Structure and function of PCD/DCoH, an enzyme with regulatory properties.

The bifunctional protein PCD/DCoH is both an enzyme involved in the phenylalanine hydroxylation system and a transcription coactivator forming a 2:2 heterotetrameric complex with the nuclear transcription factor HNF1. The discovery of a bacterial homologue and the expression pattern during Xenopus embryogenesis suggest a regulatory function not only restricted to HNF1. The crystal structures of the tetrameric rat and the dimeric bacterial PCD/DCoH have led to the proposal of substrate and HNF1 binding sites. The saddle-shaped beta-sheet surfaces of the DCoH dimers likely represent binding sites for as yet unknown macromolecular interaction partners. Possible mechanisms for DCoH-induced transcriptional regulation are discussed in the light of the three-dimensional structures.