The Rubenstein lab has several ongoing projects focused on the cellular biology and physiology of ERp29, its clients, and related proteins in diseases of protein biogenesis and trafficking.
There are several, albeit rare, CF-causing missense mutations in residues that are located on CFTR’s ER luminal/extracellular face. We are currently testing the hypothesis that these mutant CFTRs have defective biogenesis and function due to an inability to interact with ERp29. We are also investigating how an interaction of ERp29 with CFTR’s luminal face promotes interaction of CFTR with cellular trafficking machinery located in the cytoplasm.
ENaC plays a critical role in the regulation of blood pressure, as well as in airway mucociliary clearance through its role in maintaining airway surface liquid volume. We have demonstrated that ERp29 promotes the biogenesis of ENaC through the Gogi where it undergoes cleavage of its extracellular loops; this cleavage increases the open probability of the channel at the plasma membrane. We have also demonstrated a novel role for the KDEL-Receptor in this process. Interestingly, ENaC can also avoid cleavage of its extracellular loops en route to the plasma membrane, and transit through this alternate pathway is enhanced by the depletion of ERp29 and the KDEL-Receptor. We are currently investigating the identity of this alternate trafficking pathway.
We have recently published data suggesting that ERp29 promotes insulin biosynthesis by directing proinsulin from the ER to the Golgi for further processing. We are testing the hypothesis that rare, diabetes-causing mutations in proinsulin cause defects in insulin biosynthesis by interfering with the interaction of proinsulin with ERp29. Through this work, we also aim to determine the path by which proinsulin is secreted when it avoids being cleaved to insulin and C-peptide.
The most common mutation of A1AT that causes A1AT Deficiency, the PiZ mutation, leads to an A1AT protein that fails to exit the ER and is not secreted. In some model systems, 4-phenylbutyrate (4PBA) restores secretion of PiZ-A1AT. We are therefore testing the hypothesis that the increased expression of ERp29 with 4PBA treatment is the mediator of this effect.
TMEM116 is a transmembrane protein of as yet unknown function. It is interesting in the context of ERp29 in that the expression of both ERp29 and TMEM116 are regulated by the same bidirectional promoter. Based on the hypothesis that proteins regulated by the same bidirectional promoter may have related functions, we are interested in gaining insight into the function of TMEM116, and whether this function is related to that of ERp29.