MAIGO2 is involved in abscisic acid-mediated response to abiotic stresses and Golgi-to-ER retrograde transport

ZmBET5L1 inhibits primary root growth and decreases osmotic stress tolerance by mediating vesicle aggregation and tethering in maize

Improving osmotic stress tolerance is critical to help crops to thrive and maintain high yields in adverse environments. Here, we characterized a core subunit of the transport protein particle (TRAPP) complex, ZmBET5L1, in maize using knowledge-driven data mining and genome editing. We found that ZmBET5L1 can interact with TRAPP I complex subunits and act as a tethering factor to mediate vesicle aggregation and targeting from the endoplasmic reticulum to the Golgi apparatus. ZmBET5L1 knock-out increased the primary root elongation rate under 20% polyethylene glycol-simulated osmotic stress and the survival rate under drought stress compared to wild-type seedlings. In addition, we found that ZmBET5L1 moderates PIN1 polar localization and auxin flow to maintain normal root growth. ZmBET5L1 knock-out optimized auxin flow to the lateral side of the root and promoted its growth to generate a robust root, which may be related to improved osmotic stress tolerance. Together, these findings demonstrate that ZmBET5L1 inhibits primary root growth and decreases osmotic stress tolerance by regulating vesicle transport and auxin distribution. This study has improved our understanding of the role of tethering factors in response to abiotic stresses and identified desirable variants for breeding osmotic stress tolerance in maize.

A New Optimized Version of a Colorectal Cancer-Targeted Immunotoxin Based on a Non-Immunogenic Variant of the Ribotoxin α-Sarcin

Due to its incidence and mortality, cancer remains one of the main risks to human health and lifespans. In order to overcome this worldwide disease, immunotherapy and the therapeutic use of immunotoxins have arisen as promising approaches. However, the immunogenicity of foreign proteins limits the dose of immunotoxins administered, thereby leading to a decrease in its therapeutic benefit. In this study, we designed two different variants of non-immunogenic immunotoxins (IMTXA33αSDI and IMTXA33furαSDI) based on a deimmunized variant of the ribotoxin α-sarcin. The inclusion of a furin cleavage site in IMTXA33furαSDI would allow a more efficient release of the toxic domain to the cytosol. Both immunotoxins were produced and purified in the yeast Pichia pastoris and later functionally characterized (both in vitro and in vivo), and immunogenicity assays were carried out. The results showed that both immunotoxins were functionally active and less immunogenic than the wild-type immunotoxin. In addition, IMTXA33furαSDI showed a more efficient antitumor effect (both in vitro and in vivo) due to the inclusion of the furin linker. These results constituted a step forward in the optimization of immunotoxins with low immunogenicity and enhanced antitumor activity, which can lead to potential better outcomes in cancer treatment.

MAG2 and MAL Regulate Vesicle Trafficking and Auxin Homeostasis With Functional Redundancy

Auxin is a central phytohormone and controls almost all aspects of plant development and stress response. Auxin homeostasis is coordinately regulated by biosynthesis, catabolism, transport, conjugation, and deposition. Endoplasmic reticulum (ER)-localized MAIGO2 (MAG2) complex mediates tethering of arriving vesicles to the ER membrane, and it is crucial for ER export trafficking. Despite important regulatory roles of MAG2 in vesicle trafficking, the mag2 mutant had mild developmental abnormalities. MAG2 has one homolog protein, MAG2-Like (MAL), and the mal-1 mutant also had slight developmental phenotypes. In order to investigate MAG2 and MAL regulatory function in plant development, we generated the mag2-1 mal-1 double mutant. As expected, the double mutant exhibited serious developmental defects and more alteration in stress response compared with single mutants and wild type. Proteomic analysis revealed that signaling, metabolism, and stress response in mag2-1 mal-1 were affected, especially membrane trafficking and auxin biosynthesis, signaling, and transport. Biochemical and cell biological analysis indicated that the mag2-1 mal-1 double mutant had more serious defects in vesicle transport than the mag2-1 and mal-1 single mutants. The auxin distribution and abundance of auxin transporters were altered significantly in the mag2-1 and mal-1 single mutants and mag2-1 mal-1 double mutant. Our findings suggest that MAG2 and MAL regulate plant development and auxin homeostasis by controlling membrane trafficking, with functional redundancy.

