Structural Elements Involved in Specific K+ Channel Functions

K+ channels represent a diverse group of ion channels that have been found in most eukaryotic cells of the animal and plant kingdom. The functions of these channels, such as the regulation of neuronal excitability and hormone release, and movements of the stomatal pores in plants (12, 17, 36, 38, 42a, 75, 87), depend on the specific manner in which a particular K+ channel opens and closes, and its selective penneation by K+ ions. A wide range of these channel properties has been observed, which are probably crucial for subserv­ ing different physiological functions. For example, some K+ channels are activated or modulated by second messengers such as Ca2+, A TP, inositol triphosphate, and G proteins, while others are activated by changes in mem­ brane potential (11, 18, 51, 64, 81, 84, 96). Whereas some K+ channels, such as the inward rectifiers, are voltage-dependent because their pores are blocked in a voltage-dependent manner by certain cations (e.g. Mg2+) (61, 62), other channels appear to be intrinsically sensitive to voltage so that changes of membrane potential induce conformational changes in these chan­ nels that lead to channel opening (36). These voltage-gated K+ channels differ in the range over which changes in membrane potential produce channel activation and in the rates of transitions between the open (conducting) and various non-conducting states of the channel (36, 81, 84).