The Hydrophobic Internal Region of Bovine Prion Protein Shares Structural and Functional Properties with HIV Type 1 Fusion Peptide

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Abstract
The conserved fusion peptide at the N-terminus of HIV-1 envelope glycoprotein gp41 is involved in the virus-cell fusion reaction and in the cytopathic effects promoted by expression in single cells. The conserved bovine prion protein 121KHVAGAAAAGAVVGGLGGYMLGSAMSR147 transmembrane region (BPrPtm) contains a sequence rich in Gly residues [i.e., 130GAVVGGLGGYMLGSAMSR147 (BPrPmi)] that shows homology with HIV-1 fusion peptide. As in the case of the latter peptide, analysis of the BPrPmi interfacial hydrophobicity confirms that this stretch bears an intrinsic tendency to partitioning from the aqueous phase into membranes. Experimental analyses of BPrPmi-lipid interactions suggest several similarities between this sequence and HIV-1 fusion peptide. Infrared spectroscopy reveals a conformational plasticity of the membrane-associated prion sequence comparable to that displayed by the viral sequence. The adoption of a mainly α-helical structure correlates with the formation of lytic pores. This helical structure can be converted into a β-sheet conformation by addition of calcium, a process that is accompanied by vesicle membrane fusion. In contrast, transmembrane BPrPtm associates with membranes in a nonlytic, mainly helical structure but also containing some random coil. Upon addition of calcium, the random coils disappear while the helical conformation remains. In the absence of membranes both prion and HIV-1 sequences form amyloid-type fibers. It is proposed that during the pathological processes induced by secreted PrPSc and its proteolytic fragments, conformational polymorphism displayed by membrane-inserted BPrPmi may play a role at perturbing the general architecture of the membrane lipid bilayer and inducing protein-protein aggregation at membrane surfaces. These findings suggest the existence of common mechanisms underlying cytotoxicity by PrP and HIV-1 gp41.