A neurotransmitter is a chemical that is released into the synaptic cleft, the space between a presynaptic neuron and its target cell or organ, and that binds to specialized proteins called receptors on the postsynaptic target. Some of these specialized receptors are G proteins, also known as guanosine nucleotide-binding proteins. G proteins are a group or family of proteins or GTPases (enzymes that hydrolyze guanosine triphosphate, GTP) that sense chemical stimuli outside of its cell. Once this external stimulus occurs, it results in the G protein changing its shape and activating molecular pathways or second-messenger cascades within the cell. This means a G protein can be thought of as a molecular switch.
In the 1960s through the early 1970s, American biochemists Alfred Goodman Gilman (1941—) and Martin Rodbell (1925—1998) were studying the process of how adrenaline stimulated cells. During their research, they discovered that G proteins were the target receptor for adrenaline. Specifically, with the binding of adrenaline to a G protein, the G protein would undergo a conformational change and stimulate another protein or enzyme (such as adenylate cyclase that makes the second messenger cyclic adenosine monophosphate or cAMP) located within the cell. This would start a cascade of other molecular signaling inside the cell, thus changing its activity. For this contribution to science, Gilman and Rodbell won the 1994 Nobel Prize in Physiology or Medicine.
Anatomy and Function
G proteins are named because of their ability to bind GTP and break apart (hydrolyze) GTP to guanosine diphosphate (GDP), which releases some energy within the cell. A G protein is considered active when GTP is bound to the complex and is inactive when GDP is bound. G proteins are divided into two classes based on their function: monomeric small GTPases and heterotrimeric G protein complexes. The latter class consists of several different subunits including alpha (α), beta (β), and gamma (γ). G proteins are generally located just beneath the cell membrane within the cytoplasm and are coupled with a receptor that spans the membrane. This makes up a G protein-coupled receptor (GPCR) that can be found in many cell types. Within the central nervous system, these GPCRs are usually associated with sensory systems like olfaction and gustation or with producing longer lasting or modulated responses in neurons. For instance, the G protein Golf is necessary for odorant signal transduction and is found in olfactory neuroepithelium (Jones & Reed, 1989) while other G proteins (TAS1Rs) are necessary for taste transduction of sweet, bitter, and umami in taste cells (Sanemastu et al., 2014). Lastly, G proteins and GPCRs are known to regulate many cell activities such as cell contractility and motility, controlling transcription, and secretion, which can lead to regulating larger functions like embryonic development, learning and memory, and homeostasis, to name just a few.
Jennifer L. Hellier
See also: Bitter Sensation; Olfactory Sensory Neurons; Olfactory System; Sensory Receptors; Sweet Sensation; Taste System; Type I Taste Cells; Type II Taste Cells; Type III Taste Cells; Umami
Jones, D. T., & R. R. Reed. (1989). Golf: An olfactory neuron specific-G protein involved in odorant signal transduction. Science, 244(4906), 790—795.
Lans, Hannes, & Gert Jansen. (2006). Multiple sensory G proteins in the olfactory, gustatory and nociceptive neurons modulate longevity in Caenorhabditis elegans. Developmental Biology, 303(2), 474—482.
Sanematsu, Keisuke, Ryusuke Yoshida, Noriatsu Shigemura, & Yuzo Ninomiya. (2014). Structure, function, and signaling of taste G-protein coupled receptors. Current Pharmaceutical Biotechnology, 15(1), 1—11. http://dx.doi.org/10.2174/1389201015666140922105911