Functional and topological analysis of PSENEN, the fourth subunit of the gamma-secretase complex
The {gamma}-secretase complexes are intramembrane cleaving proteases involved in the generation of the A{beta} peptides in Alzheimers Disease. The complex consists of four subunits, with Presenilin, harboring the catalytic site. Here, we study the role of the smallest subunit, PSENEN or Presenilin enhancer 2 (PEN-2), encoded by the gene Psenen, in vivo and in vitro. We find a profound Notch-deficiency phenotype in Psenen-/- embryos confirming the essential role of PSENEN in the {gamma}-secretase complex. We used Psenen-/- fibroblasts to explore the structure-function of PSENEN by the Scanning Cysteine Accessibility Method. Glycine22 and Proline27 which border the membrane domains 1 and 2 of PSENEN are involved in complex formation and stabilization of {gamma}-secretase. The hairpin structured hydrophobic membrane domains 1 and 2 are exposed to a water-containing cavity in the complex, while transmembrane domain 3 is not water exposed. We finally demonstrate the essential role of PSENEN for the cleavage activity of the complex. PSENEN is more than a structural component of the {gamma}-secretase complex and might contribute to the catalytic mechanism of the enzyme.