To warrant the quality of the secretory proteome stringent control systems operate in the endoplasmic reticulum (ER)-Golgi user interface preventing the launch of nonnative items. The ERp44 set up control cycle couples secretion fidelity and efficiency downstream of the calnexin/calreticulin and BiP-dependent quality control cycles. Hematoxylin (Hydroxybrazilin) Graphical Abstract Introduction Protein folding in the endoplasmic reticulum (ER) is monitored by stringent quality control mechanisms that prevent release of immature or misfolded ER client proteins to travel further along the secretory pathway. Notable examples include the binding-and-release cycles of BiP recognizing exposed hydrophobic stretches at the surface of nonnative conformers and of Hematoxylin (Hydroxybrazilin) calnexin/calreticulin which exploit glucosylation status of N-linked glycans on ER client proteins to exert quality control (Ellgaard and Helenius 2003 ER client proteins are rich in intra- and intermolecular disulfide bonds that are generally essential for their function. Numerous oxidoreductases reside in the ER to catalyze oxidative protein folding (Ellgaard and Ruddock 2005 Also the process of disulfide bond formation is intimately linked to the quality control systems that prevent nonnative conformers to exit from the ER (Anelli and Sitia 2008 The same mechanisms that exert ER quality control on the folding of subunits also monitor certain steps in their oligomeric assembly. For instance in the case of antibodies BiP binds the first constant domain (CH1) of Ig heavy chain Pik3r2 (H) and thereby retains it in the ER until it is displaced by the Ig light chain (L) to?assemble into H2L2 structures called “monomers” in the immunological jargon (Feige et?al. 2009 Haas and Wabl 1983 Hendershot and Sitia 2005 The assembly process of μ2L2 monomers into IgM pentamers-(μ2L2)5J-and hexamers-(μ2L2)6-is favored by ERGIC-53 (Anelli et?al. 2007 which resides preferentially in the ER-to-Golgi intermediate compartment (ERGIC) (Schindler et?al. 1993 Interaction of μ2L2 monomers with ERGIC-53 occurs upon release by BiP (Anelli et?al. 2007 suggesting that these assemblies escape from the grasp of ER quality control and travel to more distal compartments of the early secretory pathway (Cortini and Sitia Hematoxylin (Hydroxybrazilin) 2010 This notion is perhaps not so surprising as the μ2L2 monomers even if not polymerized are correctly folded and partially assembled and therefore no longer substrate to the ER chaperoning machinery. The only sign that betrays their unpolymerized state is the free tail-piece cysteine which in mature IgM polymers is disulfide linked to corresponding cysteine residues in other μ2L2 monomers (Sitia et?al. 1990 Members of the PDI family of oxidoreductases associate with free cysteines of orphan assembly subunits and thereby facilitate their so-called thiol-mediated retention (Fra et?al. 1993 Reddy et?al. 1996 Sitia et?al. 1990 We previously identified ERp44 as a PDI family member that covalently binds Ero1 oxidases and facilitates their intracellular localization (Anelli et?al. 2002 2003 Otsu et?al. 2006 ERp44 is special Hematoxylin (Hydroxybrazilin) for having a single cysteine (C29) in its conserved active site (CRFS) consistent with a dedicated role in thiol-mediated retention not only of Ero1 oxidases but also of orphan subunits of otherwise disulfide-linked oligomers including IgM (Anelli et?al. 2003 2007 and adiponectin (Qiang et?al. 2007 Wang et?al. 2007 Moreover ERp44 localizes predominantly to the ERGIC (Anelli et?al. 2007 Gilchrist et?al. 2006 unlike other PDI family members which reside in the ER (Ellgaard and Ruddock 2005 Still the C-terminal ?RDEL motif of ERp44 allows its capture by KDEL receptors (KDEL-R) in distal stations of the early secretory compartment presumably for retrieval to the ER (Anelli et?al. 2003 It is yet unclear how ERp44 cycles between its predominant localizations in the distal early secretory pathway and the ER and how such cycling would relate to thiol-mediated retention. Here we show that the pH gradient between cisGolgi and ER controls association of ERp44 both with its clients and with the KDEL-R. Our results recommend a model where the simultaneous unmasking of your client and KDEL-R binding interfaces can be facilitated by dislocation from the ERp44 C-terminal tail (C-tail) which likely requires protonation state from the energetic site cysteine (C29) at cisGolgi-equivalent pH. Therefore both ERp44 activity and its own shuttling between your ER and cisGolgi are pH controlled to drive an excellent control cycle focused on the monitoring of secretory proteins set up..