PIAS proteins promote SUMO-1 conjugation to STAT1

A UL26-PIAS1 complex antagonizes anti-viral gene expression during Human Cytomegalovirus infection

Viral disruption of innate immune signaling is a critical determinant of productive infection. The Human Cytomegalovirus (HCMV) UL26 protein prevents anti-viral gene expression during infection, yet the mechanisms involved are unclear. We used TurboID-driven proximity proteomics to identify putative UL26 interacting proteins during infection to address this issue. We find that UL26 forms a complex with several immuno-regulatory proteins, including several STAT family members and various PIAS proteins, a family of E3 SUMO ligases. Our results indicate that UL26 prevents STAT phosphorylation during infection and antagonizes transcriptional activation induced by either interferon α (IFNA) or tumor necrosis factor α (TNFα). Additionally, we find that the inactivation of PIAS1 sensitizes cells to inflammatory stimulation, resulting in an anti-viral transcriptional environment similar to ΔUL26 infection. Further, PIAS1 is important for HCMV cell-to-cell spread, which depends on the presence of UL26, suggesting that the UL26-PIAS1 interaction is vital for modulating intrinsic anti-viral defense.

PAD4 controls tumor immunity via restraining the MHC class II machinery in macrophages

Tumor-associated macrophages (TAMs) shape tumor immunity and therapeutic efficacy. However, it is poorly understood whether and how post-translational modifications (PTMs) intrinsically affect the phenotype and function of TAMs. Here, we reveal that peptidylarginine deiminase 4 (PAD4) exhibits the highest expression among common PTM enzymes in TAMs and negatively correlates with the clinical response to immune checkpoint blockade. Genetic and pharmacological inhibition of PAD4 in macrophages prevents tumor progression in tumor-bearing mouse models, accompanied by an increase in macrophage major histocompatibility complex (MHC) class II expression and T cell effector function. Mechanistically, PAD4 citrullinates STAT1 at arginine 121, thereby promoting the interaction between STAT1 and protein inhibitor of activated STAT1 (PIAS1), and the loss of PAD4 abolishes this interaction, ablating the inhibitory role of PIAS1 in the expression of MHC class II machinery in macrophages and enhancing T cell activation. Thus, the PAD4-STAT1-PIAS1 axis is an immune restriction mechanism in macrophages and may serve as a cancer immunotherapy target.

RanBP2/Nup358 Mediates Sumoylation of STAT1 and Antagonizes Interferon-α-Mediated Antiviral Innate Immunity

Type I interferon (IFN-I)-induced signaling plays a critical role in host antiviral innate immune responses. Despite this, the mechanisms that regulate this signaling pathway have yet to be fully elucidated. The nucleoporin Ran Binding Protein 2 (RanBP2) (also known as Nucleoporin 358 KDa, Nup358) has been implicated in a number of cellular processes, including host innate immune signaling pathways, and is known to influence viral infection. In this study, we documented that RanBP2 mediates the sumoylation of signal transducers and activators of transcription 1 (STAT1) and inhibits IFN-α-induced signaling. Specifically, we found that RanBP2-mediated sumoylation inhibits the interaction of STAT1 and Janus kinase 1 (JAK1), as well as the phosphorylation and nuclear accumulation of STAT1 after IFN-α stimulation, thereby antagonizing the IFN-α-mediated antiviral innate immune signaling pathway and promoting viral infection. Our findings not only provide insights into a novel function of RanBP2 in antiviral innate immunity but may also contribute to the development of new antiviral therapeutic strategies.

N-MYC-interacting protein enhances type II interferon signaling by inhibiting STAT1 sumoylation

Signaling desensitization is key to limiting signal transduction duration and intensity. Signal transducer and activator of transcription 1 (STAT1) can mediate type II interferon (IFNγ)-induced immune responses, which are enhanced and inhibited by STAT1 phosphorylation and sumoylation, respectively. Here, we identified an N-MYC interacting protein, NMI, which can enhance STAT1 phosphorylation and STAT1-mediated IFNγ immune responses by binding and sequestering the E2 SUMO conjugation enzyme, UBC9, and blocking STAT1 sumoylation. NMI facilitates UBC9 nucleus-to-cytoplasm translocation in response to IFNγ, thereby inhibiting STAT1 sumoylation. STAT1 phosphorylation at Y701 and sumoylation at K703 are mutually exclusive modifications that regulate IFNγ-dependent transcriptional responses. NMI could not alter the phosphorylation level of sumoylation-deficient STAT1 after IFNγ treatment. Thus, IFNγ signaling is modulated by NMI through sequestration of UBC9 in the cytoplasm, leading to inhibition of STAT1 sumoylation. Hence, NMI functions as a switch for STAT1 activation/inactivation cycles by modulating an IFNγ-induced desensitization mechanism.

SUMOylation of Rho-associated protein kinase 2 induces goblet cell metaplasia in allergic airways

Allergic asthma is characterized by goblet cell metaplasia and subsequent mucus hypersecretion that contribute to the morbidity and mortality of this disease. Here, we explore the potential role and underlying mechanism of protein SUMOylation-mediated goblet cell metaplasia. The components of SUMOylaion machinery are specifically expressed in healthy human bronchial epithelia and robustly upregulated in bronchial epithelia of patients or mouse models with allergic asthma. Intratracheal suppression of SUMOylation by 2-D08 robustly attenuates not only allergen-induced airway inflammation, goblet cell metaplasia, and hyperreactivity, but IL-13-induced goblet cell metaplasia. Phosphoproteomics and biochemical analyses reveal SUMOylation on K1007 activates ROCK2, a master regulator of goblet cell metaplasia, by facilitating its binding to and activation by RhoA, and an E3 ligase PIAS1 is responsible for SUMOylation on K1007. As a result, knockdown of PIAS1 in bronchial epithelia inactivates ROCK2 to attenuate IL-13-induced goblet cell metaplasia, and bronchial epithelial knock-in of ROCK2(K1007R) consistently inactivates ROCK2 to alleviate not only allergen-induced airway inflammation, goblet cell metaplasia, and hyperreactivity, but IL-13-induced goblet cell metaplasia. Together, SUMOylation-mediated ROCK2 activation is an integral component of Rho/ROCK signaling in regulating the pathological conditions of asthma and thus SUMOylation is an additional target for the therapeutic intervention of this disease.

