Figure 1. Mechanosensory complex for Caenorhabditis elegans body touch sensory neurons. (a) MEC-4 and MEC-10 are transmembrane degenerin channel subunits expressed by the sensory neuron (lower left box). The exact composition of the channel complex has not been determined. MEC-2 is a stomatin-like protein proposed to couple the channel complex to the tubulin cytoskeleton (including MEC-12 and MEC-7) of the sensory neuron. The MEC-5, MEC-9 and MEC-1 proteins couple the cuticle (skin) to the channel complex and are embedded within the mantle underlying the cuticle and secreted by the hypodermal cell. Movement of the mantle versus the cytoskeleton due to touch is thought to cause the channel complex to open. Many of the interactions shown are based on genetic evidence. (b) Genes directly involved in signal transduction of touch in the C. elegans body touch sensory neurons. The names of the genes are listed along with prominent homologies or motifs.

Figure 2. Phototransduction in vertebrates and Drosophila. (a) Transduction in vertebrates. Light activates the rhodopsin–retinal complex (Rh to Rh^*). The activated complex is coupled to transducin; guanosine triphosphate (GTP)-bound -transducin activates phosphodiesterase which converts cyclic guanosine monophosphate (cGMP) into 5¢-GMP. The resulting decrease in cGMP concentration causes dependent sodium/calcium channels to close, hyperpolarizing the cell and decreasing steady state synaptic neurotransmitter (glutamate) release. (b) Transduction in Drosophila. Light activates the rhodopsin–retinal complex (Rh to Rh^*). The activated complex is coupled to transducin; GTP-bound -transducin activates phospholipase C which converts phosphatidylinositol bisphosphate (PIP₂) into inositol triphosphate (IP₃) and diacylglycerol (DAG). Intracellular calcium rises activate trp and trp-like channels which depolarize the photoreceptor and cause synaptic neurotransmitter release. GDP, guanosine diphosphate; PDE, phosphodiesterese; PKC, protein kinase C; PLC, phospholipase C; rdgC, retinal degeneration phosphates; Ro, rhodopsin; trp, transient receptor potential.

Figure 3. Signal transduction pathways for olfaction and gustation. (a) Transduction pathway for olfaction. Binding of odorants to G protein-coupled receptors (GPCR) leads to the activation of adenylyl cyclase (AC) via G_olf. The resultant increase in cyclic adenosine monophosphate (cAMP) levels gates the opening of a cyclic nucleotide-gated (CNG) channel leading to depolarization. A calcium–calmodulin complex interacts with the channel to modulate the response. (b) Transduction pathway for bitter compounds. Bitter compounds activate phosphodiesterase (PDE) via gustducin (Gust). Lowered levels of cAMP result in the opening of a cyclic nucleotide-blocked cation channel. Bitter compounds also result in an increase in cellular inositol triphosphate (IP₃) levels, leading to the release of calcium from intracellular stores. The subunits of gustducin may activate phospholipase C (PLC) in this pathway. (Adapted from Kinnamon, 1996). (c) Transduction of sweet compounds. Natural sugars result in the activation of an adenylyl cyclase and an increase in cAMP levels. Increased cAMP causes the activation of protein kinase A (PKA) and phosphorylation and closure of basolateral  K⁺ channels. Synthetic sweeteners stimulate PLC to produce IP₃ and diacylglycerol (DAG). DAG activates protein kinase C (PKC) which may phosphorylate the same  K⁺ channels. The role of gustducin in sweet taste transduction is unclear. (Adapted from Kinnamon, 1996). Proposed pathways are indicated by arrows.