Vivek Malhotra

Coordinated RNA and protein synthesis is fundamental to cell function and survival. Here, we show that TANGO6, a nucleocytoplasmic RNA binding protein, is an integral component of the proteostasis control axis, regulating both transcription and translation. In the nucleus, TANGO6 is recruited to gene promoters alongside RNA polymerase II, where it controls the transcriptional activation of essential genes, including those required for G ₁ -S cell cycle progression and cell differentiation. Cytoplasmic TANGO6 associates with polysomes to promote protein synthesis. Under stress conditions, the organization of TANGO6 changes; notably, heat stress triggers its translocation to the nucleus, thereby depleting its cytoplasmic pool. Thus, TANGO6 assists in both transcriptional and translation reprogramming to support cell survival. Overall, the nucleocytoplasmic shuttling of TANGO6 is a central mechanism for controlling the cellular proteome, establishing its essential role in cell viability from homeostasis to stress adaptation.

Procollagen I (PC1) is assembled into a trimer within the lumen of the endoplasmic reticulum (ER). In vitro, collagen trimers form rigid molecules reaching lengths of up to 400 nm, and this conformation is presumed to represent their assembled state in vivo. Here, we demonstrate that endogenous PC1 assembles into biomolecular condensates in the ER of activated human hepatic stellate cells. PC1 condensates form in response to increased collagen synthesis and are part of a multicomponent system enriched in the chaperones Hsp47 and calreticulin, as well as the disulfide isomerases PDIA1 and PDIA6, but notably lacking the unfolded protein sensor BiP. PC1 condensates localize to ER exit sites, a process mediated by TANGO1, and dissipate upon ER stress. We propose that this organization enables the accommodation of large quantities of PC1 in the ER lumen without triggering degradation. Furthermore, we suggest that PC1 within condensates is exported in a manner resembling liquid extrusion rather than as a rigid trimer.

The secretion of proteins that do not follow the well-characterized endoplasmic reticulum (ER)-Golgi apparatus pathway, known as unconventional protein secretion (UCPS), is gradually revealing its complexities. Our study has identified an ER-based tubulovesicular network, termed ER tubular body (ER-TB), as a central compartment in this process. We demonstrate that ER-TBs are formed by two reticulophagy receptors, ATL3 and RTN3L, under conditions of cellular stress. In addition to their role in stress-induced secretion, the activation of UCPS via ER-TBs facilitates cell surface trafficking of trafficking-deficient transmembrane proteins such as ΔF508-CFTR. Furthermore, their involvement in ER remodeling and vesicle trafficking suggests a potential role in viral replication, particularly in the formation of membrane compartments utilized by positive-strand RNA viruses. By uncovering ER-TBs as key cellular structures in stress-induced UCPS and demonstrating their regulation by autophagy-related factors, our findings offer valuable insights into protein homeostasis, viral pathogenesis, and potential therapeutic strategies for diseases linked to trafficking defects.

COPII-coat proteins play a crucial role in generating small vesicles at endoplasmic reticulum (ER) exit sites (ERES). However, they also assemble at the necks of tunnels and tubules connecting the ER exit site (donor) to the ERGIC/cis-Golgi cisterna (acceptor). How can COPII support these two traffic mechanisms? Through cell-free reconstitutions, we have found that the apposition of donor and acceptor membranes is important for the assembly of the outer layer of the COPII coat (Sec13/31) but had minimal impact on Sar1-induced COPII inner layer (Sec23/24) recruitment. The expression of the adaptor protein p125A, which binds to phosphatidylinositol 4-phosphate (PI4P), Sec31 and Sec23, stabilized the contact between the donor and acceptor membranes and promoted COPII outer layer assembly. A p125A-chimera expressing a Golgi-targeted PI4P-binding domain could also support outer layer assembly. Notably, the C-terminal helical domain of Sec31A, which interacts with p125A, was essential for its assembly at ERES. In cells lacking p125A, the assembly of the COPII outer layer was selectively destabilized. Transcriptome and secretome analyses reveal selective adjustments to extracellular matrix remodeling and collagen secretion, which corresponded with selective inhibition of fibrillar collagen traffic from the ER. Thus, p125A connects the outer COPII layer at ERES with PI4P-rich ERGIC/cis-Golgi membranes, coordinating COPII assembly with tunnel-driven collagen traffic.

Palade’s original model proposed that secretory cargo is transported between stable compartments via vesicles. However, recent findings challenge this view, suggesting that secretory pathway compartments are dynamic, with cargo itself dictating whether transfer occurs via vesicles or through the continuity and maturation of compartmental structures. At the heart of this process is TANGO1, a key component of a molecular machine that works in concert with COPII proteins to construct export routes tailored to the size and quantity of secretory cargo.

Compartment for unconventional protein secretion (CUPS), a compartment for secretion of signal sequence–lacking proteins, forms through COPI-independent extraction of membranes from early Golgi cisternae, lacks Golgi-specific glycosyltransferases, and requires phosphatidylinositol 4-phosphate (PI4P) for biogenesis, as well as phosphatidylinositol 3-phosphate for stability. Our findings demonstrate that Drs2, a PI4P effector from the trans-Golgi network (TGN), is essential for CUPS formation, specifically through its interaction with Rcy1, and Rcy1 is crucial for the unconventional secretion. Using 4D super-resolution confocal live imaging microscopy, we observed that CUPS interact with a modified TGN that contains Drs2 in addition to proteins Tlg2 and Snc2, which are necessary for membrane fusion. Notably, while CUPS remain stable, the modified TGN undergoes remodeling during the later stages of unconventional secretion. In summary, we suggest that CUPS and the modified TGN, without the function of COPII and COPI, participate in collecting and sorting unconventionally secreted proteins, reflecting the role of Golgi membranes in receiving cargo from the ER during conventional secretion.

Loss of TANGO2 in humans precipitates metabolic crises during periods of heightened energy demand, such as fasting, infections, or high fever. TANGO2 has been implicated in various functions, including lipid metabolism and heme transport, and its cellular localization remains uncertain. In our study, we demonstrate that TANGO2 localizes to the mitochondrial lumen via a structural region containing LIL residues. Mutations in these LIL residues cause TANGO2 to relocate to the periphery of lipid droplets. We further show that purified TANGO2 binds acyl-coenzyme A, and mutations in the highly conserved NRDE sequence of TANGO2 inhibit this binding. Collectively, our findings suggest that TANGO2 serves as an acyl-coenzyme A binding protein. These insights may provide new avenues for addressing the severe cardiomyopathies and rhabdomyolysis associated with defective TANGO2 in humans.

Export of secretory cargoes from the endoplasmic reticulum (ER) requires COPII proteins, which were first identified for their ability to coat small vesicles that bud from the ER. Recent data indicate that COPII proteins can also organize into a collar at the necks of tubules, as well as phase-separate into liquid-like condensates. Thus, COPII assemblies seem to be tailored to accommodate variations in the size and quantities of cargo secreted.