Gustducin is a G protein associated with taste and the gustatory system, found in some taste receptor cells. Research on the discovery and isolation of gustducin is recent. It is known to play a large role in the transduction of bitter, sweet and umami stimuli. Its pathways (especially for detecting bitter stimuli) are many and diverse. An intriguing feature of gustducin is its similarity to transducin. These two G proteins have been shown to be structurally and functionally similar, leading researchers to believe that the sense of taste evolved in a similar fashion to the sense of sight. Gustducin is a heterotrimeric protein composed of the products of the GNAT3 (α-subunit), GNB1 (β-subunit) and GNG13 (γ-subunit).
Discovery Gustducin was discovered in 1992 when degenerate oligonucleotide primers were synthesized and mixed with a taste tissue cDNA library. The DNA products were amplified by the polymerase chain reaction method, and eight positive clones were shown to encode the α subunits of G-proteins, (which interact with G-protein-coupled receptors). Of these eight, two had previously been shown to encode rod and cone α-transducin. The eighth clone, α-gustducin, was unique to the gustatory tissue.
Comparisons with transducin Upon analyzing the amino-acid sequence of α-gustducin, it was discovered that α-gustducins and α-transducins were closely related. This work showed that α-gustducin's protein sequence gives it 80% identity to both rod and cone a-transducin. Despite the structural similarities, the two proteins have very different functionalities. However, the two proteins have similar mechanism and capabilities. Transducin removes the inhibition from cGMP Phosphodiesterase, which leads to the breakdown of cGMP. Similarly, α-gustducin binds the inhibitory subunits of taste cell cAMP Phosphodiesterase which causes a decrease in cAMP levels. Also, the terminal 38 amino acids of α-gustducin and α-transducin are identical. This suggests that gustducin can interact with opsin and opsin-like G-coupled receptors. Conversely, this also suggests that transducin can interact with taste receptors. The structural similarities between gustducin and transducin are so great that comparison with transducin were used to propose a model of gustducin's role and functionality in taste transduction. Other G protein α-subunits have been identified in TRCs (e.g. Gαi-2, Gαi-3, Gα14, Gα15, Gαq, Gαs) with function that has not yet been determined.
Location While gustducin was known to be expressed in some taste receptor cells (TRCs), studies with rats showed that gustducin was also present in a limited subset of cells lining the stomach and intestine. These cells appear to share several feature of TRCs. Another study with humans brought to light two immunoreactive patterns for α-gustducin in human circumavallate and foliate taste cells: plasmalemmal and cytosolic. These two studies showed that gustducin is distributed through gustatory tissue and some gastric and intestinal tissue and gustducin is presented either in the cytoplasm or in apical membranes on TRC surfaces. Research showed that bitter-stimulated type 2 taste receptors (T2R/TRB) are only found in taste receptor cells positive for the expression of gustducin. α-Gustducin is selectively expressed in ~25–30% of TRCs
Evolution of the gustducin-mediated signaling model Due to its structural similarity to transducin, gustducin was predicted to activate a phosphodiesterase (PDE). Phosphodieterases were found in taste tissues and their activation was tested in vitro with both gustducin and transducin. This experiment revealed transducin and gustducin were both expressed in taste tissue (1:25 ratio) and that both G proteins are capable of activating retinal PDE. Furthermore, when present with denatonium and quinine, both G proteins can activate taste specific PDEs. This indicated that both gustducin and transducin are important in the signal transduction of denatonium and quinine. The 1992 research also investigated the role of gustducin in bitter taste reception by using "knock-out" mice lacking the gene for α-gustducin. A taste test with knock-out and control mice revealed that the knock-out mice showed no preference between bitter and regular food in most cases. When the α-gustducin gene was re-inserted into the knock-out mice, the original taste ability returned. However, the loss of the α-gustducin gene did not completely remove the ability of the knock-out mice to taste bitter food, indicating that α-gustducin is not the only mechanism for tasting bitter food. It was thought at the time that an alternative mechanism of bitter taste detection could be associated with the βγ subunit of gustducin. This theory was later validated when it was discovered that both peripheral and central gustatory neurons typically respond to more than one type of taste stimulant, although a neuron typically would favor one specific stimulant over others. This suggests that, while many neurons favor bitter taste stimuli, neurons that favor other stimuli such as sweet and umami may be capable of detecting bitter stimuli in the absence of bitter stimulant receptors, as with the knock-out mice.
Second messengers IP3 and cAMP Until recently, the nature of gustducin and its second messengers was unclear. It was clear, however, that gustducin transduced intracellular signals. Spielman was one of the first to look at the speed of taste reception, utilizing the quenched-flow technique. When the taste cells were exposed to the bitter stimulants denatonium and sucrose octaacetate, the intracellular response - a transient increase of IP3 - occurred within 50-100 millisecond of stimulation. This was not unexpected, as it was known that transducin was capable of sending signals within rod and cone cells at similar speeds. This indicated that IP3 was one of the second messengers used in bitter taste transduction. It was later discovered that cAMP also causes an influx of cations during bitter and some sweet taste transduction, leading to the conclusion that it also acted as a second messenger to gustducin.
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