TRPM4 and TRPM5 Channels Share Crucial Amino Acid Residues for Ca(2+) Sensitivity but Not Significance of PI(4,5)P(2)

Transient receptor potential melastatin member 4 (TRPM4) and 5 (TRPM5) channels are Ca(2+)-activated nonselective cation channels. Intracellular Ca(2+) is the most important regulator for them to open, though PI(4,5)P(2), a membrane phosphoinositide, has been reported to regulate their Ca(2+)-sensitivities. We previously reported that negatively-charged amino acid residues near and in the TRP domain are necessary for the normal Ca(2+) sensitivity of TRPM4. More recently, a cryo-electron microscopy structure of Ca(2+)-bound (but closed) TRPM4 was reported, proposing a Ca(2+)-binding site within an intracellular cavity formed by S2 and S3. Here, we examined the functional effects of mutations of the amino acid residues related to the proposed Ca(2+)-binding site on TRPM4 and also TRPM5 using mutagenesis and patch clamp techniques. The mutations of the amino acid residues of TRPM4 and TRPM5 reduced their Ca(2+)-sensitivities in a similar way. On the other hand, intracellular applications of PI(4,5)P(2) recovered Ca(2+)-sensitivity of desensitized TRPM4, but its effect on TRPM5 was negligible. From these results, the Ca(2+)-binding sites of TRPM4 and TRPM5 were shown to be formed by the same amino acid residues by functional analyses, but the impact of PI(4,5)P(2) on the regulation of TRPM5 seemed to be smaller than that on the regulation of TRPM4.