Functional analysis of ctenophore Shaker K+ channels: N-type inactivation in the animal roots

Biophys J. 2024 Jul 16;123(14):2038-2049. doi: 10.1016/j.bpj.2024.01.027. Epub 2024 Jan 30.

Abstract

Here we explore the evolutionary origins of fast N-type ball-and-chain inactivation in Shaker (Kv1) K+ channels by functionally characterizing Shaker channels from the ctenophore (comb jelly) Mnemiopsis leidyi. Ctenophores are the sister lineage to other animals and Mnemiopsis has >40 Shaker-like K+ channels, but they have not been functionally characterized. We identified three Mnemiopsis channels (MlShak3-5) with N-type inactivation ball-like sequences at their N termini and functionally expressed them in Xenopus oocytes. Two of the channels, MlShak4 and MlShak5, showed rapid inactivation similar to cnidarian and bilaterian Shakers with rapid N-type inactivation, whereas MlShak3 inactivated ∼100-fold more slowly. Fast inactivation in MlShak4 and MlShak5 required the putative N-terminal inactivation ball sequences. Furthermore, the rate of fast inactivation in these channels depended on the number of inactivation balls/channel, but the rate of recovery from inactivation did not. These findings closely match the mechanism of N-type inactivation first described for Drosophila Shaker in which 1) inactivation balls on the N termini of each subunit can independently block the pore, and 2) only one inactivation ball occupies the pore binding site at a time. These findings suggest classical N-type activation evolved in Shaker channels at the very base of the animal phylogeny in a common ancestor of ctenophores, cnidarians, and bilaterians and that fast-inactivating Shakers are therefore a fundamental type of animal K+ channel. Interestingly, we find evidence from functional co-expression experiments and molecular dynamics that MlShak4 and MlShak5 do not co-assemble, suggesting that Mnemiopsis has at least two functionally independent N-type Shaker channels.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Ctenophora* / genetics
  • Ctenophora* / metabolism
  • Ion Channel Gating*
  • Oocytes / metabolism
  • Phylogeny
  • Shaker Superfamily of Potassium Channels* / chemistry
  • Shaker Superfamily of Potassium Channels* / genetics
  • Shaker Superfamily of Potassium Channels* / metabolism

Substances

  • Shaker Superfamily of Potassium Channels