Involved in the retention or retrieval of proteins localized in the endoplasmic reticulum from the early Golgi compartment, including the gamma-secretase complex subunit PEN-2 (PSENEN) (PubMed:17668005). Regulates intracellular trafficking of the gamma- secretase complex and amyloid-beta precursor protein (APP), thereby controlling APP levels at the cell surface and influencing the release of amyloid-beta peptides generated through gamma-secretase mediated processing of APP (PubMed:23043097). During cerebral cortex and neural stem cell development, it is required to maintain sufficient expression and activity of gamma-secretase complex at the cell surface, modulating NOTCH processing and signaling (By similarity). In addition to its role in the control of protein trafficking, the RER1-dependent Golgi apparatus-endoplasmic reticulum retrieval system may regulate proteasomal degradation of alpha-synuclein (SNCA) (PubMed:17668005, PubMed:23043097, PubMed:28877262). {ECO:0000250|UniProtKB:Q9CQU3, ECO:0000269|PubMed:17668005, ECO:0000269|PubMed:23043097, ECO:0000269|PubMed:28877262}. This is the function of RER1 (retention in endoplasmic reticulum sorting receptor 1, Ensembl gene identifier ENSG00000157916).