An exceptional structural feature of three TRPM members is the presence of entire functional enzymes in their COOH termini: TRPM2 contains a functional NUDT9 homology domain exhibiting ADP-ribose pyrophosphatase activity (209, 344), whereas both TRPM6 and TRPM7 contain a functional COOH-terminal a-kinase (an atypical serine/threonine kinase)
(162, 282, 379, 390).