Interactions between heterologous forms of prion protein: Binding, inhibition of conversion, and species barriers

The self-induced formation of the disease-associated, protease-resistant pr ion protein (PrP-res) from the normal protease-sensitive isoform (PrP-sen) appears to be a key event in the pathogenesis of transmissible spongiform e ncephalopathies. The amino acid sequence specificity of PrP-res formation c orrelates with, and may account for, the species specificity in transmissio n of transmissible spongiform encephalopathy agents in vivo. To analyze the mechanism controlling the sequence specificity of PrP-res formation, we co mpared the binding of PrP-sen to PrP-res with its subsequent acquisition of protease resistance by using cell-free systems consisting of heterologous versus homologous mouse and hamster PrP isoforms, Our studies showed that h eterologous PrP-sen can bind to PrP-res with little conversion to the prote ase-resistant state and, in doing so, can interfere with the conversion of homologous PrP-sen, The interference occurred with molar ratios of homologo us to heterologous PrP-sen molecules as low as 1:1. The interference was du e primarily to the inhibition of conversion, but not the binding, of the ho mologous PrP-sen to PrP-res, The results provide evidence that the sequence specificity of PrP-res formation in this model is determined more by the c onversion to protease resistance than by the initial binding step. These fi ndings also imply that after the initial binding, further intermolecular in teractions between PrP-sen and PrP-res are required to complete the process of conversion to the protease-resistant state.