MINIREVIEW PROLOGUE
Signaling by Heterotrimeric G Proteins Minireview Series*

Martha Vaughan

From the Pulmonary-Critical Care Medicine Branch, NHLBI, National Institutes of Health, Bethesda, Maryland 20892

    ARTICLE
Top
Article
References

The number of known forms of G protein alpha , beta , and gamma  subunits, as well as the enormous number of receptors with which they interact, and their diverse effectors, which influence virtually all kinds of cellular processes, make this an obviously important (and complicated) field. Fortunately, however, there are many similarities among the structures and functional interactions of the G protein subunits and the G protein-coupled receptors. This has facilitated progress because it is often possible to apply information obtained with one G protein and/or receptor to the characterization of another system. At the same time, there is, of course, specificity in each of the protein-protein interactions that must in the end be understood, whether defined entirely within the structures of the proteins themselves or dependent on intermediary molecules. Just as the known Galpha proteins fall into four classes based on similarities of amino acid sequences, it seems likely that details of structural determinants will define different types (categories) of functional interactions of individual alpha  and beta gamma subunits, receptors, effectors, and ancillary components. Although much remains to be learned, it appears that at least some of these interactions depend upon specific kinds of molecular complexes that are based upon anchoring or scaffolding proteins such as those operative, for example, in cAMP-dependent protein kinase (1) or G protein-coupled (2) signaling.

The minireviews in this series address only three specific aspects of the very broad and diverse range of "signaling by heterotrimeric G proteins," aspects in which recent progress has been notable. An attempt has been made to emphasize commonalities of the processes and pathways that are considered. Of necessity, however, experiments are done with specific Galpha , beta , and gamma  subunits, receptors, ligands, and other components, which accounts for the inclusion of some more complex terminology than one might wish. Because the story began in a much simpler way, a bit of history may be useful or at least interesting.

In 1957, when Sutherland and Rall (3, 4) described the basic properties of an enzyme now known as adenylyl cyclase, its activation by epinephrine, glucagon, and NaF, and the identification of its product cAMP, G proteins and hormone receptors were unknown. Ten years later the hormone-sensitive adenylyl cyclase was still viewed as a protein complex in which activity of a catalytic unit was regulated in an allosteric fashion by the interaction of a hormone ligand with a specific site on a regulatory subunit. By the end of the 1960s, however, Birnbaumer and Rodbell (5) had concluded from studies of fat cell adenylyl cyclase, which is activated by multiple hormones, that hormone receptors are distinct from the catalyst. A few years later Orly and Schramm (6) directly demonstrated the independence of receptor and cyclase, and in 1981, Shorr et al. (7) reported purification of a beta -adrenergic receptor, the first G protein-coupled receptor with seven membrane-spanning alpha -helices to be characterized.

Shortly after they deduced the separateness of receptors and cyclase, Rodbell and Birnbaumer (8) detected a previously unsuspected role for GTP in hormonal activation of the enzyme and described effects of the nucleotide on hormone binding. Pfeuffer and Helmreich (9) separated a GTP-binding protein from the adenylyl cyclase complex, and by 1977 Ross and Gilman (10) reported that a 40-kDa GTP-binding protein could be added back to an insensitive cyclase to restore activation by GTP, as well as by NaF. This protein is now known as Galpha s (formerly Gs or Ns). Its subsequent purification and characterization by Gilman and co-workers, as well as other studies in many laboratories, were facilitated by the availability of cyc- cells that lack Galpha s. These variant lymphoma cells were initially believed to be deficient in adenylyl cyclase because they survived exposure to agents that killed other lymphoma cells by increasing cAMP (11).

During the late 1970s Cassel and Selinger (12) described a GTPase activity that was stimulated by epinephrine in parallel with adenylyl cyclase activity and was inhibited by cholera toxin, which was known to activate the cyclase. They postulated that the hormone-activated receptor interacted with Gs to facilitate release of bound GDP and subsequent GTP binding. Hydrolysis of bound GTP to GDP then inactivated Gs and completed the cycle. This is the hormone-stimulated GTPase activity that is inhibited by cholera toxin-catalyzed ADP-ribosylation of Galpha s.

While the adenylyl cyclase-G protein system was being unraveled, other investigators were defining the light-activated cGMP phosphodiesterase in retinal rod outer segments, which is associated with a light-activated GTPase (transducin), and the photon receptor rhodopsin. Numerous structural and mechanistic similarities between the two systems became increasingly evident. By 1986, cDNA cloning provided deduced amino acid sequences for Galpha s and Galpha t (transducin) plus Galpha i and Galpha o as well as the recognition that each of the other two subunits, which are tightly associated as a beta gamma dimer, exists in more than one form. In a classic review, Stryer and Bourne (13) were able to synthesize a massive amount of data and suggest many of the kinds of questions that had become accessible to investigation. They predicted the imminent rewards from x-ray crystallographic analysis of G protein structure and site-specific mutagenesis for functional studies. A decade later much of this information is integrated in the first review of this series entitled "The Many Faces of G Protein Signaling" by Heidi E. Hamm, who has made major contributions in this area.

