CLONING OF A NOVEL COMPONENT OF A-TYPE K+ CHANNELS OPERATING AT SUBTHRESHOLD POTENTIALS WITH UNIQUE EXPRESSION IN HEART AND BRAIN

1. Proteins of the Kv4 or Shal-related subfamily are key components of transient K+ channels (A channels) operating at subthreshold values o f the membrane potential. We have cloned and characterized a new mamma lian Kv4 or Shal-related cDNA (Kv4.3) that predicts a protein with str ong sequence conservation with the other known members of this subfami ly. 2. Injection of Kv4.3 transcripts into Xenopus oocytes generates a n A type K+ current, with small but physiologically significant differ ences from the currents expressed by Kv4.2 and Kv4.1 mRNAs. Kv4.3 curr ents can be modified to resemble native A currents by coinjection with a low molecular weight mRNA fraction from rat brain which does not ex press detectable currents on its own. Particularly striking is a 7- to 10-fold increase in the rate of recovery from inactivation, a 5- to 1 0-fold increase in current magnitude and a 3- to 4-fold increase in se nsitivity to 4-amino pyridine (4-AP). 3. In situ hybridization histoch emistry was used to compare the expression of the three known Kv4 gene s. Kv4.2 and Kv4.3 (but not Kv4.1) are abundant in the adult rat brain , with each displaying a specific, but sometimes overlapping pattern o f expression. Moreover, a reciprocal gradient of expression of Kv4.2 a nd Kv4.3 transcripts is seen in some brain areas, such as in the pyram idal cell layers of the hippocampus and the granule cell layer of the cerebellum. Therefore Kv4 proteins may form heteromultimeric channels of distinct subunit composition in different neurons. Moreover, the re sults suggest that neurons such as pyramidal cells in the hippocampus and granule cells in the cerebellum represent heterogeneous cell popul ations in terms of their I-SA, and hence in their firing patterns. Kv4 .2 and Kv4.3 also display complementary expression in the heart, with Kv4.3 being more abundant in atria and Kv4.2 in ventricle. The existen ce of multiple Kv4 proteins forming channels of variable subunit combi nations helps explain the diversity of I-SA channels in neurons.