In order to decipher the physiologic function of APP, we searched for molecules that interact with the plasma membrane APP. Rat primary neurons were infected with a viral vector encoding APP751 and potentially interacting proteins were crosslinked to APP. The resulting APP complexes were immunopurified with APP antibodies conjugated to beads, the purified complexes were in-gel digested, and peptides were identified by mass spectrometry. Notch2 was identified as an APP-interacting protein (Oh et al., 2004, SFN Abstract). To confirm this surprising result, we performed co-immunoprecipitation studies in HEK293 cells stably overexpressing APP751 and transiently expressing several Notch2 or Notch1 constructs. Both full-length Notch proteins and a number of deletion mutants were able to bring down APP and a high molecular weight complex containing APP and Notch, as demonstrated by western blot. To illustrate the subcellular localization of these two transmembrane proteins, COS cells were transiently transfected ...