The transmembrane protease complex gamma-secretase is responsible for the generation of the neurotoxic amyloid beta-peptide (A beta) from its precursor (APP). All has a causative role in Alzheimer disease, and thus, gamma-secretase is a therapeutic target. However, since there are more than 70 gamma-secretase substrates besides APP, selective inhibition of APP processing is required. Recent data indicates the existence of several gamma-secretase associated proteins (GSAPs) that affect the selection and processing of substrates. Here, we use a gamma-secretase inhibitor for affinity purification of gamma-secretase and associated proteins from microsomes and detergent resistant membranes (DRMs) prepared from rat or human brain. By tandem mass spectrometry we identified a novel brain GSAP; erlin-2. This protein was recently reported to reside in DRMs in the ER. A proximity ligation assay, as well as co-immunoprecipitation, confirmed the association of erlin-2 with gamma-secretase. We found that a higher proportion of erlin-2 was associated with gamma-secretase in DRMs than in soluble membranes. siRNA experiments indicated that reduced levels of erlin-2 resulted in a decreased A beta production, whereas the effect on Notch processing was limited. In summary, we have found a novel brain GSAP, erlin-2, that resides in DRMs and affects All production. (C) 2012 Elsevier Inc. All rights reserved.