|Dr. Susy Kohout|
Voltage Dependent SignalingSignaling cascades are the basis for cell life. From single cells to multi-cell organisms, cells must respond to and communicate with their local environment. The higher the organism the more complicated that communication becomes with interconnected and interdependent signaling pathways. Failures in these communication pathways often lead to diseases when the cells can no longer compensate.
One of the most important sites of cell signaling is the plasma membrane, where signals from the external environment are detected, physiological and biochemical signals of the cell initiated and through which ions, water, metabolites and proteins are selectively transported. These processes involve dozens of signaling pathways, and are critical in diverse cellular processes, including setting up and discharging the membrane potential, transforming cell morphology and migration, driving cell division and differentiation. One of the key links to the regulation of these diverse phenomena is phosphatidylinositol signaling (Fig. 1). Classically, control of this pathway has been attributed to soluble kinases and phosphatases that inter-convert phosphatidylinositol phosphates (PIPs) between forms that bind to distinct effectors. Since these effectors include ion channels and transporters, some of the fastest effects of PIP signaling are changes in membrane potential. Our research focuses on a recently discovered membrane protein that operates in the reverse direction: responding to changes in voltage to alter PIP levels. This protein, a voltage sensing phosphatase (VSP) that is found in diverse tissues including the nervous system, provides a fascinating feedback loop whose properties and physiological outcomes have yet to be determined.
Voltage Sensitive Proteins
VSP belongs to a specialized family of voltage sensitive proteins that allow cells to transduce changes in membrane potential into chemical signals. Basic cellular processes, such as neuronal firing and muscle contraction, rely on these voltage dependent events. Voltage sensitive proteins respond to the changes in membrane potential via a voltage-sensing domain (VSD). The most common members of this family, the voltage-gated ion channels, use the VSD to open and close a pore domain (Fig. 2, left), which allows for the passive movement of ions down their concentration gradients. Instead of a pore, VSP has a phosphatase domain (Fig. 2, right) that dephosphorylates phosphates from PIP lipids. The phosphatase domain from VSP is quite similar to the tumor suppressor, PTEN, sharing 44% identity between the two catalytic domains.
There are many challenges to understanding how membrane proteins function in the context of their native lipid environments. To probe this class of proteins, we apply voltage clamp fluorometry (VCF). VCF combines classical electrophysiological techniques with optical imaging to create a powerful technique to study full length membrane proteins in the context of a native cellular environment. By monitoring these protein motions in real time, the protein conformations can then be correlated with protein function giving a picture of how the protein moves to achieve its function.
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EducationB.S. Chemistry, 1995, California Institute of Technology
Ph.D. Biochemistry, 2002, University of Colorado, Boulder (Advisor: Joseph J. Falke)
Postdoctoral Fellow, Molecular Biology, University of California, Berkeley (Advisor: Ehud Y. Isacoff)