The UFM1 system: Working principles, cellular functions, and pathophysiology.

Ubiquitin-fold modifier 1 (UFM1) is a ubiquitin-like protein covalently conjugated with intracellular proteins through UFMylation, a process similar to ubiquitylation. Growing lines of evidence regarding not only the structural basis of the components essential for UFMylation but also their biological properties shed light on crucial roles of the UFM1 system in the endoplasmic reticulum (ER), such as ER-phagy and ribosome-associated quality control at the ER, although there are some functions unrelated to the ER. Mouse genetics studies also revealed the indispensable roles of this system in hematopoiesis, liver development, neurogenesis, and chondrogenesis. Of critical importance, mutations of genes encoding core components of the UFM1 system in humans cause hereditary developmental epileptic encephalopathy and Schohat-type osteochondrodysplasia of the epiphysis. Here, we provide a multidisciplinary review of our current understanding of the mechanisms and cellular functions of the UFM1 system as well as its pathophysiological roles, and discuss issues that require resolution.

UFMylation: a ubiquitin-like modification.

Post-translational modifications (PTMs) add a major degree of complexity to the proteome and are essential controllers of protein homeostasis. Amongst the hundreds of PTMs identified, ubiquitin and ubiquitin-like (UBL) modifications are recognized as key regulators of cellular processes through their ability to affect protein-protein interactions, protein stability, and thus the functions of their protein targets. Here, we focus on the most recently identified UBL, ubiquitin-fold modifier 1 (UFM1), and the machinery responsible for its transfer to substrates (UFMylation) or its removal (deUFMylation). We first highlight the biochemical peculiarities of these processes, then we develop on how UFMylation and its machinery control various intertwined cellular processes and we highlight some of the outstanding research questions in this emerging field.

RPL26/uL24 UFMylation is essential for ribosome-associated quality control at the endoplasmic reticulum.

Ribosomes that stall while translating cytosolic proteins are incapacitated by incomplete nascent chains, termed "arrest peptides" (APs) that are destroyed by the ubiquitin proteasome system (UPS) via a process known as the ribosome-associated quality control (RQC) pathway. By contrast, APs on ribosomes that stall while translocating secretory proteins into the endoplasmic reticulum (ER-APs) are shielded from cytosol by the ER membrane and the tightly sealed ribosome-translocon junction (RTJ). How this junction is breached to enable access of cytosolic UPS machinery and 26S proteasomes to translocon- and ribosome-obstructing ER-APs is not known. Here, we show that UPS and RQC-dependent degradation of ER-APs strictly requires conjugation of the ubiquitin-like (Ubl) protein UFM1 to 60S ribosomal subunits at the RTJ. Therefore, UFMylation of translocon-bound 60S subunits modulates the RTJ to promote access of proteasomes and RQC machinery to ER-APs.

A single transcription factor is sufficient to induce and maintain secretory cell architecture.

We hypothesized that basic helix-loop-helix (bHLH) MIST1 (BHLHA15) is a "scaling factor" that universally establishes secretory morphology in cells that perform regulated secretion. Here, we show that targeted deletion of MIST1 caused dismantling of the secretory apparatus of diverse exocrine cells. Parietal cells (PCs), whose function is to pump acid into the stomach, normally lack MIST1 and do not perform regulated secretion. Forced expression of MIST1 in PCs caused them to expand their apical cytoplasm, rearrange mitochondrial/lysosome trafficking, and generate large secretory granules. Mist1 induced a cohort of genes regulated by MIST1 in multiple organs but did not affect PC function. MIST1 bound CATATG/CAGCTG E boxes in the first intron of genes that regulate autophagosome/lysosomal degradation, mitochondrial trafficking, and amino acid metabolism. Similar alterations in cell architecture and gene expression were also caused by ectopically inducing MIST1 in vivo in hepatocytes. Thus, MIST1 is a scaling factor necessary and sufficient by itself to induce and maintain secretory cell architecture. Our results indicate that, whereas mature cell types in each organ may have unique developmental origins, cells performing similar physiological functions throughout the body share similar transcription factor-mediated architectural "blueprints."

