The deletion of amino acids 114-121 in the TM1 domain of mouse prion protein stabilizes its conformation but does not affect the overall structure

A mutant of mouse prion protein (PrPC) carrying a deletion of residues 114-121 (PrPΔ114-121) has previously been described to lack convertibility into the scrapie-associated isoform of PrP (PrPSc) and to exhibit a dominant-negative effect on the conversion of wild-type PrPC into PrPSc in living cells. Here we report the characterization of recombinantly expressed PrPΔ114-121 by Fourier-transformation infrared spectroscopy (FTIR) and circular dichroism (CD) spectroscopy. The analysis of spectra revealed an increased antiparallel β-sheet content in the deletion mutant compared to wild-type PrPC. This additional short β-sheet stabilized the fold of the mutant protein by ΔΔG0′ = 3.4 ± 0.3 kJ mol− 1 as shown by chemical unfolding experiments using guanidine hydrochloride. Secondary structure predictions suggest that the additional β-sheet in PrPΔ114-121 is close to the antiparallel β-sheet in PrPC. The high-affinity Cu2+-binding site outside the octarepeats, which is located close to the deletion and involves His110 as a ligand, was not affected, as detected by electron paramagnetic resonance (EPR) spectroscopy, suggesting that Cu2+ binding does not contribute to the protection of PrPΔ114-121 from conversion into PrPSc. We propose that the deletion of residues 114-121 stabilizes the mutant protein. This stabilization most likely does not obstruct the interaction of PrPΔ114-121 with PrPSc but represents an energy barrier that blocks the conversion of PrPΔ114-121 into PrPSc.