The Rice Malectin Regulates Plant Cell Death and Disease Resistance by Participating in Glycoprotein Quality Control.

In animals, malectin is well known to play an essential role in endoplasmic reticulum quality control (ERQC) by interacting with ribophorin I, one unit of the oligosaccharyltransferase (OST) complex. However, the functions of malectin in plants remain largely unknown. Here, we demonstrate the rice OsMLD1 is an ER- and Golgi-associated malectin protein and physically interacts with rice homolog of ribophorin I (OsRpn1), and its disruption leads to spontaneous lesion mimic lesions, enhanced disease resistance, and prolonged ER stress. In addition, there are many more N-glycosites and N-glycoproteins identified from the mld1 mutant than wildtype. Furthermore, OsSERK1 and OsSERK2, which have more N-glycosites in mld1, were demonstrated to interact with OsMLD1. OsMLD1 can suppress OsSERK1- or OsSERK2-induced cell death. Thus, OsMLD1 may play a similar role to its mammalian homologs in glycoprotein quality control, thereby regulating cell death and immunity of rice, which uncovers the function of malectin in plants.

The SH3 domain in the fucosyltransferase FUT8 controls FUT8 activity and localization and is essential for core fucosylation.

Core fucose is an N-glycan structure synthesized by α1,6-fucosyltransferase 8 (FUT8) localized to the Golgi apparatus and critically regulates the functions of various glycoproteins. However, how FUT8 activity is regulated in cells remains largely unclear. At the luminal side and uncommon for Golgi proteins, FUT8 has an Src homology 3 (SH3) domain, which is usually found in cytosolic signal transduction molecules and generally mediates protein-protein interactions in the cytosol. However, the SH3 domain has not been identified in other glycosyltransferases, suggesting that FUT8's functions are selectively regulated by this domain. In this study, using truncated FUT8 constructs, immunofluorescence staining, FACS analysis, cell-surface biotinylation, proteomics, and LC-electrospray ionization MS analyses, we reveal that the SH3 domain is essential for FUT8 activity both in cells and in vitro and identified His-535 in the SH3 domain as the critical residue for enzymatic activity of FUT8. Furthermore, we found that although FUT8 is mainly localized to the Golgi, it also partially localizes to the cell surface in an SH3-dependent manner, indicating that the SH3 domain is also involved in FUT8 trafficking. Finally, we identified ribophorin I (RPN1), a subunit of the oligosaccharyltransferase complex, as an SH3-dependent binding protein of FUT8. RPN1 knockdown decreased both FUT8 activity and core fucose levels, indicating that RPN1 stimulates FUT8 activity. Our findings indicate that the SH3 domain critically controls FUT8 catalytic activity and localization and is required for binding by RPN1, which promotes FUT8 activity and core fucosylation.

OST4 is a subunit of the mammalian oligosaccharyltransferase required for efficient N-glycosylation.

The eukaryotic oligosaccharyltransferase (OST) is a membrane-embedded protein complex that catalyses the N-glycosylation of nascent polypeptides in the lumen of the endoplasmic reticulum (ER), a highly conserved biosynthetic process that enriches protein structure and function. All OSTs contain a homologue of the catalytic STT3 subunit, although in many cases this is assembled with several additional components that influence function. In S. cerevisiae, one such component is Ost4p, an extremely small membrane protein that appears to stabilise interactions between subunits of assembled OST complexes. OST4 has been identified as a putative human homologue, but to date neither its relationship to the OST complex, nor its role in protein N-glycosylation, have been directly addressed. Here, we establish that OST4 is assembled into native OST complexes containing either the catalytic STT3A or STT3B isoforms. Co-immunoprecipitation studies suggest that OST4 associates with both STT3 isoforms and with ribophorin I, an accessory subunit of mammalian OSTs. These presumptive interactions are perturbed by a single amino acid change in the transmembrane region of OST4. Using siRNA knockdowns and native gel analysis, we show that OST4 plays an important role in maintaining the stability of native OST complexes. Hence, upon OST4 depletion well-defined OST complexes are partially destabilised and a novel ribophorin I-containing subcomplex can be detected. Strikingly, cells depleted of either OST4 or STT3A show a remarkably similar defect in the N-glycosylation of endogenous prosaposin. We conclude that OST4 most likely promotes co-translational N-glycosylation by stabilising STT3A-containing OST isoforms.

Association of malectin with ribophorin I is crucial for attenuation of misfolded glycoprotein secretion.

We previously demonstrated that malectin associates with ribophorin I, which is a subunit of oligosaccharyltransferase in the endoplasmic reticulum (ER). When malectin and ribophorin I are overexpressed in the ER, secretion of an α1-antitrypsin (AT) variant whose folding is defective, termed null Hong Kong (AT(NHK)), is decreased. To confirm whether the interaction between malectin and ribophorin I is involved in the decreased secretion of misfolded glycoproteins, we constructed an expression vector encoding truncated malectin, which could not associate with ribophorin I and had an Lys-Asp-Glu-Leu ER-retention sequence at its C-terminus. Expression of wild-type malectin abrogated AT(NHK) secretion, whereas expression of truncated malectin did not affect AT(NHK) secretion. Both wild-type and truncated malectin bound to AT(NHK), and the level of AT(NHK) was similar in cells expressing wild-type malectin and those expressing truncated malectin. Furthermore, we previously showed that decreased secretion of misfolded AT(NHK) by malectin overexpression is restored by treatment with a proteasome inhibitor. These results clearly demonstrate that the association of malectin with ribophorin I is required to capture misfolded glycoproteins and direct them to the degradation pathway.

