Inward rectifier K+ channel
FIGURE 12-2 The major molecular motifs of ion channels that establish and regulate neuronal excitability in the central nervous system. A. The a subunits of the Ca2+ and Na+ channels share a similar presumptive six-transmembrane structure, repeated four times, in which an intramembranous segment separates transmembrane segments 5 and 6. B. The Ca2+ channel also requires several auxiliary small proteins (a2, b, g, and d). The a2 and d subunits are linked by a disulfide bond (not shown). Regulatory subunits also exist for Na+ channels. C. Voltage-sensitive K+ channels (K ) and the rapidly activating K+ channel (Ka) share a similar presumptive six-transmembrane domain similar in overall configuration to one repeat unit within the Na+ and Ca2+ channel structure, while the inwardly rectifying K+ channel protein (Kir) retains the general configuration of just loops 5 and 6. Regulatory b subunits (cystosolic) can alter Kv channel functions. Channels of these two overall motifs can form heteromultimers.
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