Final answer:
The discovery of prion proteins and the mechanism by which the infectious PrPSC form converts the normal form PrPC was critical evidence leading to the conclusion that TSEs are caused by a protein agent, specifically the prion.
Step-by-step explanation:
The experimental evidence that led to the belief that a protein agent was responsible for transmissible spongiform encephalopathies (TSEs) is grounded in the discovery and analysis of prion proteins. PrPSC, the infectious form of the normal cellular protein PrP (prion protein), binds to the normal form PrPC and converts it into the infectious PrPSC form upon entering the body. This discovery by Stanley Prusiner marked a paradigm shift as it identified a new infectious agent that does not rely on DNA or RNA for transmission, consisting solely of abnormal protein aggregates linked to brain lesions observed in affected cattle.
Diagnosis of TSEs often involves identification of characteristic amyloid plaques, vacuoles, and specifically prion proteins within brain biopsies, through histological examinations. The presence of the 14-3-3 protein and advanced assays like RT-QuIC (real-time quaking-induced conversion) further support the identification of these unique proteinaceous infectious particles. Therefore, confirming the role of prions as wholly protein-based agents of infection without a genome.