MOLECULAR-CLONING AND FUNCTIONAL EXPRESSION OF CDNA-ENCODING A 2ND CLASS OF INWARD RECTIFIER POTASSIUM CHANNELS IN THE MOUSE-BRAIN

We have cloned a second class of inward rectifier potassium channels, designated MB-IRK2, from a mouse brain cDNA library. The amino acid se quence of this clone shares 70% identity with the mouse IRIK1. Xenopus oocytes injected with cRNA derived from MB-IRIK2 expressed a K+ curre nt, which showed inward rectifying channel characteristics similar to the MB-IRK1 current. In contrast to the MB-IRK1 current, however, the MB-IRK2 current exhibited significant inactivation during hyperpolariz ing pulses. In patch clamp experiments with 140 mM K+ in the pipette, the single channel conductance of MB-IRK2 was 34.2 +/- 2.1 picosiemens (n = 5), a value significantly larger than that of MB-IRK1 (22.2 +/- 3.0 picosiemens, n = 5). Consistent with the whole cell current, the s teady-state open probability (P-o) of the MB-IRK2 channel decreased wi th hyperpolarization, whereas that of the MB-IRK1 remained constant. N orthern blot analysis revealed the mRNA for MB-IRK2 to be expressed in forebrain, cerebellum, heart, kidney, and skeletal muscle. In the bra in, the abundance of mRNA for MB-IRK2 was much higher in cerebellum th an in forebrain and vice versa in the case of MB-IRK1. These results d emonstrate that the IRK family is composed of multiple genes, which ma y play heterogenous functional roles in various organs, including the central nervous system.