UFM1 inhibits hypoxia-induced angiogenesis via promoting proteasome degradation of HIF-1α.

Angiogenesis is crucial for blood flow recovery and ischemic tissue repair of peripheral artery disease (PAD). Exploration of new mechanisms underlying angiogenesis will shed light on the treatment of PAD. Ubiquitin-fold modifier 1 (UFM1), a newly identified ubiquitin-like molecule, has been discovered to be involved in various pathophysiological processes. However, the role of UFM1 in the pathogenesis of PAD, especially in endothelial angiogenesis remains obscure, and we aimed to clarify this issue in this study. We initially found UFM1 was significantly upregulated in gastrocnemius muscles of PAD patients and hind limb ischemia mice. And UFM1 was mainly colocalized with endothelial cells in ischemic muscle tissues. Further, elevated expression of UFM1 was observed in hypoxic endothelial cells. Subsequent genetic inhibition of UFM1 dramatically enhanced migration, invasion, adhesion, and tube formation of endothelial cells under hypoxia. Mechanistically, UFM1 reduced the stability of hypoxia-inducible factor-1α (HIF-1α) and promoted the von Hippel-Lindau-mediated K48-linked ubiquitin-proteasome degradation of HIF-1α, which in turn decreased angiogenic factor VEGFA expression and suppressed VEGFA related signaling pathway. Consistently, overexpression of UFM1 inhibited the angiogenesis of endothelial cells under hypoxic conditions, whereas overexpression of HIF-1α reversed this effect. Collectively, our data reveal that UFM1 inhibits the angiogenesis of endothelial cells under hypoxia through promoting ubiquitin-proteasome degradation of HIF-1α, suggesting UFM1 might serve as a potential therapeutic target for PAD.

The UFM1 conjugation system in mammalian development.

Posttranslational modifications by ubiquitin and ubiquitin-like proteins are important in regulating cellular protein functions. UFM1 (ubiquitin-fold modifier 1), first identified almost two decades ago, is a member of the ubiquitin-like protein family. UFM1 is covalently conjugated to the target proteins in an enzymatic cascade consisting of E1 (activating), E2 (conjugating), and E3 (ligating) enzymes. At the molecular level, modification by UFM1 (UFMylation) is an important mediator of the protein function. Dysregulation of the UFM1 system, e.g., the knockout of UFMylation components, disturbs proteome homeostasis and triggers endoplasmic reticulum stress. Such changes are linked to developmental disorders, tumorigenesis, tissue injury, inflammation, and several hereditary neurological syndromes. This review will focus on the role of the UFMylation in animal development and associated congenital disorders. We will cover the hematopoietic system, liver, central nervous system, intestine, heart, kidney, immune, and skeletal system to provide insight into disease pathogenesis and shed light on possible novel therapeutic methods.

Proximity Proteomics and Biochemical Analysis Reveal a Noncanonical Function for UFM1-Specific Protease 1 in the p62 Body Formation.

Protein aggregates play crucial roles in the development of neurodegenerative diseases and p62 is one of the key proteins regulating the formation of protein aggregates. Recently, it has been discovered that depletion of several key enzymes including UFM1-activating enzyme UBA5, UFM1-conjugating enzyme UFC1, UFM1-protein ligase UFL1, and UFM1-specific protease UfSP2 in the UFM1-conjugation system induces p62 accumulation to form p62 bodies in the cytosol. However, it is unknown whether UfSP1 participates in the formation of p62 bodies and whether its enzymatic activity is required for this process. Here, the proximity labeling technique and quantitative proteomics identify SQSTM1/p62 as a UfSP1-interacting protein. Coimmunoprecipitation reveals that p62 indeed interacts with UfSP1 and the immunofluorescence experiment discloses that UfSP1 colocalizes with p62 and promotes the formation of p62-mediated protein aggregates. Mechanistic studies unveil that UfSP1 binds to the ubiquitin-associated domain of p62 and promotes the interaction between p62 and ubiquitinated proteins, thereby increasing the formation of p62 bodies. Interestingly, we further demonstrate that both the catalytic active and inactive UfSP1 promote the formation of p62 bodies through the same mechanism. Taken together, this work discovers that UfSP1 exhibits a noncanonical function independent of its protease activity in the p62 body formation.

