Dual topology of co-chaperones at the membrane of the endoplasmic reticulum

Abstract

Dual topologies of proteins at the ER membrane are known for a variety of proteins allowing the same protein to exert different functions according to the topology adopted. A dual topology of the co-chaperone ERdj4, which resides in the endoplasmic reticulum (ER), was proposed recently, a thesis that we found to align all published data and existing controversies into one whole picture. The aim of this review is to reassess all primary data available in the literature on ER-resident Hsp40 co-chaperones with respect to their topology. After careful and critical analyses of all experimental data published so far, we identified, next to ERdj4, two other co-chaperones, ERdj3 and ERdj6, that also display features of a dual topology at the ER membrane. We assume that during cellular stress subpools of some ER-resident J protein can alter their topology so that these proteins can exert different functions in order to adapt to cellular stress.

Reduced DNAJC3 Expression Affects Protein Translocation across the ER Membrane and Attenuates the Down-Modulating Effect of the Translocation Inhibitor Cyclotriazadisulfonamide

“…In vitro translation of DNAJC3/huCD4 resulted in substantial levels of translated preprotein and detectable SP cleavage ( Figure 2 B, lane 2). Of note, more than one-third of the DNAJC3 species retained the SP after translocation as determined by proteinase K (PK) treatment ( Figure 2 C, right panel), an observation that is in line with another report [ 71 ] ( Supplementary Figure S2 ). The translocated SP-containing DNAJC3 species were fully protected from PK treatment ( Figure 2 B, lanes 8–14), indicating that these proteins were localised in the lumen of the ER.…”

“…Notably, CADA inhibited the translocation of both non-cleaved and cleaved DNAJC3 species in an equal manner ( Figure 2 B, lanes 9–14 and Figure 2 D, right panel), whereas the preprotein levels in the control samples seemed to be unaffected by CADA ( Figure 2 B, lanes 2–7 and Figure 2 D, left panel). In addition, DNAJC3 has also been proposed as an uncleaved type I transmembrane protein that inserts in the ER membrane in a head-on formation with the SP as the membrane anchor and with the majority of the protein exposed to the cytosol [ 71 ]. The relatively high amount of DNAJC3 preprotein as compared to the SP-cleaved species in the non-PK samples ( Figure 2 B, lanes 2–7 and Figure 2 C, left panel), might suggest the appearance of such a membrane-anchored cytosolic protein.…”

The function of the co-chaperone ERdj4 in diverse (patho-)physiological conditions

“…ERDJ1 and ERDJ2 both bind to BiP, and in the BiP-bound complex, they permit co-translational translocation of proteins. When released from BiP, both ERDJ1 and the ERDJ2/SEC62 complex block co-translational translocation by binding near the tunnel exit of the large ribosomal subunit (see review [ 7 ]).…”

“…It has now become increasingly clear that different protein pools of ERDJ4 exist, an ER-luminal and a cytosolic pool. Under control conditions, about 70–75% of the protein resides in the ER lumen, while 25—30% resides as an integral membrane protein at the ER membrane facing the cytosol [ 9 , 12 ]; see also a recent review [ 7 ]. Specific stressors, such as heat shock, genotoxic stress, but also inflammation, direct the entire protein pool to the nucleus [ 8 , 18 – 20 ] (Fig.…”

ER-Phagy and Microbial Infection

“…In addition, the ER is the primary resident organelle for various post-translational modification enzymes and molecular chaperones. Thus, most newly generated polypeptides are transported into the ER for processes including shearing, post-translational modification, disulfide bond formation, and proper folding into mature proteins, which are subsequently sorted to different cellular sites to perform their respective functions ( Benham, 2012 ; Daverkausen-Fischer and Pröls, 2021 ). Pathogens, such as viruses, typically hijack the host’s synthetic machinery to generate proteins for their replication and packaging.…”