Figure 1. Effects of cholera toxin (CT) and pertussis toxin (PT) on the GTPase cycle of G proteins. HR, hormone receptor; P_i, inorganic phosphate. See text for details.

Figure 2. Schematic presentation of the transmembrane arrangement of the principal subunits of voltage-gated ion channels. Sodium and calcium channels share the general structure of four homologous repeated domains (I–IV) that are organized in the membrane around a central pore, the ion-conducting pathway. The potassium channel is composed of four polypeptides, each homologous to one repeat of the sodium or calcium channel domains. Each domain consists of six putative transmembrane -helical segments (S1–S6). The most highly conserved segment is S4, which contains a unique motif of a positively charged amino acid residue followed by two nonpolar residues that is repeated four to eight times in each S4 helix. These S4 structures, which participate in the voltage-sensing mechanism, are indicated in each domain by a (+) sign. The short segments (SS1 and SS2), which are invaginations of the loops between transmembrane segments S5 and S6, form part of the ion-conducting pathway, the ‘pore region’ (area bounded by a jagged line). Black triangles represent the external vestibules of the pore. The linker connecting domains III and IV in the sodium channels represents a region that is thought to participate in the fast inactivation process.