Mediates thiol-dependent retention in the early secretory pathway, forming mixed disulfides with substrate proteins through its conserved CRFS motif. Inhibits the calcium channel activity of ITPR1. May have a role in the control of oxidative protein folding in the endoplasmic reticulum. Required to retain ERO1A and ERO1B in the endoplasmic reticulum. Forms mixed disulfides with both ERO1A and ERO1B and cargo folding intermediates; the interactions with ERO1A and ERO1B result in their retention in the endoplasmic reticulum (By similarity). Directly interacts with ITPR1 in a pH-, redox state- and calcium-dependent manner, but not with ITPR2 or ITPR3 (PubMed:15652484). The strength of this interaction inversely correlates with calcium concentration (PubMed:15652484). Widely expressed.