The heterotrimeric G-Proteins (Guanine nucleotide-binding Proteins) are signal transducers that communicate signals from many hormones, neurotransmitters, chemokines, and autocrine and paracrine factors. The extracellular signals are received by members of a large superfamily of receptors with seven membrane-spanning regions, known as GPCR (G-Protein Coupled Receptor), that activate the G-Proteins, which route the signals to several distinct intracellular signaling pathways. These pathways interact with one another to form a network that regulates metabolic enzymes, ion channels, transporters, and other components of the cellular machinery controlling a broad range of cellular processes, including transcription, motility, contractility, and secretion. These cellular processes in turn regulate systemic functions such as embryonic development, gonadal development, learning and memory, and organismal homeostasis. Heterotrimeric G-Proteins are composed of an Alpha, a Beta, and a Gamma subunit. These subunits are encoded by families of related genes. According to current knowledge, 16 genes encode for G-Alpha-subunits, five genes encode for G-Beta-, and 12 genes encode for G-Gamma-subunits. Classically, G-Proteins are divided into four families based on similarity of their Alpha-subunits: G-AlphaI/O, G-AlphaS, G-AlphaQ/11, and G-Alpha12/13.
-subunits contain two domains: a GTPase domain that is involved in the binding and hydrolysis of GTP and a helical domain that buries the GTP within the core of the protein. The helical domain is the most divergent domain among G-Alpha families and may play a role in directing specificity of receptor- and effectors-G-Proteins coupling. G-Proteins cycle between an inactive, GDP (Guanosine Diphosphate)-bound form and an active, GTP (Guanosine Triphosphate)-bound form. Upon activation, GDP is released, GTP binds to
, and subsequently G-Alpha-GTP dissociates from G-Beta-Gamma and from the receptor. Both G-Alpha-GTP and G-Beta-Gamma are then free to activate downstream effectors. The duration of the signal is determined by the intrinsic GTP hydrolysis rate of the
G-Alpha-subunit and the subsequent reassociation of G-Alpha-GDP with G-Beta-Gamma (Ref.1, 2 & 3).
G-AlphaI/O proteins are widely expressed. G-AlphaI/O family is divided into 8 subtypes: GNAI1 (G-AlphaI-1), GNAI2 (G-AlphaI-2), GNAI3 (G-AlphaI-3), GNAO (G-AlphaO), GNAZ (G-AlphaZ), GNAT3 (G-Alpha-Gust), GNAT1 (G-Alpha-t-r) and GNAT2(G-Alpha-t-c). In general, G-AlphaI protein subtypes inhibit AC (Adenylate Cyclase) and decrease intracellular cAMP (Cyclic Adenosine 3,5-monophosphate) levels. Reduction in cAMP level leads to reduced PKA (Protein Kinase-A) activation. Thus, G-AlphaI negatively regulates PKA signaling. Many important Hormones and Neurotransmitters, including Epinephrine, Acetylcholine, Dopamine, and Serotonin, use the G-AlphaI pathway to evoke physiological responses. G-AlphaII can also regulate signals from c-Src to STAT3 (Signal Transducer and Activator of Transcription-3) and to the Rap
pathways. However, the physiological consequences of Gai regulation of c-Src-STAT3 and Rap pathways remain to be established. Src family kinases are rapidly and transiently activated by various G protein-coupled receptor agonists, including LPA, Thrombin, Bombesin and Bradykinin. Besides STAT3, active c-Src also mediates Ras/MAPK activation through tyrosine phosphorylation of the adaptor protein SHC and its subsequent association with the GRB2 adaptor, which directs the GDP/GTP exchanger SOS to Ras. Activated Ras leads to activation of Raf, which further activates ERK (Extracellular Signal-Regulated Kinase) via MEK1/2 (MAPK/ERK1/2). ERK signaling cascade mediates the growth-promoting effects of stimulated G-protein-linked receptors. Activated ERK enter the nuclear and activate transcription factor Elk1 thus regulating Cell proliferation (Ref.4, 5 & 6).
also regulate Rap1 pathway. G-AlphaI interacts with Rap1-GAP. Rap1-GAP specifically stimulates GTP hydrolytic activity of the monomeric G-protein Rap1 and thus is believed to function as a down-regulator of Rap1 signaling. Rap1 belongs to the Ras subfamily of small GTP-binding proteins, which is considered to control cell growth, differentiation and survival. Two isoforms of Rap1 are known, Rap1A and Rap1B: they share more than 90% sequence homology but are differently expressed in different cell types. Rap1A and Rap1B forms an active complex with BRaf (v-Raf Murine Sarcoma Viral Oncogene Homolog-B1) for MEK1/2 (MAPK/ERK Kinase-1/2) activation finally resulting in Elk1 activation via ERK (Extracellular Signal-Regulated Kinase). Rap1 also inhibits c-Raf. Recently, the intracellular RGS (Regulator of G-protein Signalling) proteins have been discovered to serve additional, mostly negative, modulatory roles in G-Alpha-mediated signal transduction. RGSproteins are able to inhibit the effects of G-AlphaI on AC activity as a result of their GAP activity. RGS14 regulates G-protein nucleotide exchange and hydrolysis by acting as a GAP through its RGS domain and as a GDI (Guanine nucleotide Dissociation Inhibitor). RGS14 exerts GDI activity on GN-AlphaI1. RGS14 inhibits guanine nucleotide exchange on GN-AlphaI1 and GN-AlphaI3, but not GN-AlphaI2. Another protein, Caveolin-1 also act as negative regulator of GN-AlphaI. The G-Proteins regulate important cellular components, such as metabolic enzymes, ion channels, and the transcriptional machinery. The resulting alterations in cellular behavior and function are manifested in many critical systemic functions, including embryonic development, learning and memory, and organismal homeostasis. This results in the propagation of regulated activities through increasingly complex layers of organization to serve as the basis of integration at the systemic level (Ref.7, 8 & 9).