Voltage Gated Ion Channels

Broadly, voltage-gated ion channels are involved in the generation of electrical signals in excitable cells such as neurons (Table I). Voltage-gated sodium channels are activated when the membrane potential reaches a certain threshold potential, and they contribute to the rapid depolarization of the membrane potential. Some invertebrate species lack voltage-gated sodium channels. In these animals, voltage-gated calcium channels may partially fulfill the roles of voltage-gated sodium channels. Voltage-gated calcium channels also mediate the entry of Ca2+ in response to depolarization. In nerve terminals, activation of voltage-gated calcium channels by axonal action potentials is a critical step in the release of synaptic vesicles.

The voltage-gated ion channels are related in structure. Potassium channels contain one domain of six transmembrane segments per subunit, whereas sodium and calcium channels contain four domains in one large a subunit. The distinguishing feature of this group of ion channels is that they are sensitive to changes in the membrane potential. A remarkable feature of these ion channels is that they contain basic amino acids interspersed in the fourth transmembrane segment (S4). These charges could potentially sense the electric field across the membrane, and movement of S4 could transmit changes in the membrane potential to the gate. Although S4 is likely a part of the

Figure 9 The three-dimensional structure of ion channels. (a) The top panels show stereoviews of the KcsA potassium channel as viewed from above the plasma membrane. KcsA is a tetramer and each of the subunits are shaded separately. The lower panel shows a side view (reprinted with permission from D.A. Doyle et al, The structure of the potassium channel: Molecular basis of K conduction and selectivity. Science 280, 73. Copyright © 1998 American Association for the Advancement of Science). (b) The side and axial views of the MscL mechanosensitive ion channel from M. tuberculosis. MscL is a pentamer (reprinted with permission from G. Chang et al, Structure of the MscL homolog from Mycobacterium tuberculosis: A gated mechanosensitive ion channel. Science 282, 2223. Copyright © 1998 American Association for the Advancement of Science).

Figure 9 The three-dimensional structure of ion channels. (a) The top panels show stereoviews of the KcsA potassium channel as viewed from above the plasma membrane. KcsA is a tetramer and each of the subunits are shaded separately. The lower panel shows a side view (reprinted with permission from D.A. Doyle et al, The structure of the potassium channel: Molecular basis of K conduction and selectivity. Science 280, 73. Copyright © 1998 American Association for the Advancement of Science). (b) The side and axial views of the MscL mechanosensitive ion channel from M. tuberculosis. MscL is a pentamer (reprinted with permission from G. Chang et al, Structure of the MscL homolog from Mycobacterium tuberculosis: A gated mechanosensitive ion channel. Science 282, 2223. Copyright © 1998 American Association for the Advancement of Science).

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