Cryptic epitopes in N-terminally truncated prion protein are exposed in the full-length molecule: dependence of conformation on pH

Prion diseases are diseases of protein conformation. Structure-dependent antibodies have been sought to probe conformations of the prion protein (PrP) resulting from environmental changes, such as differences in pH. Despite the absence of such antibodies for full-length PrP, a recombinant Fab (D13) and a Fab derived from mAb 3F4 showed pH-dependent reactivity toward epitopes within the N-terminus of N-terminally truncated PrP(90-231). Refolding and maintaining this protein at pH > or =5.2 before immobilization on an ELISA plate inhibited reactivity relative to protein exposed to pH < or =4.7. The reactivity was not affected by pH changes after immobilization, showing retention of conformation after binding to the plate surface, although guanidine hydrochloride at 1.5-2 M was able to expose the cryptic epitopes after immobilization at pH > or =5.2. The alpha-helical CD spectrum of PrP(90-231) refolded at pH 5.5 was reduced somewhat by these pH changes, with a minor shift toward beta-sheet at pH 4 and then toward coil at pH 2. No covalent changes were caused by the pH differences. This pH dependence suggests titration of an acidic region that might inhibit the N-terminal epitopes. A similar pH dependence for a monoclonal antibody reactive to the central region identified an acidic region incorporating Glu152 as a significant participant.