UBC9 deficiency enhances immunostimulatory macrophage activation and subsequent antitumor T cell response in prostate cancer

The role of tumor-associated macrophages (TAMs), along with the regulatory mechanisms underlying distinct macrophage activation states, remains poorly understood in prostate cancer (PCa). Herein, we report that PCa growth in mice with macrophage-specific Ubc9 deficiency is substantially suppressed compared with that in wild-type littermates, an effect partially ascribed to the augmented CD8+ T cell response. Biochemical and molecular analyses revealed that signal transducer and activator of transcription 4 (STAT4) is a crucial UBC9-mediated SUMOylation target, with lysine residue 350 (K350) as the major modification site. Site-directed mutation of STAT4 (K350R) enhanced its nuclear translocation and stability, thereby facilitating the proinflammatory activation of macrophages. Importantly, administration of the UBC9 inhibitor 2-D08 promoted the antitumor effect of TAMs and increased the expression of PD-1 on CD8+ T cells, supporting a synergistic antitumor efficacy once it combined with the immune checkpoint blockade therapy. Together, our results demonstrate that ablation of UBC9 could reverse the immunosuppressive phenotype of TAMs by promoting STAT4-mediated macrophage activation and macrophage-CD8+ T cell crosstalk, which provides valuable insights to halt the pathogenic process of tumorigenesis.

Host SUMOylation Pathway Negatively Regulates Protective Immune Responses and Promotes Leishmania donovani Survival

SUMOylation is one of the post-translational modifications that have recently been described as a key regulator of various cellular, nuclear, metabolic, and immunological processes. The process of SUMOylation involves the modification of one or more lysine residues of target proteins by conjugation of a ubiquitin-like, small polypeptide known as SUMO for their degradation, stability, transcriptional regulation, cellular localization, and transport. Herein, for the first time, we report the involvement of the host SUMOylation pathway in the process of infection of Leishmania donovani, a causative agent of visceral leishmaniasis. Our data revealed that infection of L. donovani to the host macrophages leads to upregulation of SUMOylation pathway genes and downregulation of a deSUMOylating gene, SENP1. Further, to confirm the effect of the host SUMOylation on the growth of Leishmania, the genes associated with the SUMOylation pathway were silenced and parasite load was analyzed. The knockdown of the SUMOylation pathway led to a reduction in parasitic load, suggesting the role of the host SUMOylation pathway in the disease progression and parasite survival. Owing to the effect of the SUMOylation pathway in autophagy, we further investigated the status of host autophagy to gain mechanistic insights into how SUMOylation mediates the regulation of growth of L. donovani. Knockdown of genes of host SUMOylation pathway led to the reduction of the expression levels of host autophagy markers while promoting autophagosome–lysosome fusion, suggesting SUMOylation-mediated autophagy in terms of autophagy initiation and autophagy maturation during parasite survival. The levels of reactive oxygen species (ROS) generation, nitric oxide (NO) production, and pro-inflammatory cytokines were also elevated upon the knockdown of genes of the host SUMOylation pathway during L. donovani infection. This indicates the involvement of the SUMOylation pathway in the modulation of protective immune responses and thus favoring parasite survival. Taken together, the results of this study indicate the hijacking of the host SUMOylation pathway by L. donovani toward the suppression of host immune responses and facilitation of host autophagy to potentially facilitate its survival. Targeting of SUMOylation pathway can provide a starting point for the design and development of novel therapeutic interventions to combat leishmaniasis.

Proteomimetics of Natural Regulators of JAK-STAT Pathway: Novel Therapeutic Perspectives

The JAK-STAT pathway is a crucial cellular signaling cascade, including an intricate network of Protein-protein interactions (PPIs) responsible for its regulation. It mediates the activities of several cytokines, interferons, and growth factors and transduces extracellular signals into transcriptional programs to regulate cell growth and differentiation. It is essential for the development and function of both innate and adaptive immunities, and its aberrant deregulation was highlighted in neuroinflammatory diseases and in crucial mechanisms for tumor cell recognition and tumor-induced immune escape. For its involvement in a multitude of biological processes, it can be considered a valuable target for the development of drugs even if a specific focus on possible side effects associated with its inhibition is required. Herein, we review the possibilities to target JAK-STAT by focusing on its natural inhibitors as the suppressor of cytokine signaling (SOCS) proteins. This protein family is a crucial checkpoint inhibitor in immune homeostasis and a valuable target in immunotherapeutic approaches to cancer and immune deficiency disorders.

The JAK/STAT signaling pathway: from bench to clinic

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway was discovered more than a quarter-century ago. As a fulcrum of many vital cellular processes, the JAK/STAT pathway constitutes a rapid membrane-to-nucleus signaling module and induces the expression of various critical mediators of cancer and inflammation. Growing evidence suggests that dysregulation of the JAK/STAT pathway is associated with various cancers and autoimmune diseases. In this review, we discuss the current knowledge about the composition, activation, and regulation of the JAK/STAT pathway. Moreover, we highlight the role of the JAK/STAT pathway and its inhibitors in various diseases.

Chronological Gene Expression of Human Gingival Fibroblasts with Low Reactive Level Laser (LLL) Irradiation

Though previously studies have reported that Low reactive Level Laser Therapy (LLLT) promotes wound healing, molecular level evidence was uncleared. The purpose of this study is to examine the temporal molecular processes of human immortalized gingival fibroblasts (HGF) by LLLT by the comprehensive analysis of gene expression. HGF was seeded, cultured for 24 h, and then irradiated with a Nd: YAG laser at 0.5 W for 30 s. After that, gene differential expression analysis and functional analysis were performed with DNA microarray at 1, 3, 6 and 12 h after the irradiation. The number of genes with up- and downregulated differentially expression genes (DEGs) compared to the nonirradiated group was large at 6 and 12 h after the irradiation. From the functional analysis results of DEGs, Biological Process (BP) based Gene Ontology (GO), BP ‘the defense response’ is considered to be an important process with DAVID. Additionally, the results of PPI analysis of DEGs involved in the defense response with STRING, we found that the upregulated DEGs such as CXCL8 and NFKB1, and the downregulated DEGs such as NFKBIA and STAT1 were correlated with multiple genes. We estimate that these genes are key genes on the defense response after LLLT.