Nontraditional translation is the key to UFMylation and beyond.

The Ubiquitin-fold modifier 1 (Ufm1) is a ubiquitin-like protein that can also be conjugated to protein substrates and subsequently alter their fates. Both UFMylation and de-UFMylation are mediated by Ufm1-specific proteases (UFSPs). In humans, it is widely believed that UFSP2 is the only active Ufm1 protease involved in Ufm1 maturation and de-UFMylation, whereas UFSP1 is thought to be inactive. Here, Liang et al. provide strong evidence showing that human UFSP1 is also an active Ufm1 protease. These results solve an age-old mystery in the human Ufm1 conjugation system and could have a greater impact not only on Ufm1 biology but also on the translation of genes employing nontraditional start codons.

Human UFSP1 translated from an upstream near-cognate initiation codon functions as an active UFM1-specific protease.

Ubiquitin-fold modifier 1 (UFM1) is a recently identified ubiquitin-like posttranslational modification with important biological functions. However, the regulatory mechanisms governing UFM1 modification of target proteins (UFMylation) and the cellular processes controlled by UFMylation remain largely unknown. It has been previously shown that a UFM1-specific protease (UFSP2) mediates the maturation of the UFM1 precursor and drives the de-UFMylation reaction. Furthermore, it has long been thought that UFSP1, an ortholog of UFSP2, is inactive in many organisms, including human, because it lacks an apparent protease domain when translated from the canonical start codon (445AUG). Here, we demonstrate using the combination of site-directed mutagenesis, CRISPR/Cas9-mediated genome editing, and mass spectrometry approaches that translation of human UFSP1 initiates from an upstream near-cognate codon, 217CUG, via eukaryotic translation initiation factor eIF2A-mediated translational initiation rather than from the annotated 445AUG, revealing the presence of a catalytic protease domain containing a Cys active site. Moreover, we show that both UFSP1 and UFSP2 mediate maturation of UFM1 and de-UFMylation of target proteins. This study demonstrates that human UFSP1 functions as an active UFM1-specific protease, thus contributing to our understanding of the UFMylation/de-UFMylation process.

Preparation of UFM1-Derived Probes through Highly Optimized Total Chemical Synthesis.

Ufmylation is involved in various cellular processes and associated with many human diseases. The understanding of this modification relies on the use of customized UFM1-derived probes for activity-based profiling of its related enzymes. This study presents a highly optimized total chemical synthesis for the generation of diverse UFM1-derived probes including UFM1-PA, Biotin-UFM1-PA and UFM1-AMC, in which a UFM1 C-terminal valine hydrazide was readily prepared by hydrazide-based ligation and used as a versatile handle for the installation of enzyme-sensitive warheads and fluorescent reporters. The resulting probes display high reactivity and selectivity for UFM1-specific enzymes in cell lysates. This strategy facilitates the generation and diversity of the UFM1-derived toolkit that can be employed to profile UFM1-specific enzymes, thereby shining insights into the dynamics of ufmylation.

Screening UFMylation-associated genes in heart tissues of Ufm1-transgenic mice.

