β-Sheet Structure within the Extracellular Domain of C99 Regulates Amyloidogenic Processing

Familial mutations in C99 can increase the total level of the soluble Aβ peptides produced by proteolysis, as well as the Aβ42/Aβ40 ratio, both of which are linked to the progression of Alzheimer's disease. We show that the extracellular sequence of C99 forms β-sheet structure upon interaction with membrane bilayers. Mutations that disrupt this structure result in a significant increase in Aβ production and, in specific cases, result in an increase in the amount of Aβ42 relative to Aβ40. Fourier transform infrared and solid-state NMR spectroscopic studies reveal a central β-hairpin within the extracellular sequence comprising Y10-E11-V12 and L17-V18-F19 connected by a loop involving H13-H14-Q15. These results suggest how familial mutations in the extracellular sequence influence C99 processing and provide a structural basis for the development of small molecule modulators that would reduce Aβ production.