A PRION PROTEIN CYCLES BETWEEN THE CELL-SURFACE AND AN ENDOCYTIC COMPARTMENT IN CULTURED NEUROBLASTOMA-CELLS

The prion protein (PrP(C)) is a glycolipid-anchored, cell surface prot ein of unknown function, a posttranslationally modified isoform of whi ch has been implicated in the pathogenesis of spongiform encephalopath ies in man and animals. We report here the novel observation that chPr P, the chicken homologue of mammalian PrP(C), constitutively cycles be tween the cell surface and an endocytic compartment with a transit tim e of approximately 60 min, as demonstrated by surface iodination and i mmunofluorescence microscopy. Most (>95%) of the internalized protein is returned to the cell surface intact, and the remainder is proteolyt ically cleaved within a highly conserved region in the NH2-terminal ha lf of the molecule. Pulse-chase labeling experiments indicate that whi le this cleavage is slow, with a rate of approximately 1%/h, the COOH- terminal fragment produced is stable and accumulates on the cell surfa ce for as long as 24 h. The cleavage is likely to take place in an aci dified endocytic compartment, since it is reduced by lysosomotropic am ines and inhibitors of lysosomal proteases. Our results raise the poss ibility that chPrP, and perhaps other PrP(C)s, function as cell surfac e receptors, and they suggest cellular pathways that might be involved in the generation of the pathogenic isoform.