Spontaneous deamidation and isomerization of Asn108 in prion peptide 106-126 and in full-length prion protein

In prion-related encephalopathies, the cellular prion protein (PrPC) underg oes a change in conformation to become the scrapie prion protein (PrPSc) wh ich forms infectious deposits in the brain. Conceivably, the conformational transition of PrPC to PrPSc might be linked with posttranslational alterat ions in the covalent structure of a fraction of the PrP molecules. We teste d a synthetic peptide corresponding to residues 106-126 of human PrP for th e occurrence of spontaneous chemical modifications. The only asparagine res idue, Asn108, was deamidated to aspartic acid and isoaspartic acid with a h alf-life of about 12 days. The same posttranslational modifications were fo und in recombinant murine full-length protein. On aging, 0.8 mol of isoaspa rtyl residue per mole of protein was detected by the protein-L-isoaspartyl methyltransferase assay (t(1/2) similar to 30 days), Mass spectrometry and Edman degradation of Lys-C fragments identified Asn108 in the amino-termina l flexible part of the protein to be partially converted to aspartic acid a nd isoaspartic acid. A second modification was the partial isomerization of Asp226' which is only present in rodents. (C) 1999 Academic Press.