Mutations causing neurodegeneration in caenorhabdits elegans drastically alter the pH sensitivity in inactivation of the mammalin H+-gated Na+ Channel MDEG1

Abstract

The mammalian degenerin MDEG1 belongs to the nematode degenerin/epithelia Na+ channel superfamily. It is constitutively activated by the same mutations that cause gain-of-function of the Caenorhabditis elegans degenerins and neurodegeneration. ASIC and DRASIC, which were recently cloned, are structural homologues of MDEG1 and behave as H+-gated cation channels. MDEG1 is also a H+-activated Na+ channel, but it differs from ASIC in its lower pH sensitivity and alower kinetics. In addition to the generation of a constitutive current, mutations in MDEG1 also alter the properties of the H+-gated current. Replacement of Gly-430 in MDEG1 by bulkier amino acids, such as Val, Phe, or Thr, drastically increases the H+ sensitivity of the channel (half-maximal pH (pH(m)) ~4.4 for MDEG1, pH(m) ~6.7 for the differnet mutants). Furthermore, these replacements completely suppress the inactivation observed with the wild-type channel and increase the sensitivity of the H+-gated channel to blockade by amiloride by a factor of 10 without modification of its conductance and ionic selectivity. These results as well as those obtained with other mutants clearly indicate that the region surrounding Gly-430, situated just before the second transmembrane segment, is essential for pH sensitivity and gating.