Sequence similarity between opioid peptide precursors and DNA-binding proteins

The opioid peptide precursors, preprodynorphin and preproenkephalin show structure similarity with a transcription factor, hunchback and the putative helix-loop-helix DNA-binding proteins, lil-1, tal and twist. Segments with similarity contain the three enkephalin sequences in preprodynorphin and one in preproenkephalin which are present within heptapeptide repeats characteristic of an α-helical coiled-coil structure distinctive of an amphipathic helix-loop-helix DNA-binding motif, Hunchback and the opioid prohormones also have cystein-rich regions characteristic of zinc-finger domains in common.