UFMylation is a ubiquitination-like modification that is related to endoplasmic reticulum stress and unfolded protein response. A recent study reported that Ufl1, a key enzyme of UFMylation, protects against heart failure, indicating that UFMylation may be associated with heart function regulation. In the present study, we initially constructed a Flag-6×His-tagged Ufm1ΔSC transgenic (Tg-Ufm1) mouse model that enables UFMylation studies in vivo. Tg-Ufm1 mice showed significant activation of UFMylation in hearts. By using this model, we identified 38 potential Ufm1-binding proteins in heart tissues through LC‒MS/MS methods. We found that these proteins were associated with mitochondria, metabolism and chaperone binding. By using transcriptomic screening, we identified Tnfaip2 as a novel UFMylation-associated gene. Overexpression of Ufm1 significantly upregulated the protein expression of Tnfaip2, whereas isoproterenol treatment decreased Tnfaip2 expression in Tg-Ufm1 mice. These data may provide novel clues for UFMylation in cardiac hypertrophy.

Ribosomal protein RPL26 is the principal target of UFMylation.

Ubiquitin fold modifier 1 (UFM1) is a small, metazoan-specific, ubiquitin-like protein modifier that is essential for embryonic development. Although loss-of-function mutations in UFM1 conjugation are linked to endoplasmic reticulum (ER) stress, neither the biological function nor the relevant cellular targets of this protein modifier are known. Here, we show that a largely uncharacterized ribosomal protein, RPL26, is the principal target of UFM1 conjugation. RPL26 UFMylation and de-UFMylation is catalyzed by enzyme complexes tethered to the cytoplasmic surface of the ER and UFMylated RPL26 is highly enriched on ER membrane-bound ribosomes and polysomes. Biochemical analysis and structural modeling establish that UFMylated RPL26 and the UFMylation machinery are in close proximity to the SEC61 translocon, suggesting that this modification plays a direct role in cotranslational protein translocation into the ER. These data suggest that UFMylation is a ribosomal modification specialized to facilitate metazoan-specific protein biogenesis at the ER.

Overexpression of UBA5 in Cells Mimics the Phenotype of Cells Lacking UBA5.

Ufmylation is a posttranslational modification in which the modifier UFM1 is attached to target proteins. This conjugation requires the concerted work of three enzymes named UBA5, UFC1, and UFL1. Initially, UBA5 activates UFM1 in a process that ends with UFM1 attached to UBA5's active site Cys. Then, in a trans-thiolation reaction, UFM1 is transferred from UBA5 to UFC1, forming a thioester bond with the latter. Finally, with the help of UFL1, UFM1 is transferred to the final destination-a lysine residue on a target protein. Therefore, not surprisingly, deletion of one of these enzymes abrogates the conjugation process. However, how overexpression of these enzymes affects this process is not yet clear. Here we found, unexpectedly, that overexpression of UBA5, but not UFC1, damages the ability of cells to migrate, in a similar way to cells lacking UBA5 or UFC1. At the mechanistic level, we found that overexpression of UBA5 reverses the trans-thiolation reaction, thereby leading to a back transfer of UFM1 from UFC1 to UBA5. This, as seen in cells lacking UBA5, reduces the level of charged UFC1 and therefore harms the conjugation process. In contrast, co-expression of UBA5 with UFM1 abolishes this effect, suggesting that the reverse transfer of UFM1 from UFC1 to UBA5 depends on the level of free UFM1. Overall, our results propose that the cellular expression level of the UFM1 conjugation enzymes has to be tightly regulated to ensure the proper directionality of UFM1 transfer.

A Concerted Action of UBA5 C-Terminal Unstructured Regions Is Important for Transfer of Activated UFM1 to UFC1.

Ubiquitin fold modifier 1 (UFM1) is a member of the ubiquitin-like protein family. UFM1 undergoes a cascade of enzymatic reactions including activation by UBA5 (E1), transfer to UFC1 (E2) and selective conjugation to a number of target proteins via UFL1 (E3) enzymes. Despite the importance of ufmylation in a variety of cellular processes and its role in the pathogenicity of many human diseases, the molecular mechanisms of the ufmylation cascade remains unclear. In this study we focused on the biophysical and biochemical characterization of the interaction between UBA5 and UFC1. We explored the hypothesis that the unstructured C-terminal region of UBA5 serves as a regulatory region, controlling cellular localization of the elements of the ufmylation cascade and effective interaction between them. We found that the last 20 residues in UBA5 are pivotal for binding to UFC1 and can accelerate the transfer of UFM1 to UFC1. We solved the structure of a complex of UFC1 and a peptide spanning the last 20 residues of UBA5 by NMR spectroscopy. This structure in combination with additional NMR titration and isothermal titration calorimetry experiments revealed the mechanism of interaction and confirmed the importance of the C-terminal unstructured region in UBA5 for the ufmylation cascade.