Disulfidptosis-related Protein RPN1 may be a Novel Anti-osteoporosis Target of Kaempferol.

BACKGROUND: Osteoporosis (OP) is an age-related skeletal disease. Kaempferol can regulate bone mesenchymal stem cells (BMSCs) osteogenesis to improve OP, but its mechanism related to disulfidptosis, a newly discovered cell death mechanism, remains unclear. OBJECTIVE: The study aimed to investigate the biological function and immune mechanism of disulfidptosis- related ribophorin I (RPN1) in OP and to experimentally confirm that RPN1 is the target for the treatment of OP with kaempferol. METHODS: Differential expression analysis was conducted on disulfide-related genes extracted from the GSE56815 and GSE7158 datasets. Four machine learning algorithms identified disease signature genes, with RPN1 identified as a significant risk factor for OP through the nomogram. Validation of RPN1 differential expression in OP patients was performed using the GSE56116 dataset. The impact of RPN1 on immune alterations and biological processes was explored. Predictive ceRNA regulatory networks associated with RPN1 were generated via miRanda, miRDB, and TargetScan databases. Molecular docking estimated the binding model between kaempferol and RPN1. The targeting mechanism of kaempferol on RPN1 was confirmed through pathological HE staining and immunohistochemistry in ovariectomized (OVX) rats. RESULTS: RPN1 was abnormally overexpressed in the OP cohort, associated with TNF signaling, hematopoietic cell lineage, and NF-kappa B pathway. Immune infiltration analysis showed a positive correlation between RPN1 expression and CD8+ T cells and resting NK cells, while a negative correlation with CD4+ naive T cells, macrophage M1, T cell gamma delta, T cell follicular helper cells, activated mast cells, NK cells, and dendritic cells, was found. Four miRNAs and 17 lncRNAs associated with RPN1 were identified. Kaempferol exhibited high binding affinity (-7.2 kcal/mol) and good stability towards the RPN1. The experimental results verified that kaempferol could improve bone microstructure destruction and reverse the abnormally high expression of RPN1 in the femur of ovariectomized rats. CONCLUSION: RPN1 may be a new diagnostic biomarker in patients with OP, and may serve as a new target for kaempferol to improve OP.

RPN1: a pan-cancer biomarker and disulfidptosis regulator.

Background: Elevated expression of SLC7A11, in conjunction with glucose deprivation, has revealed disulfidptosis as an emerging cell death modality. However, the prevalence of disulfidptosis across tumor cell lines, irrespective of SLC7A11 levels, remains uncertain. Additionally, deletion of the ribophorin I (RPN1) gene imparts resistance to disulfidptosis, yet the precise mechanism linking RPN1 to disulfidptosis remains elusive. The aim of this study is to determine the mechanism of RPN1-induced disulfidptosis and to determine the possibility of RPN1 as a pan-cancer marker. Methods: We hypothesized the widespread occurrence of disulfidptosis in various tumor cells, and proposed that RPN1-mediated disulfidptosis may be executed through cell skeleton breakdown. Experimental validation was conducted via flow cytometry, immunofluorescence, and western blot techniques. Furthermore, given RPN1's status as an emerging cell death marker, we utilized bioinformatics to analyze its expression in tumor tissues, clinical relevance, mechanisms within the tumor microenvironment, and potential for immunotherapy. Results: Conducting experiments on breast cancer (MDA-MB-231) and lung cancer (A549) cell lines under glucose-starved conditions, we found that RPN1 primarily induces cell skeleton breakdown to facilitate disulfidptosis. RPN1 demonstrated robust messenger RNA (mRNA) expression across 16 solid tumors, validated by data from 12 tumor types in the Gene Expression Omnibus (GEO). Across 12 cancer types, RPN1 exhibited significant diagnostic potential, particularly excelling in accuracy for glioblastoma (GBM). Elevated RPN1 expression in tumor tissues was found to correlate with improved overall survival (OS) in certain cancers [diffuse large B-cell lymphoma (DLBC) and thymoma (THYM)] but poorer prognosis in others [adrenocortical carcinoma (ACC), kidney chromophobe (KICH), brain lower grade glioma (LGG), liver hepatocellular carcinoma (LIHC), and pancreatic adenocarcinoma (PAAD)]. RPN1 is enriched in immune-related pathways and correlates with immune scores in tumor tissues. In urothelial carcinoma (UCC), RPN1 demonstrates potential in predicting the efficacy of anti-programmed cell death ligand 1 (PD-L1) immune therapy. Conclusions: This study underscores RPN1's role in facilitating disulfidptosis, its broad relevance as a pan-cancer biomarker, and its association with the efficacy of anti-PD-L1 immune therapy.