Inclusion of a Furin Cleavage Site Enhances Antitumor Efficacy against Colorectal Cancer Cells of Ribotoxin α-Sarcin- or RNase T1-Based Immunotoxins

Immunotoxins are chimeric molecules that combine the specificity of an antibody to recognize and bind tumor antigens with the potency of the enzymatic activity of a toxin, thus, promoting the death of target cells. Among them, RNases-based immunotoxins have arisen as promising antitumor therapeutic agents. In this work, we describe the production and purification of two new immunoconjugates, based on RNase T1 and the fungal ribotoxin α-sarcin, with optimized properties for tumor treatment due to the inclusion of a furin cleavage site. Circular dichroism spectroscopy, ribonucleolytic activity studies, flow cytometry, fluorescence microscopy, and cell viability assays were carried out for structural and in vitro functional characterization. Our results confirm the enhanced antitumor efficiency showed by these furin-immunotoxin variants as a result of an improved release of their toxic domain to the cytosol, favoring the accessibility of both ribonucleases to their substrates. Overall, these results represent a step forward in the design of immunotoxins with optimized properties for potential therapeutic application in vivo.

Communications Between the Endoplasmic Reticulum and Other Organelles During Abiotic Stress Response in Plants

To adapt to constantly changing environmental conditions, plants have evolved sophisticated tolerance mechanisms to integrate various stress signals and to coordinate plant growth and development. It is well known that inter-organellar communications play important roles in maintaining cellular homeostasis in response to environmental stresses. The endoplasmic reticulum (ER), extending throughout the cytoplasm of eukaryotic cells, is a central organelle involved in lipid metabolism, Ca²⁺ homeostasis, and synthesis and folding of secretory and transmembrane proteins crucial to perceive and transduce environmental signals. The ER communicates with the nucleus via the highly conserved unfolded protein response pathway to mitigate ER stress. Importantly, recent studies have revealed that the dynamic ER network physically interacts with other intracellular organelles and endomembrane compartments, such as the Golgi complex, mitochondria, chloroplast, peroxisome, vacuole, and the plasma membrane, through multiple membrane contact sites between closely apposed organelles. In this review, we will discuss the signaling and metabolite exchanges between the ER and other organelles during abiotic stress responses in plants as well as the ER-organelle membrane contact sites and their associated tethering complexes.

Proteome of olive non-glandular trichomes reveals protective protein network against (a)biotic challenge

Olive is one of the most important fruit crop trees in the history of Mediterranean because of the high quality oil. Olive oil has a well-balanced fatty acid composition along with biophenols, which make it exceptional in human diet and provide an exceptional value to the olive oil. Leaf non-glandular peltate trichomes are specialized cell types representing a protective barrier against acute environmental conditions. To characterize the proteome of this highly differentiated cell type, we performed a comparative proteomic analysis among isolated trichomes and trichome-less leaves. Proteins were separated and identified using the 2-DE MALDI-TOF/MS method. A number of enzymes involved in abiotic and biotic stress responses are present and may be responsible for the adaptation to prolonged adverse environmental conditions. The results show that this highly differentiated cell type is physiologically active fulfilling the demands of the trichomes in furnishing the leaf with a highly protective mechanism.

Misregulation of ER-Golgi Vesicle Transport Induces ER Stress and Affects Seed Vigor and Stress Response

Seeds of higher plants accumulate numerous storage proteins to use as nitrogen resources for early plant development. Seed storage proteins (SSPs) are synthesized as large precursors on the rough endoplasmic reticulum (rER), and are delivered to protein storage vacuoles (PSVs) via vesicle transport, where they are processed to mature forms. We previously identified an Arabidopsis ER-localized tethering complex, MAG2 complex, which might be involved in Golgi to ER retrograde transport. The MAG2 complex is composed of 4 subunits, MAG2, MIP1, MIP2, and MIP3. Mutants with defective alleles for these subunits accumulated SSP precursors inside the ER lumen. Here, we report that the mag2-1 mip3-1 and mip2-1 mip3-1 double mutant have more serious vesicle transport defects than the mag2-1, mip2-1, and mip3-1 single mutants, since they accumulate more SSP precursors than the corresponding single mutants, and ER stress is more severe than the single mutants. The mag2-1 mip3-1 and mip2-1 mip3-1 double mutants show growth and developmental defects rather than the single mutants. Both single and double mutant seeds are found to have lower protein content and decreased germinating vigor than wild type seeds. All the mutants are sensitive to abscisic acid (ABA) and salt stress, and exhibit alteration in ABA signaling pathway. Our study clarified that ER-Golgi vesicle transport affects seed vigor through controlling seed protein quality and content, as well as plant response to environmental stress via influencing ABA signaling pathway.

Multisubunit tethering complexes in higher plants

Tethering complexes mediate the initial, specific contact between donor and acceptor membranes. This review focuses on the modularity and function of multisubunit tethering complexes (MTCs) in higher plants. One emphasis is on molecular interactions of plant MTCs. Here, a number of insights have been gained concerning interactions between different tethering complexes, and between tethers and microtubule-associated proteins. The roles of tethering complexes in abiotic stress responses appear indirect, but in the context of biotic stress responses it has been suggested that some tethers are direct targets of pathogen effectors or virulence factors. In light of the central roles tethering complexes play in plant development, an emerging concept is that tethers may be co-opted for plant adaptive responses.