The UFM1 Pathway Impacts HCMV US2-Mediated Degradation of HLA Class I.

To prevent accumulation of misfolded proteins in the endoplasmic reticulum, chaperones perform quality control on newly translated proteins and redirect misfolded proteins to the cytosol for degradation by the ubiquitin-proteasome system. This pathway is called ER-associated protein degradation (ERAD). The human cytomegalovirus protein US2 induces accelerated ERAD of HLA class I molecules to prevent immune recognition of infected cells by CD8+ T cells. Using US2-mediated HLA-I degradation as a model for ERAD, we performed a genome-wide CRISPR/Cas9 library screen to identify novel cellular factors associated with ERAD. Besides the identification of known players such as TRC8, p97, and UBE2G2, the ubiquitin-fold modifier1 (UFM1) pathway was found to affect degradation of HLA-I. UFMylation is a post-translational modification resembling ubiquitination. Whereas we observe ubiquitination of HLA-I, no UFMylation was detected on HLA-I or several other proteins involved in degradation of HLA-I, suggesting that the UFM1 pathway impacts ERAD in a different manner than ubiquitin. Interference with the UFM1 pathway seems to specifically inhibit the ER-to-cytosol dislocation of HLA-I. In the absence of detectable UFMylation of HLA-I, UFM1 may contribute to US2-mediated HLA-I degradation by misdirecting protein sorting indirectly. Mass spectrometry analysis of US2-expressing cells showed that ribosomal proteins are a major class of proteins undergoing extensive UFMylation; the role of these changes in protein degradation may be indirect and remains to be established.

Morphological and Elastic Transition of Polystyrene Adsorbed Layers on Silicon Oxide.

Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. More in detail, standard contact mode, force versus distance spectroscopy and ultrasonic force microscopy have been employed to obtain spatially-resolved maps and, thus, observe the physisorption of polystyrene on native silicon oxide substrate in function of time. Thick films, spin coated from a toluene solution, have been annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires 24 h. The pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion measurements show that the films present a reduced snap-off and the formation of a meniscus between tip and surface for annealing time up to 8 h. On the other hand, elastic measurements allow us to observe a progressive increase of the elastic modulus, with a complete transition for annealing time above 20 h. This is indication that a dense packing of the polystyrene molecules occurs, in line with the predictions of current models on the kinetics of irreversible adsorption. LAY DESCRIPTION: Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. Thick polystyrene films, spin coated from a toluene solution, have been thermally annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires twenty-four hours. The adsorption pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion and elastic measurements have allowed us to observe a progressive increase of the packing density of the polystyrene molecules, in agreement with the predictions of current models on the kinetics of irreversible adsorption.

Generation of the UFM1 Toolkit for Profiling UFM1-Specific Proteases and Ligases.

Ubiquitin-fold modifier 1 (UFM1) is a reversible post-translational modifier that is covalently attached to target proteins through an enzymatic cascade and removed by designated proteases. Abnormalities in this process, referred to as Ufmylation, have been associated with a variety of human diseases. Given this, the UFM1-specific enzymes represent potential therapeutic targets; however, understanding of their biological function has been hampered by the lack of chemical tools for activity profiling. To address this unmet need, a diversifiable platform for UFM1 activity-based probes (ABPs) utilizing a native chemical ligation (NCL) strategy was developed, enabling the generation of a variety of tools to profile both UFM1 conjugating and deconjugating enzymes. The use of the probes is demonstrated in vitro and in vivo for monitoring UFM1 enzyme reactivity, opening new research avenues.

