Identification of Structural Calcium Binding Sites in Membrane-Bound Presenilin 1 and 2

Variants of presenilin (PS1 and PS2) are the main genetic risk factors of familial Alzheimer's disease and thus central to the disease etiology. Although mostly studied as catalytic units of γ-secretase controlling Aβ production, presenilins also affect calcium levels, which are disturbed in Alzheimer's disease. We investigated the interaction of calcium with both PS1 and PS2 using all-atom molecular dynamics (MD) simulations in realistic membrane models, with the specific aim to identify any Ca²⁺ sites. We did not observe any complete Ca²⁺ leak event, but we identified four persistent Ca²⁺ sites in membrane-bound PS1 and PS2: One in HL2 near the C-terminal of TM6, one in HL2 toward the N-terminal of TM7, a site at the catalytic aspartate on TM7, and a site at the PALP motif on TM9. The sites feature negatively charged glutamates and aspartates typical of calcium binding. Structural homology to diaspartate calcium transport proteins and mutation studies of calcium efflux support our identified calcium sites. Calcium consistently dampens HL2 motions in all comparisons (PS1, protonated PS1, PS2, protonated PS2). Due to their location in HL2 and the active site, we propose that the calcium sites control autoproteolytic maturation of presenilin by a pH-dependent conformational restriction of the HL2 recognition loop, which also regulates calcium transport proteins such as inositol 1,4,5-triphosphate receptor and ryanodine receptor. Our structural dynamics could provide a possible molecular basis for the need of both calcium and presenilin for lysosome proteolytic function, perhaps relevant also to other protein misfolding diseases.