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. 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. Interacts with HSP90B1 and PDIA3 in a calcium-dependent manner. Expressed predominantly in the spleen and lymph nodes. Abundantly expressed in marginal zone B and B1 cells. High expression in mesenteric adipose tissue (MAT). Expressed also in pancreas, perigonadal adipose tissue (PAT), uterus, subcutaneous adipose tissue, heart, muscle, ovary and liver. Very low expression is detected in brown adipose tissue. In PAT, significantly higher expression in stromal-vascular cell than in adipocytes. Expressed in macrophage RAW 264.7 cell line. Down-regulated in For-knockout female MAT at 5 months (obese state) followed by steep up-regulation at 9 months (prediabetic condition) when mutants progress towards the metabolic syndrome. Up-regulated during plasma cell differentiation. Forms an interchain disulfide bond with IgM monomers. Belongs to the MZB1 family.