Structural and Functional Analysis of a Novel Interaction Motif within UFM1-activating Enzyme 5 (UBA5) Required for Binding to Ubiquitin-like Proteins and Ufmylation.

The covalent conjugation of ubiquitin-fold modifier 1 (UFM1) to proteins generates a signal that regulates transcription, response to cell stress, and differentiation. Ufmylation is initiated by ubiquitin-like modifier activating enzyme 5 (UBA5), which activates and transfers UFM1 to ubiquitin-fold modifier-conjugating enzyme 1 (UFC1). The details of the interaction between UFM1 and UBA5 required for UFM1 activation and its downstream transfer are however unclear. In this study, we described and characterized a combined linear LC3-interacting region/UFM1-interacting motif (LIR/UFIM) within the C terminus of UBA5. This single motif ensures that UBA5 binds both UFM1 and light chain 3/γ-aminobutyric acid receptor-associated proteins (LC3/GABARAP), two ubiquitin (Ub)-like proteins. We demonstrated that LIR/UFIM is required for the full biological activity of UBA5 and for the effective transfer of UFM1 onto UFC1 and a downstream protein substrate both in vitro and in cells. Taken together, our study provides important structural and functional insights into the interaction between UBA5 and Ub-like modifiers, improving the understanding of the biology of the ufmylation pathway.

The eukaryotic ancestor had a complex ubiquitin signaling system of archaeal origin.

The origin of the eukaryotic cell is one of the most important transitions in the history of life. However, the emergence and early evolution of eukaryotes remains poorly understood. Recent data have shown that the last eukaryotic common ancestor (LECA) was much more complex than previously thought. The LECA already had the genetic machinery encoding the endomembrane apparatus, spliceosome, nuclear pore, and myosin and kinesin cytoskeletal motors. It is unclear, however, when the functional regulation of these cellular components evolved. Here, we address this question by analyzing the origin and evolution of the ubiquitin (Ub) signaling system, one of the most important regulatory layers in eukaryotes. We delineated the evolution of the whole Ub, Small-Ub-related MOdifier (SUMO), and Ub-fold modifier 1 (Ufm1) signaling networks by analyzing representatives from all major eukaryotic, bacterial, and archaeal lineages. We found that the Ub toolkit had a pre-eukaryotic origin and is present in three extant archaeal groups. The pre-eukaryotic Ub toolkit greatly expanded during eukaryogenesis, through massive gene innovation and diversification of protein domain architectures. This resulted in a LECA with essentially all of the Ub-related genes, including the SUMO and Ufm1 Ub-like systems. Ub and SUMO signaling further expanded during eukaryotic evolution, especially labeling and delabeling enzymes responsible for substrate selection. Additionally, we analyzed protein domain architecture evolution and found that multicellular lineages have the most complex Ub systems in terms of domain architectures. Together, we demonstrate that the Ub system predates the origin of eukaryotes and that a burst of innovation during eukaryogenesis led to a LECA with complex posttranslational regulation.

The MPN domain of Caenorhabditis elegans UfSP modulates both substrate recognition and deufmylation activity.

Ubiquitin-fold modifier 1 (Ufm1) specific protease (UfSP) is a novel cysteine protease that activates Ufm1 from its precursor by processing the C-terminus to expose the conserved Gly necessary for substrate conjugation and de-conjugates Ufm1 from the substrate. There are two forms: UfSP1 and UfSP2, the later with an additional domain at the N-terminus. Ufm1 and both the conjugating and deconjugating enzymes are highly conserved. However, in Caenorhabditis elegans there is one UfSP which has extra 136 residues at the N terminus compared to UfSP2. The crystal structure of cUfSP reveals that these additional residues display a MPN fold while the rest of the structure mimics that of UfSP2. The MPN domain does not have the metalloprotease activity found in some MPN-domain containing protein, rather it is required for the recognition and deufmylation of the substrate of cUfSP, UfBP1. In addition, the MPN domain is also required for localization to the endoplasmic reticulum.