Hormones and neurotransmitters interact with specific receptors and thereby trigger a multicomponent signal transduction system that produces their biological effects. These receptors are localised in the plasma membrane and control one or several target systems via the activation of an intermediary guanine nucleotide-binding regulatory protein or G-protein. G-proteins serve as signal transducers, linking extracellulary oriented receptors to membrane bound effectors. Of the many members of the G-protein family that have already been discovered, at least two, Gs and Gi, are involved in the coupling of hormone receptors to adenylate cyclase. This enzyme is located in the plasma membrane and catalyses the transformation of ATP to cyclic AMP. G-proteins share a number of properties: all are membrane-bound, have an oligomeric structure and consist of one α, one β and one γ subunit.
The α subunits of the G-proteins are clearly different, although they appear to be very homologous. Their molecular weight ranges from 39 kDa to 52 kDa. Each α subunit has a central guanine-nucleotide binding site, as well as a binding site for the βγ complex and the receptor. The β subunits appear to be very homologous. Their molecular weight ranges from 35 kDa to 36kDa. One important function of the β subunit is the ability to bind to the α subunit, a property that is shared by the β subunits of Gs, Gi and Gt. The function of the γ subunit is yet unknown. Its molecular weight ranges from 8 kDa to 10 kDa, and it always binds tightly to the β subunit, forming the βγ-complex [85, 86].
Hitherto described types of G-proteins are summarised in table 1.6 [72].
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© 2001 Alexander Binder