The extracellular loop of P2X channel proteins contains a sequence stretch (positions 170-330) that exhibits similarities with the catalytic domains of class II aminoacyl-tRNA synthetases as shown by secondary structure predictions and sequence alignments. The arrangement of several conserved cysteines (positions 110-170) shows similarities with metal binding regions of metallothioneins and zinc finger motifs. Thus, for the extracellular part of P2X channel proteins a metal binding domain and an antiparallel six-stranded beta-pleated sheet containing the ATP binding site are very probable. The putative channel forming H5 part (positions 320-340) shows similarities with the enzyme motif 1 responsible for aggregation of subunits to the holoenzyme.