Tethering Complexes in the Arabidopsis Endomembrane System

Targeting of endomembrane transport containers is of the utmost importance for proper land plant growth and development. Given the immobility of plant cells, localized membrane vesicle secretion and recycling are amongst the main processes guiding proper cell, tissue and whole plant morphogenesis. Cell wall biogenesis and modification are dependent on vectorial membrane traffic, not only during normal development, but also in stress responses and in plant defense against pathogens and/or symbiosis. It is surprising how little we know about these processes in plants, from small GTPase regulation to the tethering complexes that act as their effectors. Tethering factors are single proteins or protein complexes mediating first contact between the target membrane and arriving membrane vesicles. In this review we focus on the tethering complexes of the best-studied plant model—Arabidopsis thaliana. Genome-based predictions indicate the presence of all major tethering complexes in plants that are known from a hypothetical last eukaryotic common ancestor (LECA). The evolutionary multiplication of paralogs of plant tethering complex subunits has produced the massively expanded EXO70 family, indicating a subfunctionalization of the terminal exocytosis machinery in land plants. Interpretation of loss of function (LOF) mutant phenotypes has to consider that related, yet clearly functionally-specific complexes often share some common core subunits. It is therefore impossible to conclude with clarity which version of the complex is responsible for the phenotypic deviations observed. Experimental interest in the analysis of plant tethering complexes is growing and we hope to contribute with this review by attracting even more attention to this fascinating field of plant cell biology.

α-sarcin and RNase T1 based immunoconjugates: the role of intracellular trafficking in cytotoxic efficiency

Toxins have been thoroughly studied for their use as therapeutic agents in search of an improvement in toxic efficiency together with a minimization of their undesired side effects. Different studies have shown how toxins can follow different intracellular pathways which are connected with their cytotoxic action inside the cells. The work herein presented describes the different pathways followed by the ribotoxin α-sarcin and the fungal RNase T1, as toxic domains of immunoconjugates with identical binding domain, the single chain variable fragment of a monoclonal antibody raised against the glycoprotein A33. According to the results obtained both immunoconjugates enter the cells via early endosomes and, while α-sarcin can translocate directly into the cytosol to exert its deathly action, RNase T1 follows a pathway that involves lysosomes and the Golgi apparatus. These facts contribute to explaining the different cytotoxicity observed against their targeted cells, and reveal how the innate properties of the toxic domain, apart from its catalytic features, can be a key factor to be considered for immunotoxin optimization.

Moonlighting functions of the NRZ (mammalian Dsl1) complex

The yeast Dsl1 complex, which comprises Dsl1, Tip20, and Sec39/Dsl3, has been shown to participate, as a vesicle-tethering complex, in retrograde trafficking from the Golgi apparatus to the endoplasmic reticulum. Its metazoan counterpart NRZ complex, which comprises NAG, RINT1, and ZW10, is also involved in Golgi-to-ER retrograde transport, but each component of the complex has diverse cellular functions including endosome-to-Golgi transport, cytokinesis, cell cycle checkpoint, autophagy, and mRNA decay. In this review, we summarize the current knowledge of the metazoan NRZ complex and discuss the "moonlighting" functions and intercorrelation of their subunits.

MAG2 and three MAG2-INTERACTING PROTEINs form an ER-localized complex to facilitate storage protein transport in Arabidopsis thaliana

In Arabidopsis thaliana, MAIGO 2 (MAG2) is involved in protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus via its association with the ER-localized t-SNARE components SYP81/AtUfe1 and SEC20. To characterize the molecular machinery of MAG2-mediated protein transport, we explored MAG2-interacting proteins using transgenic A. thaliana plants expressing TAP-tagged MAG2. We identified three proteins, which were designated as MAG2-INTERACTING PROTEIN 1-3 [MIP1 (At2g32900), MIP2 (At5g24350) and MIP3 (At2g42700)]. Both MIP1 and MAG2 localized to the ER membrane. All of the mag2, mip1, mip2 and mip3 mutants exhibited a defect in storage protein maturation, and developed abnormal storage protein body (MAG body) structures in the ER of seed cells. These observations suggest that MIPs are closely associated with MAG2 and function in protein transport between the ER and Golgi apparatus. MIP1 and MIP2 contain a Zeste-White 10 (ZW10) domain and a Sec39 domain, respectively, but have low sequence identities (21% and 23%) with respective human orthologs. These results suggest that the plant MAG2-MIP1-MIP2 complex is a counterpart of the triple-subunit tethering complexes in yeast (Tip20p-Dsl1p-Sec39p) and humans (RINT1-ZW10-NAG). Surprisingly, the plant complex also contained a fourth member (MIP3) with a Sec1 domain. There have been no previous reports showing that a Sec1-containing protein is a subunit of ER-localized tethering complexes. Our results suggest that MAG2 and the three MIP proteins form a unique complex on the ER that is responsible for efficient transport of seed storage proteins.