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). Helps to diversify peripheral B-cell functions by regulating Ca(2+) stores, antibody secretion, and integrin activation (By similarity). 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 (By similarity). 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. Interacts with HSP90B1 and PDIA3 in a calcium-dependent manner. Forms an interchain disulfide bond with IgM monomers. Belongs to the MZB1 family.