Although phosphatidic acid is one of the most important glycerophospholipids in biomembranes, very little was known about how it regulates proteins. Here, Edgar E. Kooijman explains some research that brings us closer to solving this problem.
|Despite its extremely simple chemical structure, little was known about the specific regulation of proteins by PtdOH.
Apart from phosphoinositides, phosphatidic acid (PtdOH) is arguably one of the most important glycerophospholipids found in biomembranes. It is the glycero-phospholipid precursor (1) and has been implicated in processes from membrane dynamics to signaling (2, 3). However, little was known regarding the specific regulation of proteins by PtdOH, despite its extremely simple chemical structure.
Several years ago, while working on elucidating the role of PtdOH in membrane dynamics, we initiated a biophysical study into the charge carried by the phosphomonoester headgroup of PtdOH (4). At the time, we were interested merely in the degree of ionization, as this likely was to affect the molecular shape of PtdOH. However, we found that lyso-PtdOH carried more charge than PtdOH at constant pH in a phosphatidylcholine matrix, despite identical phosphomonoester headgroups. Furthermore, phosphatidylethanolamine increased the overall charge of both PtdOH and LPtdOH. A breakthrough in our understanding came from experiments with a LPtdOH compound lacking the free hydroxyl group in the backbone of LPtdOH. This so-called dehydroxy-LPtdOH behaved identically to PtdOH, implicating the hydroxyl in the difference in ionization behavior. This also indicated that the effect of PtdEth likely was due to its primary amine compared with the quaternary amine of phosphatidylcholine. Further studies with model membrane-spanning α-helical peptides eventually led us to introduce the electrostatic/hydrogen bond switch to describe the ionization and protein interaction mechanism of PtdOH (5).
The model describes the effect of hydrogen bonds on the degree of ionization of PtdOH. Upon losing its “first” proton, the remaining proton becomes more tightly bound, not only by covalent interactions, but also by the electrostatic charge of the phosphate. Interestingly, hydrogen bonds formed with the phosphate of PtdOH destabilize (most likely through a competition for electrons) the “second” proton, facilitating its removal. The further deprotonation leads not only to an increased negative charge but also creates an additional H-bond acceptor. We thus proposed that proteins recognize and interact with PtdOH based on the novel mechanism. This electrostatic/hydrogen bond switch model not only describes the ionization properties of PtdOH, but of every phosphomonoester moiety. Recent work on other lipids, such as cer-1-p and polyphosphoinositides, confirmed the model.
The implications of the model are numerous. It predicts a role for PtdOH at basic sites (at the headgroup/acyl-chain interface) of transmembrane proteins (6) and predicts pH-dependent binding of peripheral membrane proteins. Indeed, the latter property recently was confirmed by Chris Loewen and co-workers (7). They showed that Opi1 in yeast binds PtdOH at the endoplasmic reticulum in a pH-dependent manner in vivo, as predicted by the electrostatic/hydrogen bond switch. More importantly, they showed that the binding of PtdOH by Opi1 is regulated by the metabolic state of yeast and that PtdOH thus ties metabolism to membrane biogenesis. The pH-dependent interaction of Opi1, and potentially other proteins, likely is a subtle function of the number and position (structure) of basic residues in the binding domain, as well as local lipid composition in the membrane. The exciting work by Loewen and co-workers raises intriguing questions as to which other proteins are regulated in this fashion and how other lipids, such as cer-1-p and the PIPs, might exploit this property of their phosphomonoester headgroup.
1. Athenstaedt, K., and Daum, G. (1999) Phosphatidic Acid, a Key Intermediate in Lipid Metabolism. Eur. J. Biochem. 266, 1 – 16.
2. Arisz, S. A., Testerink, C., and Munnik, T. (2009) Plant PtdOH Signaling via Diacylglycerol Kinase. Biochim. Biophys. Acta. 1791, 869 – 875.
3. Stace, C., Manifava, M., Delon, C., Coadwell, J., Cockcroft, S., and Ktistakis, N. T. (2008) PtdOH Binding of Phosphatidylinositol 4-Phosphate 5-Kinase. Adv. Enzyme Regul. 48, 55 – 72.
4. Kooijman, E. E., Carter, K. M., van Laar, E. G., Chupin, V., Burger, K. N., and de Kruijff, B. (2005) What Makes the Bioactive Lipids Phosphatidic Acid and Lysophosphatidic Acid so Special? Biochemistry 44, 17007 – 17015.
5. Kooijman, E. E., Tieleman, D. P., Testerink, C., Munnik, T., Rijkers, D. T., Burger, K. N., and de Kruijff, B. (2007) An Electrostatic/Hydrogen Bond Switch as the Basis for the Specific Interaction of Phosphatidic Acid with Proteins. J. Biol. Chem. 282, 11356 – 11364.
6. Raja, M., Spelbrink, R. E., de Kruijff, B., and Killian, J. A. (2007) Phosphatidic Acid Plays a Special Role in Stabilizing and Folding of the Tetrameric Potassium Channel KcsA. FEBS Lett. 581, 5715 – 5722.
7. Young, B. P., Shin, J. J. H., Orij, R., Chao, J. T., Li, S. C., Guan, X. L., Khong, A., Jan, E., Wenk, M. R., Prinz, W. A., Smits, G. J., and Loewen, C. J. R. (2010) Phosphatidic Acid Is a pH Biosensor That Links Membrane Biogenesis to Metabolism. Science 329, 1085 – 1088.
Edgar E. Kooijman (firstname.lastname@example.org) is an assistant professor in the biological sciences department at Kent State University.