γ‐Secretase is involved in the production of amyloid β‐peptide, which is the principal component of amyloid plaques in the brains of patients with Alzheimer disease. γ‐Secretase is a complex composed of presenilin (PS), nicastrin, anterior pharynx‐defective phenotype 1 (Aph1) and PS enhancer 2 (Pen2). We previously proposed a mechanism of complex assembly by which unassembled subunits are retained in the endoplasmic reticulum (ER) and only the fully assembled complex is exported from the ER. We have now identified Retention in endoplasmic reticulum 1 (Rer1) as a protein that is involved in the retention/retrieval of unassembled Pen2 to the ER. Direct binding of unassembled Pen2 to Rer1 is mediated by the first transmembrane domain of Pen2, and a conserved asparagine in this domain is required. Downregulation of Rer1 leads to increased surface localization of Pen2, whereas overexpression of Rer1 stabilizes unassembled Pen2. To our knowledge, Rer1 is the first identified interaction partner of mammalian transmembrane‐based retention/retrieval signals.

γ-Secretase is critically involved in the Notch pathway and in Alzheimer's disease. The four subunits of γ-secretase assemble in the endoplasmic reticulum (ER) and unassembled subunits are retained/retrieved to the ER by specific signals. We here describe a novel ERretention/retrieval signal in the transmembrane domain (TMD) 4 of presenilin 1, a subunit of γ-secretase. TMD4 also is essential for complex formation, conferring a dual role for this domain. Likewise, TMD1 of Pen2 is bifunctional as well. It carries an ER-retention/retrieval signal and is important for complex assembly by binding to TMD4. The two TMDs directly interact with each other and mask their respective ER-retention/retrieval signals, allowing surface transport of reporter proteins. Our data suggest a model how assembly of Pen2 into the nascent γ-secretase complex could mask TMD-based ER-retention/retrieval signals to allow plasma membrane transport of fully assembled γ-secretase.

Background: ␥-Secretase is composed of four subunits with 19 transmembrane domains. Results: Transmembrane domain (TMD) 4 of presenilin 1 contains polar amino acids that are involved in ER retention, assembly, and stability. Conclusion: TMD4 is a crucial interaction site in the ␥-secretase complex. Significance: Understanding TMD-TMD interactions in molecular detail is crucial for understanding of ␥-secretase and other membrane protein complexes.

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