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. {ECO:0000269|PubMed:19818709, ECO:0000269|PubMed:23591866}. This is the function of IGBP1 (immunoglobulin binding protein 1, ENSG00000089289).