Functional domains in presenilin 1. The Tyr-288 residue controls γ-secretase activity and endoproteolysis

Processing of the Alzheimer amyloid precursor protein (APP) into the amyloid β-protein and the APP intracellular domain is a proteolysis event mediated by the γ-secretase complex where presenilin (PS) proteins are key constituents. PS is subjected to an endoproteolytic cleavage, generating a stable heterodimer composed of an N-terminal and a C-terminal fragment. Here we aimed at further understanding the role of PS in endoproteolysis, in proteolytic processing of APP and Notch, and in assembly of the γ-secretase complex. By using a truncation protocol and alanine scanning, we identified Tyr-288 in the PS1 N-terminal fragment as critical for PS-dependent intramembrane proteolysis. Further mutagenesis of the 288 site identified mutants differentially affecting endoproteolysis and γ-secretase activity. The Y288F mutant was endoproteolyzed to the same extent as wild type PS but increased the amyloid β-protein 42/40 ratio by ∼75%. In contrast, the Y288N mutant was also endoproteolytically processed but was inactive in reconstituting γ-secretase in PS null cells. The Y288D mutant was deficient in both endoproteolysis and γ-secretase activity. All three mutant PS1 molecules were incorporated into γ-secretase complexes and stabilized Pen-2 in PS null cells. Thus, mutations at Tyr-288 do not affect γ-secretase complex assembly but can differentially control endoproteolysis and γ-secretase activity.