Session 6: Signal transduction in the light membrane structure
How might membrane proteins influence the localization of important signalling lipids, such as phosphoinositides?
Stuart McLaughlin
The cell uses several biophysical principles, such as reduction of dimensionality and electrostatics, to choreograph the flow of information during signal transduction; for example, through the calcium/phosphoinositide second messenger system1,2. These principles require no expenditure of energy by the cell and have been termed "cheap tricks"3. For example, pleckstrin homology (PH) and other domains attach phosphoinositide-specific phospholipase C (PLC) to the inner leaflet of the plasma membrane, producing a reduction of dimensionality or local concentration effect that allows the PLC catalytic domains to experience a ~1,000-fold higher effective concentration of their phosphoinositide substrates.
Many signal transduction proteins not only bind to membranes, but also use non-specific electrostatics to modulate the lateral organization of multivalent anionic phosphoinositides (for example, phosphatidylinositol-4,5-bisphosphate (PIP2); see REF. 3). Theory predicts, and experiments confirm, that any unstructured cluster of more than four basic residues located at the membrane-solution interface will sequester or laterally concentrate phosphoinositides. Specifically, the Poisson-Boltzmann equation predicts that these basic clusters produce a local positive electrostatic potential of ~+25 mV, whereas the potential over most of the inner leaflet of a mammalian cell plasma membrane is ~-25 mV because it typically contains 20-30% monovalent acidic lipids, such as phosphatidylserine. This ~50 mV potential difference allows the basic clusters to sequester tetravalent PIP2 electrostatically, even though it is present at only a low molar fraction in the membrane (~1%). Fluorescence resonance energy transfer (FRET), electron paramagnetic resonance (EPR), self-quenching and PLC activity experiments confirm the theoretical predictions4. Furthermore, calcium/calmodulin (Ca/CaM) can rapidly release the electrostatically sequestered PIP2 from several important signal transduction proteins by binding strongly (Kd ~10 nM) to the basic/hydrophobic clusters. For example, Ca/CaM binds to the basic effector domain of the natively unstructured peripheral protein MARCKS, suggesting a new function for this widely distributed protein.
References
1. Berridge, M. J. et al. Calcium signaling: dynamics, homeostasis and remodeling. Nature Rev. Mol. Cell Biol. 4, 517-529 (2003)
2. Berridge, M. J. Unlocking the secrets of cell signaling. Annu. Rev. Physiol. 67, 1-21 (2005)
3. McLaughlin, S. et al. PIP2 and proteins: interactions, organization and information flow. Annu. Rev. Biophys. Biomol. Struct. 31, 151-175 (2002)
4. Gambhir, A. et al. Electrostatic sequestration of PIP2 on phospholipid membranes by basic/aromatic regions of proteins. Biophys. J. 86, 2188-2207 (2004)
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