Biosynthesis of Prion Protein Nucleocytoplasmic Isoforms by Alternative Initiation of Translation

Abstract The cellular prion protein PrPC is synthesized as a family of four distinct forms. Of these, CytPrP is a minor member that segregates outside of the secretory route and can generate cytotoxic forms. Using signal sequence mutants, we found that CytPrP is translated from a downstream AUG (coding for Met-8 in human PrP or Met-15 in Syrian hamster PrP). Shortening of the signal sequence dictated the spillage of this isoform into the cytosol, from where it accessed the nucleus or formed insoluble cytosolic aggregates if the proteasome is inhibited. The PrP isoform isolated from the nuclear fractions of cell and brain homogenates was partially SUMO-1-conjugated. Expression of HaPrP(M15) in cells caused an antiproliferative phenotype due to a cell cycle arrest at the G0/G1 phase. The identification of this PrP isoform and its properties provides novel insight into PrPC physiological and pathological functions.