A highly conserved tryptophane residue indispensable for cloned rat neuronal P2X receptor activation

The role of a tryptophane residue (Trp(256)) in the extracellular loop of a neuronal P2X receptor clone (P2X(2) receptor/channel) was investigated using site-directed mutagenesis and Xenopus oocyte expression. When Trp(256) was replaced with leucine, serine or phenylalanine (W256L, W256S or W256F), a current response to adenosine triphosphate (ATP) mediated through the P2X2 receptor/channel was abolished. When replaced with tyrosine (W256Y), the response was not abolished, but a reduced current response to ATP was observed. The insertion of a tryptophane residue in W256L at positions close to position 256 failed to recover the responsiveness to ATP. These results suggest that an amino acid residue with a side chain of an aromatic ring with a hydroxy group (tryptophane or tyrosine) is necessary exactly at position 256 for P2X(2) receptor/channel activation.