Eukaryotes have evolved various quality control mechanisms to promote proteostasis in the ER. Selective removal of certain ER domains via autophagy (termed as ER-phagy) has emerged as a major quality control mechanism. However, the degree to which ER-phagy is employed by other branches of ER-quality control remains largely elusive. Here, we identify a cytosolic protein, C53, that is specifically recruited to autophagosomes during ER-stress, in both plant and mammalian cells. C53 interacts with ATG8 via a distinct binding epitope, featuring a shuffled ATG8 interacting motif (sAIM). C53 senses proteotoxic stress in the ER lumen by forming a tripartite receptor complex with the ER-associated ufmylation ligase UFL1 and its membrane adaptor DDRGK1. The C53/UFL1/DDRGK1 receptor complex is activated by stalled ribosomes and induces the degradation of internal or passenger proteins in the ER. Consistently, the C53 receptor complex and ufmylation mutants are highly susceptible to ER stress. Thus, C53 forms an ancient quality control pathway that bridges selective autophagy with ribosome-associated quality control in the ER.

Membrane proteins on the tonoplast are indispensible for vacuolar functions in plants. However, how these proteins are transported to the vacuole and how they become separated from plasma membrane proteins remain largely unknown. In this study, we used Arabidopsis thaliana vacuolar ion transporter1 (VIT1) as a reporter to study the mechanisms of tonoplast targeting. We showed that VIT1 reached the tonoplast through a pathway involving the endoplasmic reticulum (ER), Golgi, trans-Golgi network (TGN), prevacuolar compartment, and tonoplast. VIT1 contains a putative N-terminal dihydrophobic type ER export signal, and its N terminus has a conserved dileucine motif (EKQTLL), which is responsible for tonoplast targeting. In vitro peptide binding assays with synthetic VIT1 N terminus identified adaptor protein complex-1 (AP1) subunits that interacted with the dileucine motif. A deficiency of AP1 gamma adaptins in Arabidopsis cells caused relocation of tonoplast proteins containing the dileucine motif, such as VIT1 and inositol transporter1, to the plasma membrane. The dileucine motif also effectively rerouted the plasma membrane protein SCAMP1 to the tonoplast. Together with subcellular localization studies showing that AP1 gamma adaptins localize to the TGN, we propose that the AP1 complex on the TGN mediates tonoplast targeting of membrane proteins with the dileucine motif.

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Secretory proteins traffic from endoplasmic reticulum (ER) to Golgi via the coat protein complex II (COPII) vesicle, which consists of five cytosolic components (Sar1,. In eukaryotes, COPII transport has diversified due to gene duplication, creating multiple COPII paralogs. Evidence has accumulated, revealing the functional heterogeneity of COPII paralogs in protein ER export. Sar1B, the small GTPase of COPII machinery, seems to be specialized for large cargo secretion in mammals. Arabidopsis contains five Sar1 and seven Sec23 homologs, and AtSar1a was previously shown to exhibit different effects on α-amylase secretion. However, mechanisms underlying the functional diversity of Sar1 paralogs remain unclear in higher organisms. Here, we show that the Arabidopsis Sar1 homolog AtSar1a exhibits distinct localization in plant cells. Transgenic Arabidopsis plants expressing dominant-negative AtSar1a exhibit distinct effects on ER cargo export. Mutagenesis analysis identified a single amino acid, Cys84, as being responsible for the functional diversity of AtSar1a. Structure homology modeling and interaction studies revealed that Cys84 is crucial for the specific interaction of AtSar1a with AtSec23a, a distinct Arabidopsis Sec23 homolog. Structure modeling and coimmunoprecipitation further identified a corresponding amino acid, Cys484, on AtSec23a as being essential for the specific pair formation. At the cellular level, the Cys484 mutation affects the distinct function of AtSec23a on vacuolar cargo trafficking. Additionally, dominant-negative AtSar1a affects the ER export of the transcription factor bZIP28 under ER stress. We have demonstrated a unique plant pair of COPII machinery function in ER export and the mechanism underlying the functional diversity of COPII paralogs in eukaryotes.coat protein complex II | Sar1 | Sec23 | ER export | functional diversity

In planar cell polarity (PCP), the epithelial cells are polarized along the plane of the cell surface perpendicular to the classical apical-basal axis, a process mediated by several conserved signaling receptors. Two PCP-signaling proteins, VANGL planar cell polarity protein 2 (Vangl2) and Frizzled6 (Fzd6), are located asymmetrically on opposite boundaries of the cell. Examining sorting of these two proteins at the -Golgi network (TGN), we demonstrated previously that the GTP-binding protein ADP-ribosylation factor-related protein 1 (Arfrp1) and the clathrin-associated adaptor protein complex 1 (AP-1) are required for Vangl2 transport from the TGN. In contrast, TGN export of Frizzled6 does not depend on Arfrp1 or AP-1. Here, to further investigate the TGN sorting process in mammalian cells, we reconstituted release of Vangl2 and Frizzled6 from the TGN into vesicles Immunoblotting of released vesicles indicated that Vangl2 and Frizzled6 exit the TGN in separate compartments. Knockdown analysis revealed that a clathrin adaptor, epsinR, regulates TGN export of Frizzled6 but not of Vangl2. Protein interaction analysis suggested that epsinR forms a stable complex with clathrin and that this complex interacts with a conserved polybasic motif in the Frizzled6 cytosolic domain to package Frizzled6 into transport vesicles. Moreover, we found that Frizzled6-epsinR binding dissociates epsinR from AP-1, which may separate these two cargo adaptors from each other to perform distinct cargo-sorting functions. Our results suggest that Vangl2 and Frizzled6 are packaged into separate vesicles that are regulated by different clathrin adaptors at the TGN, which may contribute to their asymmetric localizations.