The Function of SUMOylation and Its Role in the Development of Cancer Cells under Stress Conditions: A Systematic Review

Malignant tumors still pose serious threats to human health due to their high morbidity and mortality. Recurrence and metastasis are the most important factors affecting patient prognosis. Chemotherapeutic drugs and radiation used to treat these tumors mainly interfere with tumor metabolism, destroy DNA integrity, and inhibit protein synthesis. The upregulation of small ubiquitin-like modifier (SUMO) is a prevalent posttranslational modification (PTM) in various cancers and plays a critical role in tumor development. The dysregulation of SUMOylation can protect cancer cells from stresses exerted by external or internal stimuli. SUMOylation is a dynamic process finely regulated by SUMOylation enzymes and proteases to maintain a balance between SUMOylation and deSUMOylation. An increasing number of studies have reported that SUMOylation imbalance may contribute to cancer development, including metastasis, angiogenesis, invasion, and proliferation. High level of SUMOylation is required for cancer cells to survive internal or external stresses. Downregulation of SUMOylation may inhibit the development of cancer, making it an important potential clinical therapeutic target. Some studies have already begun to treat tumors by inhibiting the expression of SUMOylation family members, including SUMO E1 or E2. The tumor cells become more aggressive under internal and external stresses. The prevention of tumor development, metastasis, recurrence, and radiochemotherapy resistance by attenuating SUMOylation requires further exploration. This review focused on SUMOylation in tumor cells to discuss its effects on tumor suppressor proteins and oncoproteins as well as classical tumor pathways to identify new insights for cancer clinical therapy.

UBC-9 Acts in GABA Neurons to Control Neuromuscular Signaling in C. elegans

Regulation of excitatory to inhibitory signaling balance is essential to nervous system health and is maintained by numerous enzyme systems that modulate the activity, localization, and abundance of synaptic proteins. SUMOylation is a key post-translational regulator of protein function in diverse cells, including neurons. There, its role in regulating synaptic transmission through pre- and postsynaptic effects has been shown primarily at glutamatergic central nervous system synapses, where the sole SUMO-conjugating enzyme Ubc9 is a critical player. However, whether Ubc9 functions globally at other synapses, including inhibitory synapses, has not been explored. Here, we investigated the role of UBC-9 and the SUMOylation pathway in controlling the balance of excitatory cholinergic and inhibitory GABAergic signaling required for muscle contraction in Caenorhabditis elegans. We found inhibition or overexpression of UBC-9 in neurons modestly increased muscle excitation. Similar and even stronger phenotypes were seen with UBC-9 overexpression specifically in GABAergic neurons, but not in cholinergic neurons. These effects correlated with accumulation of synaptic vesicle-associated proteins at GABAergic presynapses, where UBC-9 and the C. elegans SUMO ortholog SMO-1 localized, and with defects in GABA-dependent behaviors. Experiments involving expression of catalytically inactive UBC-9 [UBC-9(C93S)], as well as co-expression of UBC-9 and SMO-1, suggested wild type UBC-9 overexpressed alone may act via substrate sequestration in the absence of sufficient free SUMO, underscoring the importance of tightly regulated SUMO enzyme function. Similar effects on muscle excitation, GABAergic signaling, and synaptic vesicle localization occurred with overexpression of the SUMO activating enzyme subunit AOS-1. Together, these data support a model in which UBC-9 and the SUMOylation system act at presynaptic sites in inhibitory motor neurons to control synaptic signaling balance in C. elegans. Future studies will be important to define UBC-9 targets at this synapse, as well as mechanisms by which UBC-9 and the SUMO pathway are regulated.

Interferon Response in Hepatitis C Virus-Infected Hepatocytes: Issues to Consider in the Era of Direct-Acting Antivirals

When interferons (IFNs) bind to their receptors, they upregulate numerous IFN-stimulated genes (ISGs) with antiviral and immune regulatory activities. Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus that affects over 71 million people in the global population. Hepatocytes infected with HCV produce types I and III IFNs. These endogenous IFNs upregulate a set of ISGs that negatively impact the outcome of pegylated IFN-α and ribavirin treatments, which were previously used to treat HCV. In addition, the IFNL4 genotype was the primary polymorphism responsible for a suboptimal treatment response to pegylated IFN-α and ribavirin. However, recently developed direct-acting antivirals have demonstrated a high rate of sustained virological response without pegylated IFN-α. Herein, we review recent studies on types I and III IFN responses to in HCV-infected hepatocytes. In particular, we focused on open issues related to IFN responses in the direct-acting antiviral era.

Chordate PIAS proteins act as conserved repressors of the TRAF6 self-polyubiquitination

In mammals, PIAS proteins are important SUMO E3 ligases and act as versatile regulators of over sixty different proteins, including components from the NF-κB pathways. But the PIAS functions are not well-understood due to complicated molecular mechanisms and multiple gene paralogs with overlapping roles, which is especially true in lower vertebrates where dedicated studies are scarce. As a basal chordate with a single PIAS gene, amphioxus is a convenient model to study PIAS from the evolutionary perspective. TRAF6 is a critical adaptor of the NF-κB pathways but it is not known whether TRAF6 is regulated by PIAS. Here we discover that in mammalian cells, amphioxus PIAS inhibited NF-κB activation by co-localizing and binding with TRAF6. The interaction relied on the N-terminal SAP and PINIT domains of PIAS. TRAF6 is an E3 ubiquitin ligase, which initiates downstream NF-κB signaling by promoting its self-ubiquitination. Both amphioxus SUMO1 and Ubc9 (SUMO E2 ligase) could suppress TRAF6 self-ubiquitination and NF-κB activation, suggesting that the SUMOylation activity competed away the ubiquitination activity of TRAF6. However, we show that the wild-type PIAS and the mutant PIAS without SUMO E3 ligase activity both could inhibit TRAF6-mediated NF-κB activation by reducing TRAF6 self-ubiquitination. This implies that SUMO ligase activity is not the only mechanism for PIAS to negatively regulate TRAF6. Finally, we tested the interactions between human PIAS1-4 and TRAF6. It reveals that human PIAS1, 3 and 4, but not 2, were able to repress NF-κB activation by reducing TRAF6 self-ubiquitination. Taken together, our study discovers a conserved regulatory interaction between chordate PIAS and TRAF6. It therefore sheds light on the complicated role of PIAS in immune regulation, and may help to understand the PIAS functions in other lower chordate taxa, such as jawless and jawed fishes.

