Dynamic localization of G proteins in Dictyostelium discoideum.
Extracellular stimuli exert their effects on eukaryotic cells via serpentine G-protein-coupled receptors and mediate a vast number of physiological responses. Activated receptors stimulate heterotrimeric G-proteins, consisting of three subunits, alpha, beta and gamma. In Dictyostelium discoideum, cAMP binds to the cAMP receptor cAR1, which is coupled to the heterotrimer containing the Galpha2 subunit. These studies provide in vivo evidence as to how receptors influence the localization of the G-protein complex prior to and after ligand binding. Previous work has shown that the state of the heterotrimer could be monitored by changes in fluorescence (or Förster) resonance energy transfer (FRET) between the alpha2- and beta-subunits of D. discoideum. We now report the kinetics of G-protein activation as a loss of FRET prior to and after cAMP addition by using total internal reflection fluorescence microscopy (TIRFM). We also performed photobleaching experiments to measure G-protein recovery times. Our data show that inactive and active G-proteins cycle between the cytosol and plasma membrane. These data suggest that cAR1 activation slows the membrane dissociation ('off') rate of the alpha2 subunit, while simultaneously promoting betagamma-subunit dissociation.
Elzie
Carrie A
CA
Department of Biological Sciences, Vanderbilt University, Nashville, TN 37235, USA.
Colby
Jennifer
J
Sammons
Morgan A
MA
Janetopoulos
Chris
C
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism*; D004023:Dictyostelium / Q000378:metabolism*; D004735:Energy Transfer; D031541:Fluorescence Resonance Energy Transfer; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D021122:Protein Subunits; D015398:Signal Transduction
D021122:Protein Subunits; D000242:Cyclic AMP; D020962:Heterotrimeric GTP-Binding Proteins