Functions as a chaperone protein (PubMed:9396746). Is one of the most abundant endoplasmic reticulum (ER) proteins (PubMed:9396746). Plays a role in the export of secreted proteins in the ER, the recognition of abnormally folded protein and their targeting to the ER associated-degradation (ERAD) (PubMed:9396746). Also serves as a cargo receptor for the export of transmembrane proteins (PubMed:15187134). Plays a role in the assembly of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) by stimulating the translocation of NDUFS4 and NDUFB11 from the cytosol to the mitochondria via interaction with TOMM40 (By similarity). In response to ER stress, delocalizes from the ER-mitochondria contact sites and binds BCL2 (By similarity). May be involved in CASP8-mediated apoptosis (By similarity). Homodimer and heterodimer with BCAP29 (PubMed:8612576). Binds CASP8 (isoform 9) as a complex containing BCAP31, BCAP29, BCL2 and/or BCL2L1 (By similarity). Forms a complex (via C-terminus) with TOMM40 which mediates the translocation of components of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) from the cytosol to the mitochondria; within the complex BCAP31 interacts directly with unprocessed and processed NDUFS4 and NDUFB11. Interacts with VDAC1 (By similarity). Interacts with VAMP3, VAMP1 and membrane IgD immunoglobulins (PubMed:9396746). Interacts with HACD2 (By similarity). May shuttle between the ER and the intermediate compartment/cis-Golgi complex. Associates with the mitochondria-associated endoplasmic reticulum membrane via interaction with TOMM40. Ubiquitous. Cleaved by CASP8 and other caspases. Belongs to the BCAP29/BCAP31 family.