Associates with immunoglobulin M (IgM) heavy and light chains and promotes IgM assembly and secretion. May exert its effect by acting as a molecular chaperone or as an oxidoreductase as it displays a low level of oxidoreductase activity (By similarity). Isoform 2 may be involved in regulation of apoptosis. Helps to diversify peripheral B-cell functions by regulating Ca(2+) stores, antibody secretion and integrin activation. Acts as a hormone-regulated adipokine/pro-inflammatory cytokine that is implicated in causing chronic inflammation, affecting cellular expansion and blunting insulin response in adipocytes. May have a role in the onset of insulin resistance. Part of the ER chaperone complex, a multi-protein complex in the endoplasmic reticulum containing a large number of molecular chaperones which associates with unassembled incompletely folded immunoglobulin heavy chains (By similarity). Isoform 2 interacts with CASP2 and CASP9. Interacts with HSP90B1 and PDIA3 in a calcium-dependent manner (By similarity). Diffuse granular localization in the cytoplasm surrounding the nucleus (PubMed:11350957). Widely expressed with highest levels in adult brain, small intestine and lymphoid tissues such as thymus and spleen. Expression is frequently lower in intestinal-type gastric cancer. In obese patients, more abundant in omental than in subcutaneous fat. Down-regulated in primary B-cells early after ligand-stimulated activation. Up-regulated in bacterial lipopolysaccharides (LPS)-stimulated peritoneal macrophages. Forms an interchain disulfide bond with IgM monomers. Major. Belongs to the MZB1 family. Wrong choice of frame.