Mutations causing neurodegeneration in Caenorhabditis elegans drastically alter the pH sensitivity and inactivation of the mammalian H+-gated Na+ channel MDEG1

The mammalian degenerin MDEG1 belongs to the nematode degenerin/epithelial 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 slower 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)) similar to 4.4 for MDEG1, pH(m) similar to 6.7 for the different 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.