Aph‐1 interacts at the cell surface with proteins in the active γ‐secretase complex and membrane‐tethered Notch

The activity of the γ‐secretase complex is critical for the processing of a number of transmembrane proteins, including Notch. Functional γ‐secretase activity can be reconstituted from four proteins – presenilin, nicastrin, Pen‐2 and Aph‐1 – but the role of the individual proteins remains unclear. In this report we describe the cellular localization and protein interactions of Aph‐1, with particular regard to Notch receptor processing. We found that Aph‐1 is present at the cell surface, where it interacts with Pen‐2, the mature forms of presenilin and nicastrin, and full‐length Notch. Aph‐1 also interacts with a truncated form of Notch, which is a direct substrate for γ‐secretase, but not with the Notch intracellular domain. Immunoprecipitation data for Notch and Aph‐1 showed that the Notch‐containing γ‐secretase complexes most likely form a small subset of the total number of γ‐secretase complexes. In conclusion, these data demonstrate that Aph‐1 is present at the cell surface, presumably in active γ‐secretase complexes, and interacts with the Notch receptor, both before and after ligand activation.