The role of JAK/STAT signaling pathway and its inhibitors in diseases

The JAK/STAT signaling pathway is an universally expressed intracellular signal transduction pathway and involved in many crucial biological processes, including cell proliferation, differentiation, apoptosis, and immune regulation. It provides a direct mechanism for extracellular factors-regulated gene expression. Current researches on this pathway have been focusing on the inflammatory and neoplastic diseases and related drug. The mechanism of JAK/STAT signaling is relatively simple. However, the biological consequences of the pathway are complicated due to its crosstalk with other signaling pathways. In addition, there is increasing evidence indicates that the persistent activation of JAK/STAT signaling pathway is closely related to many immune and inflammatory diseases, yet the specific mechanism remains unclear. Therefore, it is necessary to study the detailed mechanisms of JAK/STAT signaling in disease formation to provide critical reference for clinical treatments of the diseases. In this review, we focus on the structure of JAKs and STATs, the JAK/STAT signaling pathway and its negative regulators, the associated diseases, and the JAK inhibitors for the clinical therapy.

Novel Gain-of-Function Mutation in Stat1 Sumoylation Site Leads to CMC/CID Phenotype Responsive to Ruxolitinib

Mutations in the coiled-coil and DNA-binding domains of STAT1 lead to delayed STAT1 dephosphorylation and subsequently gain-of-function. The associated clinical phenotype is broad and can include chronic mucocutaneous candidiasis (CMC) and/or combined immunodeficiency (CID). We report a case of CMC/CID in a 10-year-old boy due to a novel mutation in the small ubiquitin molecule (SUMO) consensus site at the C-terminal region of STAT1 leading to gain-of-function by impaired sumoylation. Immunodysregulatory features of disease improved after Janus kinase inhibitor (jakinib) treatment. Functional testing after treatment confirmed reversal of the STAT1 hyper-phosphorylation and downstream transcriptional activity. IL-17 and IL-22 production was, however, not restored with jakinib therapy (ruxolitinib), and the patient remained susceptible to opportunistic infection. In conclusion, a mutation in the SUMO consensus site of STAT1 can lead to gain-of-function that is reversible with jakinib treatment. However, full immunocompetence was not restored, suggesting that this treatment strategy might serve well as a bridge to definitive therapy such as hematopoietic stem cell transplant rather than a long-term treatment option.

Post-Receptor Inhibitors of the GHR-JAK2-STAT Pathway in the Growth Hormone Signal Transduction

The growth hormone (GH) plays a key role in the regulation of metabolic processes in an organism. Determination of the correct structure and functioning of the growth hormone receptor (GHR) allowed for a more detailed research of its post-receptor regulators, which substantially influences its signal transduction. This review is focused on the description of the post-receptor inhibitors of the GHR-JAK2-STAT pathway, which is one of the most important pathways in the transduction of the somatotropic axis signal. The aim of this review is the short characterization of the main post-receptor inhibitors, such as: cytokine-inducible SH2-containing protein (CIS), Suppressors of Cytokine Signaling (SOCS) 1, 2 and 3, sirtuin 1 (SIRT1), protein inhibitors of activated STAT (PIAS) 1, 3 and PIAS4, protein tyrosine phosphatases (PTP) 1B and H1, Src homology 2 (SH2) domain containing protein tyrosine phosphatase (SHP) 1, 2 and signal regulatory protein (SIRP) α1. The equilibrium between these regulators activity and inhibition is of special concern because, as many studies showed, even slight imbalance may disrupt the GH activity causing serious diseases. The regulation of the described inhibitors expression and activity may be a point of interest for pharmaceutical industry.

Ubc9 overexpression and SUMO1 deficiency blunt inflammation after intestinal ischemia/reperfusion

The intestinal epithelium constitutes a crucial defense to the potentially life-threatening effects of gut microbiota. However, due to a complex underlying vasculature, hypoperfusion and resultant tissue ischemia pose a particular risk to function and integrity of the epithelium. The small ubiquitin-like modifier (SUMO) conjugation pathway critically regulates adaptive responses to metabolic stress and is of particular significance in the gut, as inducible knockout of the SUMO-conjugating enzyme Ubc9 results in rapid intestinal epithelial disintegration. Here we analyzed the pattern of individual SUMO isoforms in intestinal epithelium and investigated their roles in intestinal ischemia/reperfusion (I/R) damage. Immunostaining revealed that epithelial SUMO2/3 expression was almost exclusively limited to crypt epithelial nuclei in unchallenged mice. However, intestinal I/R or overexpression of Ubc9 caused a remarkable enhancement of epithelial SUMO2/3 staining along the crypt-villus axis. Unexpectedly, a similar pattern was found in SUMO1 knockout mice. Ubc9 transgenic mice, but also SUMO1 knockout mice were protected from I/R injury as evidenced by better preserved barrier function and blunted inflammatory responses. PCR array analysis of microdissected villus-tip epithelia revealed a specific epithelial contribution to reduced inflammatory responses in Ubc9 transgenic mice, as key chemotactic signaling molecules such as IL17A were significantly downregulated. Together, our data indicate a critical role particularly of the SUMO2/3 isoforms in modulating responses to I/R and provide the first evidence that SUMO1 deletion activates a compensatory process that protects from ischemic damage.

Novel signal transducer and activator of transcription 1 mutation disrupts small ubiquitin-related modifier conjugation causing gain of function

BACKGROUND

Sumoylation is a posttranslational reversible modification of cellular proteins through the conjugation of small ubiquitin-related modifier (SUMO) and comprises an important regulator of protein function.

OBJECTIVE

We sought to characterize the molecular mechanism of a novel mutation at the SUMO motif on signal transducer and activator of transcription 1 (STAT1).

METHODS

STAT1 sequencing and functional characterization were performed in transfection experiments by using immunoblotting and immunoprecipitation in STAT1-deficient cell lines. Transcriptional response and target gene activation were also investigated in PBMCs.

RESULTS

We identified a novel STAT1 mutation (c.2114A>T, p.E705V) within the SUMO motif (702IKTE705) in a patient with disseminated Rhodococcus species infection, Norwegian scabies, chronic mucocutaneous candidiasis, hypothyroidism, and esophageal squamous cell carcinoma. The mutation is located in the tail segment and is predicted to disrupt STAT1 sumoylation. Immunoprecipitation experiments performed in transfected cells confirmed absent STAT1 sumoylation for E705V, whereas it was present in wild-type (WT) STAT1 cells, as well as the loss-of-function mutants L706S and Y701C. Furthermore, stimulation with IFN-γ led to enhanced STAT1 phosphorylation, enhanced transcriptional activity, and target gene expression in the E705V-transfected compared with WT-transfected cells. Computer modeling of WT and mutant STAT1 molecules showed variations in the accessibility of the phosphorylation site Y701, which corresponded to the loss-of-function and gain-of-function variants.

