Associated to surface IgM-receptor; may be involved in signal transduction. Involved in regulation of the catalytic activity of the phosphatases PP2A, PP4 and PP6 by protecting their partially folded catalytic subunits from degradative polyubiquitination until they associate with regulatory subunits (By similarity). Interacts with partially folded PPP2CA, but not with the fully active protein. Interacts with PPP2CB, and with PP4 and PP6. Interacts with MID1 and MID2. Interacts with ubiquitin (By similarity). The UIM domain is required for protective effect on PP2A. Phosphorylated. Monoubiquitination by MID1 triggers calpain-mediated cleavage and switches IGBP1 activity from protective to destructive. Belongs to the IGBP1/TAP42 family.