Required for protein translocation and folding in the endoplasmic reticulum (ER). Functions as a nucleotide exchange factor for the ER lumenal chaperone HSPA5. Interacts with HSPA5. Expressed in several areas of the brain including the cerebellum, cerebral cortex, cortical neurons, glial cells of white matter, hippocampus, olfactory bulb, Purkinje cells, inferior olive and the choroids plexus. Also expressed in the eye and skeletal muscle. Expressed in the developing retina and epithelial cells of the lens at 12.5 dpc. Expressed in the developing cerebral cortex at 15.5 dpc. N-glycosylated. Ubiquitinated by the CRL2(FEM1A) and CRL2(FEM1C) complexes, which recognize the -Lys-Xaa-Xaa-Arg C-degron at the C-terminus, leading to its degradation. Mice develop adult-onset ataxia with cerebellar Purkinje cell loss. Affected cells have intracellular protein accumulations in the endoplasmic reticulum and the nucleus. Belongs to the SIL1 family.