Guanine nucleotide-binding proteins (G proteins) are involved as modulators or transducers in various transmembrane signaling systems. Could be the transducin analog, an amplifier and one of the transducers of a visual impulse that performs the coupling between opsin and cGMP-phosphodiesterase. Could mediate a subset of olfactory and gustatory responses. Additional isoforms seem to exist. Isoform D is expressed only in the retina and ocellus of the adult head. Isoform A is expressed in chemosensory cells of the olfactory and taste structures, including a subset of olfactory and gustatory neurons, and in cells of the central nervous system, including neurons in the lamina ganglionaris. Belongs to the G-alpha family. G(q) subfamily.