Co-expression of nicastrin and presenilin rescues a loss of function mutant of APH-1

Abstract

γ-Secretase is an intramembrane-cleaving aspartyl protease complex that mediates the final cleavage of β-amyloid precursor protein to liberate the neurotoxic amyloid-β peptide implicated in Alzheimer's disease. The four proteins presenilin (PS), nicastrin (NCT), APH-1, and PEN-2 are sufficient to reconstitute γ-secretase activity in yeast. Although PS seems to contribute the catalytic core of the γ-secretase complex, no distinct function could be attributed to the other components so far. In Caenorhabditis elegans, mutation of a glycine to an aspartic acid within a conserved GXXXG motif in the fourth transmembrane domain of APH-1 causes a loss of function phenotype. Surprisingly, we now found that the human homologue APH-1a carrying the equivalent mutation G122D is fully active in yeast co-expressing PS1, NCT, and PEN-2. To address this discrepancy, we expressed APH-1a G122D in HEK293 cells. As reported previously, overexpressed APH-1a G122D was not incorporated into the γ-secretase complex. Separate overexpression of PS1, NCT, or PEN-2 together with APH-1a G122D allowed the formation of heterodimers lacking the other endogenous components. Only the combined overexpression of PS1 and NCT together with APH-1a G122D facilitated the formation of a fully active γ-secretase complex. Under these conditions, APH-1a G122D supported the production of normal amounts of Aβ. We conclude that cooperative effects may stabilize a trimeric complex of APH-1a G122D together with PS1 and NCT. Upon successful complex assembly, the GXXXG motif becomes dispensable for γ-secretase activity.