2Eukaryotes have evolved various quality control mechanisms to promote proteostasis in the ER.3 Selective removal of certain ER domains via autophagy (termed as ER-phagy) has emerged as a 4 major quality control mechanism. However, the degree to which ER-phagy is employed by other 5 branches of ER-quality control remains largely elusive. Here, we identify a cytosolic protein, C53, 6 that is specifically recruited to autophagosomes during ER-stress, in both plant and mammalian cells. 7C53 interacts with ATG8 via a distinct binding epitope, featuring a shuffled ATG8 interacting motif 8 (sAIM). C53 senses proteotoxic stress in the ER lumen by forming a tripartite receptor complex with 9 the ER-associated ufmylation ligase UFL1 and its membrane adaptor DDRGK1. The 10 C53/UFL1/DDRGK1 receptor complex is activated by stalled ribosomes and induces the degradation 11 of internal or passenger proteins in the ER. Consistently, the C53 receptor complex and ufmylation 12 mutants are highly susceptible to ER stress. Thus, C53 forms an ancient quality control pathway that 13 bridges selective autophagy with ribosome-associated quality control at the ER. 14 15 65 stress, we performed an immunoprecipitation coupled to mass spectrometry (IP-MS) screen to identify 66 AIM-dependent ATG8 interactions triggered by ER stress. We hypothesized that a synthetic AIM peptide 67 that has higher affinity for ATG8 can outcompete, and thus reveal, AIM-dependent ATG8 interactors. To 68 identify this synthetic peptide, we performed a peptide array analysis that revealed the AIM wt peptide 69 (Figure S1a, b; Table S1). Using isothermal titration calorimetry (ITC), we showed that the AIM wt binds 70 4 ATG8 with nanomolar affinity (KD=~700 nM), in contrast to the AIM mutant peptide (AIM mut), which 71 does not show any binding ( Figure S1c-f) or the low micromolar-range affinities measured for most cargo 72 receptors (Zaffagnini and Martens, 2016). As plants have an expanded set of ATG8 proteins, we first tested 73 if any of the ATG8 isoforms specifically responded to ER stress induced by tunicamycin (Kellner et al., 74 2016). Tunicamycin inhibits glycosylation and leads to proteotoxic stress at the ER (Bernales et al., 2006). 75 Quantification of ATG8 puncta in transgenic seedlings expressing GFP-ATG8A-I revealed that 76 tunicamycin treatment significantly induced all nine ATG8 isoforms (Figure S2). Since all ATG8 isoforms 77 were induced, we chose ATG8A, and performed peptide competition coupled IP-MS analysis (See methods 78 for detailed description). In addition to well-known AIM dependent ATG8 interactors such as ATG4 79 (Autophagy related gene 4) and NBR1 (Neighbour of BRCA1) (Wild et al., 2014), our analyses revealed 80 that the highly conserved cytosolic protein C53 (aliases: CDK5RAP3, LZAP, IC53, HSF-27) is an AIM-81 dependent ATG8 interactor (Figure 1a, Table S2, Figure S3). 83To confirm our IP-MS results, we performed in vitro pull-down experiments. Arabidopsis thaliana (At) 84 C53 specifically interacted with GST-ATG8A, and this interac...

The fidelity of protein transport in the secretory pathway relies on the accurate sorting of proteins to their correct destinations. To deepen our understanding of the underlying molecular mechanisms, it is important to develop a robust approach to systematically reveal cargo proteins that depend on specific sorting machinery to be enriched into transport vesicles. Here, we used an in vitro assay that reconstitutes packaging of human cargo proteins into vesicles to quantify cargo capture. Quantitative mass spectrometry (MS) analyses of the isolated vesicles revealed cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner. We found that two of them, FAM84B (also known as LRAT domain containing 2 or LRATD2) and PRRC1, contain proline-rich domains and regulate anterograde trafficking. Further analyses revealed that PRRC1 is recruited to endoplasmic reticulum (ER) exit sites, interacts with the inner COPII coat, and its absence increases membrane association of COPII. In addition, we uncovered cargo proteins that depend on GTP hydrolysis to be captured into vesicles. Comparing control cells with cells depleted of the cargo receptors, SURF4 or ERGIC53, we revealed specific clients of each of these two export adaptors. Our results indicate that the vesicle formation assay in combination with quantitative MS analysis is a robust and powerful tool to uncover novel factors that mediate vesicular trafficking and to uncover cargo clients of specific cellular factors.

Endothelial barrier (EB) breaching is a frequent event during inflammation, and it is followed by the rapid recovery of microvascular integrity. The molecular mechanisms of EB recovery are poorly understood. Triggering of MHC molecules by migrating T-cells is a minimal signal capable of inducing endothelial contraction and transient microvascular leakage. Using this model, we show that EB recovery requires a CD31 receptor-induced, robust glycolytic response sustaining junction re-annealing. Mechanistically, this response involves srchomology phosphatase activation leading to Akt-mediated nuclear exclusion of FoxO1 and concomitant β-catenin translocation to the nucleus, collectively leading to cMyc transcription. CD31 signals also sustain mitochondrial respiration, however this pathway does not contribute to junction remodeling. We further show that pathologic microvascular leakage in CD31-deficient mice can be corrected by enhancing the glycolytic flux via pharmacological Akt or AMPK activation, thus providing a molecular platform for the therapeutic control of EB response.