The membrane interaction and hydrophobicity of the normal (PrPC) and infectious isoform (PrPSc/CJD) of scrapie and Creutzfeldt-Jakob disease amyloid precursor proteins was studied. The normal isoform of hamster and human scrapie amyloid precursor protein was found on the microsomal/synaptosomal membranes anchored solely by the C-terminal glycolipid. Glycolipid cleavage resulted in dissociation from the membranes and change of behavior from a highly hydrophobic to a hydrophilic protein, susceptible to proteases. In contrast, the PrPSc/CJD isoform was resistant to release by glycolipid-cleaving enzymes. A part of PrPSc/CJD was released from the membranes after prolonged trypsin treatment, yielding a further protease-resistant product of 27-30 kDa. The results demonstrate the proteolytic resistance of the membrane-bound PrPSc/CJD isoform and also indicate the presence of a different, apparently disease-induced mechanism of membrane interaction in the scrapie- and CJD-infected microsomal and synaptosomal membranes.