Copper (II) binding to the human Doppel protein may mark its functional diversità from the prion protein

Doppel (Dpl) is the first described homologue of the prion protein, the main constituent of the agent responsible for prion diseases. The cellular prion protein (PrPC) is predominantly present in the central nervous system. Although its role is not yet completely clarified, PrPC seems to be involved in Cu2+ recycling from synaptic clefts and in preventing neuronal oxidative damage. Conversely, Dpl is expressed in heart and testis and has been shown to regulate male fertility by intervening in gametogenesis and sperm-egg interactions. Therefore, despite a high sequence homology and a similar three-dimensional fold, the functions of PrPC and Dpl appear unrelated. Here we show by electron paramagnetic resonance and fluorescence spectroscopy that the in vitro binding of copper(II) to human recombinant Dpl occurs with a different pattern from that observed for recombinant PrP. At physiological pH values, two copper(II)-binding sites with different affinities were found in Dpl. At lower pH values, two additional copper(II)-binding sites can be identified as follows: one complex is present only at pH 4, and the other is observed in the pH range 5-6. As derived from the electron paramagnetic resonance characteristics, all Dpl-copper(II) complexes have a different coordination sphere from those present in PrP. Furthermore, in contrast to the effect shown previously for PrPC, addition of Cu2+ to Dpl-expressing cells does not cause Dpl internalization. These results suggest that binding of the ion to PrPC and Dpl may contribute to the different functional roles ascribed to these highly homologous proteins.