curvature induced by PA and lysoPA. For example TREK-1 channels are activated by local association with PLD and production of PA. The dissociation constant of PA for TREK-1 is approximately 10 micromolar. The relatively weak binding combined with a low concentration of PA in the membrane allows the channel to turn off. The local high concentration for activation suggests at least some restrictions in local lipid diffusion. The bulk low concentration of PA combined with high local bursts is the opposite of PIP2 signaling. PIP2 is kept relatively high in the membrane and then transiently hydrolized near a protein in order to transiently reduce PIP2 signaling. PA signaling mirrors PIP2 signaling in that the bulk concentration of signaling lipid need not change to exert a potent local effect on a target protein. As described above,
PLD hydrolyzes PC to form PA and
choline. Because choline is very abundant in the cell, PLD activity does not significantly affect choline levels; and choline is unlikely to play any role in signaling. The role of PLD activation in numerous signaling contexts, combined with the lack of a role for choline, suggests that PA is important in signaling. However, PA is rapidly converted to DAG, and DAG is also known to be a signaling molecule. This raises the question as to whether PA has any direct role in signaling or whether it simply acts as a precursor for DAG production. If it is found that PA acts only as a DAG precursor, then one can raise the question as to why cells should produce DAG using two enzymes when they contain the
PLC that could produce DAG in a single step. PA produced by PLD or by DAGK can be distinguished by the addition of [γ-32P]ATP. This will show whether the phosphate group is newly derived from the kinase activity or whether it originates from the PC. Although PA and DAG are interconvertible, they do not act in the same pathways. Stimuli that activate PLD do not activate enzymes downstream of DAG, and vice versa. For example, it was shown that addition of PLD to membranes results in the production of [32P]-labeled PA and [32P]-labeled phosphoinositides. The addition of DAGK inhibitors eliminates the production of [32P]-labeled PA but not the PLD-stimulated production of phosphoinositides. It is possible that, though PA and DAG are interconvertible, separate pools of signaling and non-signaling lipids may be maintained. Studies have suggested that DAG signaling is mediated by polyunsaturated DAG, whereas PLD-derived PA is monounsaturated or saturated. Thus functional saturated/monounsaturated PA can be degraded by hydrolysing it to form non-functional saturated/monounsaturated DAG, whereas functional polyunsaturated DAG can be degraded by converting it into non-functional polyunsaturated PA. This model suggests that PA and DAG effectors should be able to distinguish lipids with the same headgroups but with differing acyl chains. Although some lipid-binding proteins are able to insert themselves into membranes and could hypothetically recognize the type of acyl chain or the resulting properties of the membrane, many lipid-binding proteins are cytosolic and localize to the membrane by binding only the headgroups of lipids. Perhaps the different acyl chains can affect the angle of the head-group in the membrane. If this is the case, it suggests that a PA-binding domain must not only be able to bind PA specifically but must also be able to identify those head-groups that are at the correct angle. Whatever the mechanism is, such specificity is possible. It is seen in the pig testes DAGK that is specific for polyunsaturated DAG and in two rat hepatocyte LPPs that dephosphorylate different PA species with different activities. Moreover, the stimulation of SK1 activity by PS
in vitro was shown to vary greatly depending on whether dioleoyl (C18:1), distearoyl (C18:0), or 1-stearoyl, 2-oleoyl species of PS were used. Thus it seems that, though PA and DAG are interconvertible, the different species of lipids can have different biological activities; and this may enable the two lipids to maintain separate signaling pathways. ==Measurement of PA production==