Dominant loss of responsiveness to sweet and bitter compounds caused by a single mutation in α-gustducin

Biochemical and genetic studies have implicated alpha -gustducin as a key component in the transduction of both bitter or sweet taste. Yet, alpha -gustducin-null mice are not completely unresponsive to bitter or sweet compounds. To gain insights into how gustducin mediates responses to bitter and sweet compounds, and to elicit the nature of the gustducin-independent pathways, we generated a dominant-negative form of alpha -gustducin and expressed it as a transgene from the alpha -gustducin promoter in both wild-type and alpha -gustducin-null mice. A single mutation, G352P, introduced into the C-terminal region of alpha -gustducin critical for receptor interaction rendered the mutant protein unresponsive to activation by taste receptor, but left its other functions intact. In control experiments, expression of wild-type alpha -gustducin as a transgene in alpha -gustducin-null mice fully restored responsiveness to bitter and sweet compounds, formally proving that the targeted deletion of the alpha -gustducin gene caused the taste deficits of the null mice. In contrast, transgenic expression of the G352P mutant did not restore responsiveness of the null mice to either bitter or sweet compounds. Furthermore, in the wild-type background, the mutant transgene inhibited endogenous alpha -gustducin's interactions with taste receptors, i.e., it acted as a dominant-negative. That the mutant transgene further diminished the residual bitter and sweet taste responsiveness of the alpha -gustducin-null mice suggests that other guanine nucleotide-binding regulatory proteins expressed in the alpha -gustducin lineage of taste cells mediate these responses.