Prion diseases are fatal neurodegenerative disorders associated with conformational conversion of the cellular prion protein, PrP C , into a misfolded, protease-resistant form, PrP Sc . Here we show, for the first time, the oligomerization and fibrillization of the C-terminal domain of murine PrP, mPrP-(121-231), which lacks the entire unstructured N-terminal domain of the protein. In particular, the construct we used lacks amino acid residues 106 -120 from the so-called amyloidogenic core of PrP (residues 106 -126). Amyloid formation was accompanied by acquisition of resistance to proteinase K digestion. Aggregation of mPrP-(121-231) was investigated using a combination of biophysical and biochemical techniques at pH 4.0, 5.5, and 7.0 and at 37 and 65°C. Under partially denaturing conditions (65°C), aggregates of different morphologies ranging from soluble oligomers to mature amyloid fibrils of mPrP-(121-231) were formed. Transmission electron microscopy analysis showed that roughly spherical aggregates were readily formed when the protein was incubated at pH 5.5 and 65°C for 1 h, whereas prolonged incubation led to the formation of mature amyloid fibrils. Samples incubated at 65°C at pH 4.0 or 7.0 presented an initial mixture of oligomers and protofibrils or fibrils. Electrophoretic analysis of samples incubated at 65°C revealed formation of sodium dodecyl sulfate-resistant oligomers (dimers, trimers, and tetramers) and higher molecular weight aggregates of mPrP-(121-231). These results demonstrate that formation of an amyloid form with physical properties of PrP Sc can be achieved in the absence of the flexible N-terminal domain and, in particular, of residues 106 -120 of PrP and does not require other cellular factors or a PrP Sc template.The conformational conversion of the normal cellular isoform of the prion protein, PrP C , 4 into an abnormal pathological isoform, PrP Sc , underlies a group of fatal neurodegenerative disorders known as transmissible spongiform encephalopathies or prion diseases, which includes Creutzfeldt-Jakob disease in humans, scrapie in sheep and goats, and bovine spongiform encephalopathy in cattle (1). Neuropathologically, prion diseases are characterized by neuronal loss and astrogliosis and often by spongiform degeneration of the brain and deposition of amyloid plaques (1). The unique feature of these diseases is that, in addition to sporadic and inherited forms, they may be acquired by transmission of an infectious agent. The "proteinonly" hypothesis of prion propagation postulates that the abnormal isoform, PrP Sc , acts as a transmissible agent of the disease and self-propagates its pathological conformation using PrP C as a substrate (1).PrP Sc is defined as an aggregated form of PrP that is largely resistant to proteinase K (PK) digestion under conditions in which PrP C and most other proteins are readily degraded (2). In addition to their different stabilities to proteolytic degradation, the secondary, tertiary, and quaternary structures of PrP C and PrP Sc also differ (3-7...