CONCLUSION

This is the first report of a mutation in the STAT1 sumoylation motif associated with clinical disease. These data reinforce sumoylation as a key posttranslational regulatory modification of STAT1 and identify a novel mechanism for gain-of-function STAT1 disease in human subjects.

Interferon Independent Non-Canonical STAT Activation and Virus Induced Inflammation

Interferons (IFNs) are a group of secreted proteins that play critical roles in antiviral immunity, antitumor activity, activation of cytotoxic T cells, and modulation of host immune responses. IFNs are cytokines, and bind receptors on cell surfaces to trigger signal transduction. The major signaling pathway activated by IFNs is the JAK/STAT (Janus kinase/signal transducer and activator of transcription) pathway, a complex pathway involved in both viral and host survival strategies. On the one hand, viruses have evolved strategies to escape from antiviral host defenses evoked by IFN-activated JAK/STAT signaling. On the other hand, viruses have also evolved to exploit the JAK/STAT pathway to evoke activation of certain STATs that somehow promote viral pathogenesis. In this review, recent progress in our understanding of the virus-induced IFN-independent STAT signaling and its potential roles in viral induced inflammation and pathogenesis are summarized in detail, and perspectives are provided.

Differential effects of SUMO1 and SUMO3 on PKR activation and stability

Double-stranded RNA (dsRNA)-dependent protein kinase (PKR) is a serine/threonine kinase that exerts its own phosphorylation and the phosphorylation of the α subunit of the protein synthesis initiation factor eIF-2α. PKR was identified as a target of SUMOylation and the triple PKR-SUMO deficient mutant on Lysine residues K60-K150-K440 has reduced PKR activity. We report that SUMO1 and SUMO3 expression exert differential effects on PKR localization, activation and stability. SUMO1 or SUMO3 did not alter the repartition of PKR in the cytoplasm and the nucleus. However, in SUMO3-expressing cells PKR was found more concentrated around the perinuclear membrane and was recruited from small speckles to nuclear dots. Interestingly, SUMO1 expression alone resulted in PKR and eIF-2α activation, whereas SUMO3 reduced PKR and eIF-2α activation upon viral infection or dsRNA transfection. In addition, encephalomyocarditis virus (EMCV) enhanced PKR conjugation to SUMO1 and SUMO3 but only SUMO3 expression promoted caspase-dependent EMCV-induced PKR degradation. Furthermore, the higher EMCV-induced PKR activation by SUMO1 was correlated with an inhibition of EMCV. Importantly SUMO1, by inducing PKR activation in the absence of viral infection, and SUMO3, by counteracting both PKR activation and stability upon viral infection, shed a new light on the differential effects of SUMO-modified PKR.

Responses to Cytokines and Interferons that Depend upon JAKs and STATs

Many cytokines and all interferons activate members of a small family of kinases (the Janus kinases [JAKs]) and a slightly larger family of transcription factors (the signal transducers and activators of transcription [STATs]), which are essential components of pathways that induce the expression of specific sets of genes in susceptible cells. JAK-STAT pathways are required for many innate and acquired immune responses, and the activities of these pathways must be finely regulated to avoid major immune dysfunctions. Regulation is achieved through mechanisms that include the activation or induction of potent negative regulatory proteins, posttranslational modification of the STATs, and other modulatory effects that are cell-type specific. Mutations of JAKs and STATs can result in gains or losses of function and can predispose affected individuals to autoimmune disease, susceptibility to a variety of infections, or cancer. Here we review recent developments in the biochemistry, genetics, and biology of JAKs and STATs.

Dual regulation of Stat1 and Stat3 by the tumor suppressor protein PML contributes to interferon α-mediated inhibition of angiogenesis

IFNs are effective in inhibiting angiogenesis in preclinical models and in treating several angioproliferative disorders. However, the detailed mechanisms of IFNα-mediated anti-angiogenesis are not completely understood. Stat1/2/3 and PML are IFNα downstream effectors and are pivotal regulators of angiogenesis. Here, we investigated PML's role in the regulation of Stat1/2/3 activity. In Pml knock-out (KO) mice, ablation of Pml largely reduces IFNα angiostatic ability in Matrigel plug assays. This suggested an essential role for PML in IFNα's anti-angiogenic function. We also demonstrated that PML shared a large cohort of regulatory genes with Stat1 and Stat3, indicating an important role of PML in regulating Stat1 and Stat3 activity. Using molecular tools and primary endothelial cells, we demonstrated that PML positively regulates Stat1 and Stat2 isgylation, a ubiquitination-like protein modification. Accordingly, manipulation of the isgylation system by knocking down USP18 altered IFNα-PML axis-mediated inhibition of endothelial cell migration and network formation. Furthermore, PML promotes turnover of nuclear Stat3, and knockdown of PML mitigates the effect of LLL12, a selective Stat3 inhibitor, on IFNα-mediated anti-angiogenic activity. Taken together, we elucidated an unappreciated mechanism in which PML, an IFNα-inducible effector, possess potent angiostatic activity, doing so in part by forming a positive feedforward loop with Stat1/2 and a negative feedback loop with Stat3. The interplay between PML, Stat1/Stat2, and Stat3 contributes to IFNα-mediated inhibition of angiogenesis, and disruption of this network results in aberrant IFNα signaling and altered angiostatic activity.

Canonical and Non-Canonical Aspects of JAK-STAT Signaling: Lessons from Interferons for Cytokine Responses

Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signal transduction mediates cytokine responses. Canonical signaling is based on STAT tyrosine phosphorylation by activated JAKs. Downstream of interferon (IFN) receptors, activated JAKs cause the formation of the transcription factors IFN-stimulated gene factor 3 (ISGF3), a heterotrimer of STAT1, STAT2 and interferon regulatory factor 9 (IRF9) subunits, and gamma interferon-activated factor (GAF), a STAT1 homodimer. In recent years, several deviations from this paradigm were reported. These include kinase-independent JAK functions as well as extra- and intranuclear activities of U-STATs without phosphotyrosines. Additionally, transcriptional control by STAT complexes resembling neither GAF nor ISGF3 contributes to transcriptome changes in IFN-treated cells. Our review summarizes the contribution of non-canonical JAK-STAT signaling to the innate antimicrobial immunity imparted by IFN. Moreover, we touch upon functions of IFN pathway proteins beyond the IFN response. These include metabolic functions of IRF9 as well as the regulation of natural killer cell activity by kinase-dead TYK2 and different phosphorylation isoforms of STAT1.