GTP hydrolysis is a very critical step in G protein signaling because it is a "turn off" switch. The intrinsic rates of GTP hydrolysis by G proteins differ widely. Casey and Gilman (14) wrote in a 1988 minireview that "More information is needed on interactions that may influence the rate of the GTPase reaction that is catalyzed by an alpha -subunit; interactions with effectors or with unidentified components may speed the kinetics of deactivation." In 1997, considerably more, albeit still incomplete, information was available, some of which was summarized in a minireview by Dohlman and Thorner (15). The second review of this series, "Mammalian RGS Proteins: Barbarians at the Gate" by David M. Berman and Alfred G. Gilman, brings the subject up-to-date from a slightly different perspective.

During the 1980s and 1990s, mechanisms of signaling from what had become a very large number of known G proteins, involving beta gamma as well as alpha  subunits and previously unrecognized effectors such as ion channels and phospholipase C, were being elucidated with increasing rapidity. Simultaneously, signal transduction from tyrosine kinase receptors (e.g. for insulin and growth factors), including Ras pathways with numerous other kinases and monomeric GTPases, was enjoying increased experimental attention. Only relatively recently is the extent to which these different pathways and their components communicate and interact becoming apparent. J. Silvio Gutkind, who is an important contributor to this progress, summarizes evidence for "The Pathway Connecting G Protein-coupled Receptors to the Nucleus through Divergent MAP Kinase Cascades" in the last review of this series.

Forty years ago, the marvelous complexity of biological regulatory reactions and molecules that would be revealed by the discovery of cAMP and adenylyl cyclase could not have been imagined. Thus far, Nobel Prizes for that and directly related discoveries have been awarded to Sutherland (1971), Fischer and Krebs (1992), whose extensive studies of reversible protein phosphorylation began with cAMP-dependent protein kinase, and Gilman and Rodbell (1994) for their work on heterotrimeric G proteins. There may well be more because these components are fundamental to critical signaling processes in essentially all eukaryotic cells, and much remains to be learned.

    FOOTNOTES

* This minireview will be reprinted in the 1996 Minireview Compendium, which will be available in December, 1996. 

    REFERENCES
Top
Article
References

  1. Dell'Acqua, M. L., and Scott, J. P. (1997) Protein kinase A anchoring. J. Biol. Chem. 272, 12881-12884
  2. Tsunoda, S., Sierralta, J., Sun, Y., Yumei, S., Bodner, R., Suzuki, E., Becker, A., Socolich, M., and Zuker, C. S. (1997) A multivalent PDZ-domain protein assembles signalling complexes in a G protein-coupled cascade. Nature 388, 243-249
  3. Rall, T. W., Sutherland, E. W., and Berthet, J. (1957) The relationship of epinephrine and glucagon to liver phosphorylase. IV. Effect of epinephrine and glucagon on the reactivation of phosphorylase in liver homogenates. J. Biol. Chem. 224, 463-475
  4. Sutherland, E. W., and Rall, T. W. (1958) Fractionation and characterization of a cyclic adenine ribonucleotide formed by tissue particles. J. Biol. Chem. 232, 1077-1091
  5. Birnbaumer, L., and Rodbell, M. (1969) Adenylyl cyclase in fat cells. II. Hormone receptors. J. Biol. Chem. 244, 3477-3482
  6. Orly, J., and Schramm, M. (1976) Coupling of catecholamine receptor from one cell with adenylate cyclase from another cell by cell fusion. Proc. Natl. Acad. Sci. U. S. A. 73, 4410-4414
  7. Shorr, R. G. L., Lefkowitz, R. J., and Caron, M. G. (1981) Purification of the beta -adrenergic receptor. Identification of the hormone-binding subunit. J. Biol. Chem. 256, 5820-5826
  8. Rodbell, M., Birnbaumer, L., Pohl, S. L., and Krans, H. M. J. (1971) The glucagon-sensitive adenylyl cyclase system in plasma membranes of rat liver. V. An obligatory role of guanyl nucleotides in glucagon action. J. Biol. Chem. 246, 1877-1882
  9. Pfeuffer, T., and Helmreich, E. J. M. (1975) Activation of pigeon erythrocyte membrane adenylate cyclase by guanyl nucleotide analogues and separation of nucleotide-binding protein. J. Biol. Chem. 250, 867-876
  10. Ross, E. M., and Gilman, A. G. (1977) Resolution of some components of adenylate cyclase necessary for catalytic activity. J. Biol. Chem. 252, 6966-6969
  11. Bourne, H. R., Coffino, P., and Tompkins, G. M. (1975) Selection of a variant lymphoma cell deficient in adenylate cyclase. Science 187, 750-752
  12. Cassel, D., and Selinger, Z. (1978) Mechanism of adenylate cyclase activation through the beta -adrenergic receptor: catecholamine-induced displacement of bound GDP by GTP. Proc. Natl. Acad. Sci. U. S. A. 75, 4155-4159
  13. Stryer, L., and Bourne, H. R. (1986) G proteins: a family of signal transducers. Annu. Rev. Cell Biol. 2, 391-419
  14. Casey, P. J., and Gilman, A. G. (1988) G protein involvement in receptor-effector coupling. J. Biol. Chem. 263, 2577-2580
  15. Dohlman, H. G., and Thorner, J. (1997) RGS proteins and signaling by heterotrimeric G proteins. J. Biol. Chem. 272, 3871-3874


Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.