Negatively-charged Amino Acids near and in Transient Receptor Potential (TRP) Domain of TRPM4 Channel Are One Determinant of Its Ca2+ Sensitivity [Molecular Biophysics]

November 6th, 2014 by Yamaguchi, S., Tanimoto, A., Otsuguro, K.-i., Hibino, H., Ito, S.

Transient Receptor Potential (TRP) channel Melastatin subfamily member 4 (TRPM4) is a broadly expressed nonselective monovalent cation channel. TRPM4 is activated by membrane depolarization and intracellular Ca2+, which is essential for the activation. The Ca2+ sensitivity is known to be regulated by calmodulin and membrane phosphoinositides, such as PI(4,5)P2. Although these regulators must play important roles in controlling TRPM4 activity, mutation analyses of the calmodulin binding sites have suggested that Ca2+ binds to TRPM4 directly. However, the intrinsic binding sites in TRPM4 remain to be elucidated. Here, by using patch-clamp and molecular biological techniques, we show that there are at least two functionally different divalent cation binding sites and the negatively charged amino acids near and in the TRP domain in C-terminal tail of TRPM4 (D1049 and E1062 of rat TRPM4) are required for maintaining the normal Ca2+ sensitivity of one of the binding sites. Applications of Co2+, Mn2+, or Ni2+ to the cytosolic side potentiated TRPM4 currents, increased the Ca2+ sensitivity, but were unable to evoke TRPM4 currents without Ca2+. Mutations of the acidic amino acids near and in the TRP domain, which are conserved in TRPM2, TRPM5, and TRPM8, deteriorated the Ca2+ sensitivity in the presence of Co2+ or PI(4,5)P2 but hardly affected the sensitivity to Co2+ and PI(4,5)P2. These results suggest a novel role of the TRP domain in TRPM4 as a site responsible for maintaining the normal Ca2+ sensitivity. These findings provide more insights into the molecular mechanisms of the regulation of TRPM4 by Ca2+.
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