Receptor-G Protein-Effector Coupling: Coding and Regulation of the Signal Transduction Process

Summary

Regulation of G protein mediated signal transduction is thought to occur primarily as the result of the occupancy of a receptor by the appropriate agonist. There is much regulation of this process, however, which occurs independently of straightforward agonist occupancy. It is demonstrated here that receptors themselves might have intrinsic activity and the expression of such intrinsically activated receptors might alter the activated state of a G protein and its effector within a given cell. The regulation of receptor display on the cell surface is also discussed and it is seen that the ability to sequester, internalize and restore receptors to the plasma membrane is quite different for different types of G protein coupled receptors. The suggestion that a given receptor uses specific G protein α, β, and γ subunits in order to couple receptor to effector is also explored. Such coding would allow a cell to shape its response to a given agonist. It further proposed that elements of the cytoskeleton (tubulin) form complexes with Gs, Gil or Gq and activate those proteins via the direct transfer of GTP. In this way, changes in cell shape could be conveyed into the intracellular mileu. Further, these cytoskeletal elements might act as the contact point for interplay among various signal transduction systems within the cell. Finally, it is observed that voltage-dependent calcium channels undergo a complex regulation in which G protein β subunits compete with one of the polypeptides making up the channel. Such a process allows for the neurotransmitter modulation of neurotransmitter release. Such modulation returns full circle back to the receptor-mediated activation of G proteins and the possibility that this process is modulated in the cellular interior.