Background: The common event in transmissible spongiform encephalopathies (TSEs) or prion diseases is the conversion of host-encoded protease sensitive cellular prion protein (PrPC) into strain dependent isoforms of scrapie associated protease resistant isoform (PrPSc) of prion protein (PrP). These processes are determined by similarities as well as strain dependent variations in the PrP structure. Selective self-interaction between PrP molecules is the most probable basis for initiation of these processes, potentially influenced by chaperone molecules, however the mechanisms behind these processes are far from understood. We previously determined that polymorphisms do not affect initial PrPC to PrPSc binding but rather modulate a subsequent step in the conversion process. Determining possible sites of self-interaction could elucidate which amino acid(s) or amino acid sequences contribute to binding and further conversion into other isoforms. To this end, ovine - and bovine PrP peptide-arrays consisting of 15-mer overlapping peptides were probed with recombinant sheep PrPC fused to maltose binding protein (MBP-PrP).
Results: The peptide-arrays revealed two distinct high binding areas as well as some regions of lower affinity in PrPC resulting in total in 7 distinct amino acid sequences (AAs). The first high binding area comprises sheep-PrP peptides 43-102 (AA 43-116), including the N-terminal octarepeats. The second high binding area of sheep-PrP peptides 134-177 (AA 134-191), encompasses most of the scrapie susceptibility-associated polymorphisms in sheep. This concurs with previous studies showing that scrapie associated-polymorphisms do not modulate the initial binding of PrPC to PrPSc. Comparison of ovine - and bovine peptide-array binding patterns revealed that amino acid specific differences can influence the MBP-PrP binding pattern. PrP-specific antibodies were capable to completely block interaction between the peptide-array and MBP-PrP. MBP-PrP was also capable to specifically bind to PrP in a Western blot approach. The octarepeat region of PrP seems primarily important for this interaction because proteinase K pre-treatment of PrPSc completely abolished binding.
Conclusion: Binding of MBP-PrP to PrP-specific sequences indicate that several specific self-interactions between individual PrP molecules can occur and suggest that an array of interactions between PrPC-PrPC as well as PrPC-PrPSc may be possible, which ultimately lead to variations in species barrier and strain differences.