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.