Small is not beautiful: antagonizing functions for the prion protein PrP(C) and its homologue Dpl

A conformational variant of the normal prion protein PrP(C) is believed to be identical to PrP(Sc), the agent that causes prion diseases. Recently, a novel protein, named Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrP(C). In specific strains of PrP(C)-deficient mouse lines, Dpl is overexpressed and causes a neurological disease. Dpl neurotoxicity is counteracted and prevented by PrP(C), but the mechanism of antagonistic PrP(C)-Dpl interaction remains elusive. In contrast to its homologue PrP(C), initial studies suggest that Dpl is dispensable for prion disease progression and for the generation of PrP(Sc). Although we are only beginning to understand its function, the discovery of Dpl has already provided some answers to long-standing questions and is transforming our understanding of prion biology.