Endoplasmic reticulum (ER)-anchored protein that mediates the formation of contact sites between the ER and endosomes via interaction with FFAT motif-containing proteins such as STARD3 or WDR44 (PubMed:32344433, PubMed:33124732). Interacts with STARD3 in a FFAT motif phosphorylation dependent manner (PubMed:33124732). Via interaction with WDR44 participates in neosynthesized protein export (PubMed:32344433). Participates in the endoplasmic reticulum unfolded protein response (UPR) by inducing ERN1/IRE1 activity (PubMed:16891305, PubMed:20940299). Involved in cellular calcium homeostasis regulation (PubMed:22131369). {ECO:0000269|PubMed:16891305, ECO:0000269|PubMed:20940299, ECO:0000269|PubMed:22131369, ECO:0000269|PubMed:32344433, ECO:0000269|PubMed:33124732}. This is the function of Ensembl gene identifier ENSG00000124164 (VAPB, VAMP associated protein B and C).