Introduction to Sumoylation

Reversible post-translational modification is a rapid and efficient system to control the activity of pre-existing proteins. Modifiers range from small chemical moieties, such as phosphate groups, to proteins themselves as the modifier. The patriarch of the protein modifiers is ubiquitin which plays a central role in protein degradation and protein targeting. Over the last 20 years, the ubiquitin family has expanded to include a variety of ubiquitin-related small modifier proteins that are all covalently attached to a lysine residue on target proteins via series of enzymatic reactions. Of these more recently discovered ubiquitin-like proteins, the SUMO family has gained prominence as a major regulatory component that impacts numerous aspects of cell growth, differentiation, and response to stress. Unlike ubiquitinylation which often leads to proteins turn over, sumoylation performs a variety of function such as altering protein stability, modulating protein trafficking, directing protein-protein interactions, and regulating protein activity. This chapter will introduce the basic properties of SUMO proteins and the general tenets of sumoylation.

The implication of SUMO in intrinsic and innate immunity

Since its discovery, SUMOylation has emerged as a key post-translational modification involved in the regulation of host-virus interactions. SUMOylation has been associated with the replication of a large number of viruses, either through the direct modification of viral proteins or through the modulation of cellular proteins implicated in antiviral defense. SUMO can affect protein function via covalent or non-covalent binding. There is growing evidence that SUMO regulates several host proteins involved in intrinsic and innate immunity, thereby contributing to the process governing interferon production during viral infection; as well as the interferon-activated Jak/STAT pathway. Unlike the interferon-mediated innate immune response, intrinsic antiviral resistance is mediated by constitutively expressed antiviral proteins (defined as restriction factors), which confer direct viral resistance through a variety of mechanisms. The aim of this review is to evaluate the role of SUMO in intrinsic and innate immunity; highlighting the involvement of the TRIM family proteins, with a specific focus on the mechanism through which SUMO affects i- interferon production upon viral infection, ii-interferon Jak/STAT signaling and biological responses, iii-the relationship between restriction factors and RNA viruses.

Nuclear unphosphorylated STAT3 correlates with a worse prognosis in human glioblastoma

Glioblastoma (GBM) is currently the most aggressive form of brain tumor identified, and STAT3 is known to play an important role in gliomagenesis. Moreover, while several studies have used pharmacological approaches to modulate STAT3 activity, the results have been contradictory. In this study, expressions of STAT3, pSTAT3 (Y705), and pSTAT3 (S727) were evaluated using immunohistochemistry assays of tissue microarrays containing non-neoplastic tissue (NN, n=12), grade II astrocytomas (n=33), grade III astrocytomas (n=12), and GBM (n=85) specimens. In GBM specimens, STAT3 was overexpressed and exhibited greater nuclear localization compared with lower grade astrocytomas and NN. Conversely, nuclear localization of pSTAT3 (Y705) and pSTAT3 (S727) exhibited a similar phenotype in both GBMs and NNs. MET was also detected as a non-canonical pathway marker for STAT3. For tumors with higher levels of STAT3 nuclear localization, and not pSTAT3 (Y705) and pSTAT3 (S727), these specimens exhibited increased levels of MET expression. Thus, a non-canonical pathway may mediate a proportion of the STAT3 that translocates to the nucleus. Moreover, tumors which exhibited greater nuclear localization of STAT3 corresponded with patients that presented with lower rates of recurrence-free survival and overall survival. In contrast, the phosphorylated forms of STAT3 did not correlate with patient survival. These findings suggest that phosphorylation-independent mechanisms may mediate the nuclear translocation and activation of STAT3. Further studies are needed to identify the mechanisms involved, especially those that provide targets to achieve efficient inhibition and control of GBM progression.

HCV infection, IFN response and the coding and non-coding host cell genome

HCV is an ideal model to study how the infected cell is altered to allow the establishment of a chronic infection. After infection, the transcriptome of the cell changes in response to the virus or to the antiviral pathways induced by infection. The cell has evolved to sense HCV soon after infection and to activate antiviral pathways. In turn, HCV has evolved to block the antiviral pathways induced by the cell and, at the same time, to use some for its own benefit. In this review, we summarize the proviral and antiviral factors induced in HCV infected cells. These factors can be proteins and microRNAs, but also long noncoding RNAs (lncRNAs) that are induced by infection. Interestingly, several of the lncRNAs upregulated after HCV infection have oncogenic functions, suggesting that upregulation of lncRNAs could explain, at least in part, the increased rate of liver tumors observed in HCV-infected patients. Other lncRNAs induced by HCV infection may regulate the expression of coding genes required for replication or control genes involved in the cellular antiviral response. Given the evolutionary pressure imposed by viral infections and that lncRNAs are specially targeted by evolution, we believe that the study of proviral and antiviral lncRNAs may lead to unexpected discoveries that may have a strong impact on basic science and translational research.

Small Ubiquitin-like Modifier Alters IFN Response

IFNs orchestrate immune defense through induction of hundreds of genes. Small ubiquitin-like modifier (SUMO) is involved in various cellular functions, but little is known about its role in IFN responses. Prior work identified STAT1 SUMOylation as an important mode of regulation of IFN-γ signaling. In this study, we investigated the roles of SUMO in IFN signaling, gene expression, protein stability, and IFN-induced biological responses. We first show that SUMO overexpression leads to STAT1 SUMOylation and to a decrease in IFN-induced STAT1 phosphorylation. Interestingly, IFNs exert a negative retrocontrol on their own signaling by enhancing STAT1 SUMOylation. Furthermore, we show that expression of each SUMO paralog inhibits IFN-γ-induced transcription without affecting that of IFN-α. Further, we focused on IFN-induced gene products associated to promyelocytic leukemia (PML) nuclear bodies, and we show that neither IFN-α nor IFN-γ could increase PML and Sp100 protein expression because they enhanced their SUMO3 conjugation and subsequent proteasomal degradation. Because it is known that SUMO3 is important for the recruitment of RING finger protein 4, a poly-SUMO-dependent E3 ubiquitin ligase, and that PML acts as a positive regulator of IFN-induced STAT1 phosphorylation, we went on to show that RING finger protein 4 depletion stabilizes PML and is correlated with a positive regulation of IFN signaling. Importantly, inhibition of IFN signaling by SUMO is associated with a reduction of IFN-induced apoptosis, cell growth inhibition, antiviral defense, and chemotaxis. Conversely, inhibition of SUMOylation results in higher IFN-γ-induced STAT1 phosphorylation and biological responses. Altogether, our results uncover a new role for SUMO in the modulation of IFN response.

