Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges

Abstract

: Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.

Keywords: EGAD; ER stress; ERAD; Golgi stress; protein quality control.

Figure 1

Molecular machines involved in the control of protein homeostasis in the early secretory pathway. After ribosomal-dependent messenger RNA (mRNA) translation (1), newly synthesized polypeptide chains are imported in the endoplasmic reticulum (ER) lumen. Here, proteins encounter diverse chaperones, modifying enzymes and complexes, such as CANX/CRT, necessary for maturation and folding (2). Once ready, mature proteins are embedded into COPII vesicles (3) and transported into the Golgi compartment (4). Here other chaperones and enzymes complete proteins maturation. Finally, once completely ready proteins are translocated in diverse subcellular compartments (extracellular space, mitochondria, lysosomes, etc.) (5). In the other hand, COPI vesicles mediate Golgi-to-ER retrograde transport of immature proteins (6). Proteins that are not fully mature and do not pass ER/Golgi dependent quality control systems are retrieved back in the cytosol through the ER-associated degradation (ERAD) machinery (i), targeted by ubiquitin system (7) and finally degraded by proteasome (8). Alternatively, newly synthetized polypeptide chains, in some conditions, do not reach the ER lumen and are directly ubiquitinated and degraded by proteasome, through a process referred to as pre-empty quality control (pre-QC) (ii).

Figure 2

Organelle homeostasis imbalance signaling pathways. ER stress signaling (with the unfolded protein response (UPR) sensors IRE1, PERK and ATF6; left) and Golgi stress response (right). Left panel: unfolded protein accumulation is sense by three ER resident receptors: IRE1, PERK and ATF6. IRE1 activation imposes the induction of two different activities: kinasic and RNAsic activity. The first controls phosphorylation of the pro-apoptotic factor JNK, the latter mediates IRE1 dependent mRNA decay (RIDD) and the unconventional splicing of XBP1 mRNA. PERK activation induces eiF2a phosphorylation, thus inducing global mRNA translation inhibition and translation of specific mRNA, such as ATF4 mRNA. ATF6 activation induces its translocation into the Golgi, where it is the substrate of two proteases. This cleavage promotes the release of an active transcription factor known as ATF6f. All together these factors mediate the transcriptional program unfolded protein response. Right panel: unfolded protein accumulation also induces a Golgi-dependent response. The main receptors are still needed to be defined (X, Y and Z), but currently two main effectors were identified: TF3/MLX and CREB3. The first regulates genes necessary to increase Golgi function, the latter transcribe for ARF4, thus inducing apoptosis.

Figure 3

ER and Golgi degradation pathways, namely ERAD (upper panel) and endosome and Golgi-related stress-responsive associated degradation pathway (EGAD; lower panel). Upper panel: ERAD-mediated process consists of four steps: recognition, translocation, ubiquitination and elimination. To reach the cytoplasm, ER-resident proteins need to pass by the ER membrane, this process is mediated by the retrotranslocon. SEC61 is the major component of the translocation channel that imports polypeptides into the ER. Derlin proteins participate in ERAD and are part of the translocon component. Derlin-1 interacts with p97 and VCP-interacting membrane protein 1 (VIMP1) complex necessary for the recruitment and the transport of unfolded protein to the cytosol, thanks to the help of ubiquitin fusion degradation protein 1 (UFD1) and nuclear protein localization 4 (NPL4). UBE1 is an E1 ubiquitin-activating enzyme controls polypeptides degradation. The HRD1 complex is composed of HRD1, HRD3, Derlin1, OS9, USA1, UBX2 and p97 (ERAD-L); Doa10 complex consists of Doa10, UBX2 and p97 (ERAD-C). Lower panel: EGAD (endosome and Golgi-related stress-responsive associated degradation pathway) controls changes in the size, subcellular localization, and organization of the Golgi. The Dsc complex is composed of six components: the membrane proteins Tul1, Dsc2, Dsc3, Dsc4 and Ubx3, and the cytosolic AAA+ ATPase Cdc48 (the homologous for the mammalian p97/VCP).

Figure 4

Targeting different steps in the early secretory pathway. Schematic presentation of proteostasis targeting compounds used in clinic for the treatment of diverse diseases.