Characterization of major peptides in Creutzfeldt-Jakob disease and scrapie.

In Creutzfeldt-Jakob disease three major peptides cosediment with the infectious agent. These distinct peptides are not present in identical fractions from uninfected brain, and bind to polyclonal antibodies raised against "prion protein" purified by protease treatment. Three similar distinct peptides are also found in scrapie-infected brain fractions purified without the use of proteases. To clarify the relationships between these distinct peptides and prion protein, peptides were analyzed on immunoblots after cleavage with various glycosidases. There are two different 34-kDa peptides. One binds to ricin and cannot be detected by nonequilibrium pH gradient electrophoresis, presumably due to its highly acidic or basic pI. A second basic 34-kDa glycopeptide (Gp34) contains multiple terminal sialic acid residues responsible for charge heterogeneity (pI values, 7.2-7.8) and is reduced to a single spot with a pI value of 7.8 after neuraminidase treatment. After (but not before) neuraminidase treatment, secondary D-galactose-like sugars are detectable on Gp34, and a small number of N-acetylglucosamine residues probably represent the third sugar residue in an N-linked chain. When virtually all sugar residues are removed with endoglycosidase H the molecular weight of Gp34 is reduced by only approximately equal to 2 kDa. The residual peptide strongly binds antibody. A third acidic 24- to 26-kDa species (p26) also binds polyclonal antibodies but, in contrast to Gp34, was unaffected by any glycosidase treatment. Protease-treated peptides showed a very broad array of pI spots, consistent with a heterogeneous protein origin. None of the nonproteolyzed peptides show a clear relationship to prion protein. The number of sugar residues on Gp34 is not consistent with those estimated for prion protein. Although p26 could be the source of the "prion sequence," p26 does not appear to be glycosylated. Regardless, it is likely that all the major peptides described thus far are accumulated or modified normal gene products and are not integral components of the infectious agent.