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G
Proteins and Transmembrane Signaling
Our laboratory focuses on the trimeric G proteins, which relay signals
from transmembrane receptors for sensory or hormonal stimuli to regulation
of effector enzymes and ion channels.
Structure/function of receptor-G protein coupling:
Sharing a 7-transmembrane-helix topology, each of 1,000 serpentine receptors
can couple specifically with one of 16 known trimeric G proteins, triggering
exchange of GTP for GDP bound to the G-alpha subunit. To understand this
process, we have identified interaction surfaces of the receptors and
G protein subunits, characterized mutant G-alpha subunits with specific
defects in susceptibility to receptor activation, and initiated a combined
genetic and biochemical analysis of the conformational switch in the seven-helix
bundle of serpentine receptors that is turned on by agonist ligands and
is responsible for relaying the signal to the G protein.
Neutrophil:
Neutrophils, circulating blood cells important for host defense against
infection and tissue injury, can crawl up very shallow gradients of attractants,
in which the concentration difference from front to back of the cell is
~2%. In such gradients the cell‰s internal signaling gradient is very
much steeper (as assessed by localization of actin polymers and other
markers). Thus, the neutrophil‰s internal compass mechanism can somehow
amplify the external gradient of stimulus. We have found that neutrophils
asymmetrically accumulate PI(3,4,5)P3 and other lipid products of PI3-kinases
(PI3Ks) in plasma membrane at the leading edge, where it is thought to
control activation of Rho GTPases and thereby direct actin assembly and
cell polarity. Our experiments using PI3K inhibitors, toxins, and dominant
negative Rho GTPase mutants suggest that the steep internal gradient depends
upon a positive feedback loop in which PI(3,4,5)P3 accumulation activates
Rac and Cdc42 and is in turn stimulated by these Rho GTPases. These Rho
GTPases feed back to activate PI(3,4,5)P3 accumulation by a mechanism
that depends, at least in part, upon formation of new actin polymers.
Three Rho GTPases Ö Rac, Cdc42, and Rho itself Ö appear to play distinctive
roles in regulating polarity and assembly of the neutrophil cytoskeleton.
Now we seek to identify specific components of the molecular wiring diagram
that controls cell polarity and directed migration.
Selected Publications:
Baranski, T. J., Herzmark, P., Lichtarge, O., Gerber, B. O., Trueheart,
J., Meng, E. C., Iiri, T., Sheikh, S. P. and Bourne, H. R. (1999). C5a
receptor activation: Genetic identification of critical residues in four
transmembrane helices. J. Biol. Chem. 274, 15757-15765.
Servant, G., Weiner, O. D., Neptune, E. R., Sedat, J. W. and Bourne, H.
R. (1999). Dynamics of a chemoattractant receptor in living neutrophils
during chemotaxis. Mol. Biol. Cell 10, 1163-1178.
Weiner, O. D., Servant, G., Welch, M. D., Mitchison, T. J., Sedat, J.
W. and Bourne, H. R. (1999). Spatial control of actin polymerization during
neutrophil chemotaxis. Nature Cell Biology 1, 75-81.
Fishburn, C. S., Pollitt, S. K. and Bourne, H. R. (2000). Localization
of a peripheral membrane protein: Gbg targets Gaz. Proc. Natl. Acad. Sci.
USA 97, 1085-1090.
Servant, G., Weiner, O. D., Herzmark, P., Balla, T., Sedat, J. W. and
Bourne, H. R. (2000). Polarization of chemoattractant receptor signaling
during neutrophil chemotaxis. Science 287, 1037-1040.
Contact Information:
Email: bourne@cmp.ucsf.edu
Phone: 415/ 476-8161
Address: Box 0450, Room S 1212
The University of California, San Francisco, CA 94143, (415) 476-9000
Copyright 2003, The Regents of the University of California.
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