Age-Associated Failure To Adjust Type I IFN Receptor Signaling Thresholds after T Cell Activation

With increasing age, naive CD4 T cells acquire intrinsic defects that compromise their ability to respond and differentiate. Type I IFNs, pervasive constituents of the environment in which adaptive immune responses occur, are known to regulate T cell differentiation and survival. Activated naive CD4 T cells from older individuals have reduced responses to type I IFN, a defect that develops during activation and that is not observed in quiescent naive CD4 T cells. Naive CD4 T cells from young adults upregulate the expression of STAT1 and STAT5 after activation, lowering their threshold to respond to type I IFN stimulation. The heightened STAT signaling is critical to maintain the expression of CD69 that regulates lymphocyte egress and the ability to produce IL-2 and to survive. Although activation of T cells from older adults also induces transcription of STAT1 and STAT5, failure to exclude SHP-1 from the signaling complex blunts their type I IFN response. In summary, our data show that type I IFN signaling thresholds in naive CD4 T cells after activation are dynamically regulated to respond to environmental cues for clonal expansion and memory cell differentiation. Naive CD4 T cells from older adults have a defect in this threshold calibration. Restoring their ability to respond to type I IFN emerges as a promising target to restore T cell responses and to improve the induction of T cell memory.

SUMO2 overexpression enhances the generation and function of interleukin-17-producing CD8⁺ T cells in mice

Small ubiquitin-like modifier (SUMO) 2 is a small protein that controls the activity and stability of other proteins by SUMOylation. In this study, T cell-specific SUMO2 overexpressing transgenic mice were generated to study the effect of SUMO2 on T lymphocytes. SUMO2 overexpression promoted differentiation of interleukin (IL)-17-producing CD8(+) T cells, and significantly suppressed the growth of EL4 tumor cells in vivo. Moreover, the tumor tissue from SUMO2-overexpressing mice had higher interferon (IFN)-γ and granzyme B mRNA levels. Although SUMO2 overexpression did not increase IFN-γ or granzyme B production in cytotoxic T lymphocytes, IL-12 treatment restored and increased IFN-γ secretion in IL-17-producing CD8(+) T cells. SUMO2 overexpression also increased gene expression of chemokines, CCL4, and CXCL10, which attract cytotoxic T lymphocytes to tumor tissues. Additionally, SUMO2-overexpressing T cells exhibited increased STAT3 phosphorylation, implying a SUMO2 target which up-regulates STAT3 activity governing IL-17A-producing CD8(+) T cell differentiation and antitumor immune responses.

Interferons and viruses: an evolutionary arms race of molecular interactions

Over half a century has passed since interferons (IFNs) were discovered and shown to inhibit virus infection in cultured cells. Since then, researchers have steadily brought to light the molecular details of IFN signaling, catalogued their pleiotropic effects on cells, and harnessed their therapeutic potential for a variety of maladies. While advances have been plentiful, several fundamental questions have yet to be answered and much complexity remains to be unraveled. We explore the current knowledge surrounding four main questions: are type I IFN subtypes differentially produced in response to distinct pathogens? How are IFN subtypes distinguished by cells? What are the mechanisms and consequences of viral antagonism? Lastly, how can the IFN response be harnessed to improve vaccine efficacy?

Promoter occupancy of STAT1 in interferon responses is regulated by processive transcription

Interferons regulate immunity by inducing DNA binding of the transcription factor STAT1 through Y701 phosphorylation. Transcription by STAT1 needs to be restricted to minimize the adverse effects of prolonged immune responses. It remains unclear how STAT1 inactivation is regulated such that the transcription output is adequate. Here we show that efficient STAT1 inactivation in macrophages is coupled with processive transcription. Ongoing transcription feeds back to reduce the promoter occupancy of STAT1 and, consequently, the transcriptional output. Once released from the promoter, STAT1 is ultimately inactivated by Y701 dephosphorylation. We observe similar regulation for STAT2 and STAT3, suggesting a conserved inactivation mechanism among STATs. These findings reveal that STAT1 promoter occupancy in macrophages is regulated such that it decreases only after initiation of the transcription cycle. This feedback control ensures the fidelity of cytokine responses and provides options for pharmacological intervention.

Interferon-stimulated genes: a complex web of host defenses

Interferon-stimulated gene (ISG) products take on a number of diverse roles. Collectively, they are highly effective at resisting and controlling pathogens. In this review, we begin by introducing interferon (IFN) and the JAK-STAT signaling pathway to highlight features that impact ISG production. Next, we describe ways in which ISGs both enhance innate pathogen-sensing capabilities and negatively regulate signaling through the JAK-STAT pathway. Several ISGs that directly inhibit virus infection are described with an emphasis on those that impact early and late stages of the virus life cycle. Finally, we describe ongoing efforts to identify and characterize antiviral ISGs, and we provide a forward-looking perspective on the ISG landscape.

JAK-STAT signaling in stem cells

Adult stem cells are essential for the regeneration and repair of tissues in an organism. Signals from many different pathways converge to regulate stem cell maintenance and differentiation while preventing overproliferation. Although each population of adult stem cells is unique, common themes arise by comparing the regulation of various stem cell types in an organism or by comparing similar stem cell types across species. The JAK-STAT signaling pathway, identified nearly two decades ago, is now known to be involved in many biological processes including the regulation of stem cells. Studies in Drosophila first implicated JAK-STAT signaling in the control of stem cell maintenance in the male germline stem cell microenvironment, or niche; subsequently it has been shown play a role in other niches in both Drosophila and mammals. In this chapter, we will address the role of JAK-STAT signaling in stem cells in the germline, intestinal, hematopoietic and neuronal niches in Drosophila as well as the hematopoietic and neuronal niches in mammals. We will comment on how the study of JAK-STAT signaling in invertebrate systems has helped to advance our understanding of signaling in vertebrates. In addition to the role of JAK- STAT signaling in stem cell niche homeostasis, we will also discuss the diseases, including cancers, that can arise when this pathway is misregulated.

Structure-function analysis indicates that sumoylation modulates DNA-binding activity of STAT1

Background

STAT1 is an essential transcription factor for interferon-γ-mediated gene responses. A distinct sumoylation consensus site (ψKxE) ⁷⁰²IKTE⁷⁰⁵ is localized in the C-terminal region of STAT1, where Lys703 is a target for PIAS-induced SUMO modification. Several studies indicate that sumoylation has an inhibitory role on STAT1-mediated gene expression but the molecular mechanisms are not fully understood.

Results

Here, we have performed a structural and functional analysis of sumoylation in STAT1. We show that deconjugation of SUMO by SENP1 enhances the transcriptional activity of STAT1, confirming a negative regulatory effect of sumoylation on STAT1 activity. Inspection of molecular model indicated that consensus site is well exposed to SUMO-conjugation in STAT1 homodimer and that the conjugated SUMO moiety is directed towards DNA, thus able to form a sterical hindrance affecting promoter binding of dimeric STAT1. In addition, oligoprecipitation experiments indicated that sumoylation deficient STAT1 E705Q mutant has higher DNA-binding activity on STAT1 responsive gene promoters than wild-type STAT1. Furthermore, sumoylation deficient STAT1 E705Q mutant displayed enhanced histone H4 acetylation on interferon-γ-responsive promoter compared to wild-type STAT1.

Conclusions

Our results suggest that sumoylation participates in regulation of STAT1 responses by modulating DNA-binding properties of STAT1.

SUMO-specific protease 1 is critical for early lymphoid development through regulation of STAT5 activation

Small ubiquitin-like modifier (SUMO) modification has emerged as an important regulatory mechanism during embryonic development. However, it is not known whether SUMOylation plays a role in the development of the immune system. Here, we show that SUMO-specific protease 1 (SENP1) is essential for the development of early T and B cells. STAT5, a key regulator of lymphoid development, is modified by SUMO-2 and is specifically regulated by SENP1. In the absence of SENP1, SUMO-2 modified STAT5 accumulates in early lymphoid precursors, resulting in a block in its acetylation and subsequent signaling. These results demonstrate a crucial role of SENP1 in the regulation of STAT5 activation during early lymphoid development.

Biology and significance of the JAK/STAT signalling pathways

Since its discovery two decades ago, the activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway by numerous cytokines and growth factors has resulted in it becoming one of the most well-studied intracellular signalling networks. The field has progressed from the identification of the individual components to high-resolution crystal structures of both JAK and STAT, and an understanding of the complexities of the molecular activation and deactivation cycle which results in a diverse, yet highly specific and regulated pattern of transcriptional responses. While there is still more to learn, we now appreciate how disruption and deregulation of this pathway can result in clinical disease and look forward to adoption of the next generation of JAK inhibitors in routine clinical treatment.

Evidence for a functional link between Dd-STATa and Dd-PIAS, a Dictyostelium PIAS homologue

Several mammalian protein families inhibit the activity of signal transducer and activator of transcription (STAT) proteins. The protein inhibitor of activated STAT (PIAS) was initially identified through its ability to interact with human STAT proteins. We isolated a gene (pisA) encoding a Dictyostelium orthologue of PIAS, Dd-PIAS, which possesses almost all the representative motifs and domains of mammalian PIAS proteins. A Dd-PIAS null mutant strain displays a normal terminal morphology but with accelerated development once cells are aggregated. In contrast, Dd-PIAS overexpressor strains demonstrate delayed aggregation, almost no slug phototaxis, impaired slug motility, and a prolonged slug migration period. This strain is a near phenocopy of the Dd-STATa null mutant, although it eventually forms a fruiting body, albeit inefficiently. The expression of several Dd-STATa-activated genes is upregulated in the Dd-PIAS null mutant and there is ectopic expression of pstAB makers. The concentration of a PIAS-green fluorescent protein (GFP) fusion protein, expressed under the PIAS promoter, is greatest in the pstO cells and gradually decreases with proximity to the tip of the slug and culminant: a pattern diametrically opposite to that of Dd-STATa. Our results suggest a functional interrelationship between Dd-PIAS and Dd-STATa that influences gene expression and development.

SUMOylation in carcinogenesis

SUMOylation is a post-translational modification characterized by covalent and reversible binding of small ubiquitin-like modifier (SUMO) to a target protein. In mammals, four different isoforms, termed SUMO-1, -2, -3 and -4 have been identified so far. SUMO proteins are critically involved in the modulation of nuclear organization and cell viability. Their expression is significantly increased in processes associated with carcinogenesis such as cell growth, differentiation, senescence, oxidative stress and apoptosis. Little is known about the role of SUMOylation in cancer development. Therefore the present review focuses on possible implications of SUMOylation in carcinogenesis highlighting its impact as an important regulatory cell cycle protein. Moreover, novel opportunities for therapeutic approaches are discussed. The differential expression levels, the target protein preferences and the function of the SUMO pathway in different cancer subtypes raises unexpected issues questioning our understanding of the implication of SUMO in carcinogenesis.

Sumoylation of the P protein at K254 plays an important role in growth of parainfluenza virus 5

The P protein of parainfluenza virus 5 (PIV5) is an essential cofactor of the viral RNA-dependent RNA polymerase. Phosphorylation of the P protein can positively or negatively regulate viral gene expression, depending on the precise phosphorylation sites. Sumoylation, a process of adding small ubiquitin-like modifier (SUMO) to proteins posttranslationally, plays an important role in regulating protein function. In this study, we have found that the P protein of PIV5 was sumoylated with SUMO1 in both transfected and infected cells. The K254 residue of the P protein is within a consensus sumoylation motif. Mutation of the P protein at K254 to arginine (P-K254R) reduced PIV5 minigenome activity, as well as the sumoylation level of the P protein. Incorporation of K254R into a recombinant PIV5 (rPIV5-P-K254R) resulted in a virus that grew to a lower titer and had lower levels of viral RNA synthesis and protein expression than wild-type PIV5, suggesting that sumoylation of the P protein at K254 is important for PIV5 growth. Biochemical studies did not reveal any defect of P-K254R in its interactions with viral proteins NP and L or formation of homotetramers. We propose that sumoylation of the P protein at K254 regulates PIV5 gene expression through a host protein.