Protein SUMOylation and phase separation: partners in stress?

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Protein SUMOylation is a prevalent stress-response posttranslational modification crucial for maintaining cellular homeostasis. Herein, we report that protein SUMOylation modulates cellular signaling mediated by cAMP, an ancient and universal stress-response second messenger. We identify K561 as a primary SUMOylation site in exchange protein directly activated by cAMP (EPAC1) via site-specific mapping of SUMOylation using mass spectrometry. Sequence and site-directed mutagenesis analyses reveal that a functional SUMO-interacting motif in EPAC1 is required for the binding of SUMO-conjugating enzyme UBC9, formation of EPAC1 nuclear condensate, and EPAC1 cellular SUMOylation. Heat shock-induced SUMO modification of EPAC1 promotes Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

Predicting nuclear G-quadruplex RNA-binding proteins with roles in transcription and phase separation

RNA-binding proteins are central for many biological processes and their characterization has demonstrated a broad range of functions as well as a wide spectrum of target structures. RNA G-quadruplexes are important regulatory elements occurring in both coding and non-coding transcripts, yet our knowledge of their structure-based interactions is at present limited. Here, using theoretical predictions and experimental approaches, we show that many chromatin-binding proteins bind to RNA G-quadruplexes, and we classify them based on their RNA G-quadruplex-binding potential. Combining experimental identification of nuclear RNA G-quadruplex-binding proteins with computational approaches, we build a prediction tool that assigns probability score for a nuclear protein to bind RNA G-quadruplexes. We show that predicted G-quadruplex RNA-binding proteins exhibit a high degree of protein disorder and hydrophilicity and suggest involvement in both transcription and phase-separation into membrane-less organelles. Finally, we present the G4-Folded/UNfolded Nuclear Interaction Explorer System (G4-FUNNIES) for estimating RNA G4-binding propensities at http://service.tartaglialab.com/new_submission/G4FUNNIES .

Spray-type modifications: an emerging paradigm in post-translational modifications

A post-translational modification (PTM) occurs when a nucleophilic residue (e.g., lysine of a target protein) attacks electrophilic substrate molecules (e.g., acyl-AMP), involving writer enzymes or even occurring spontaneously. Traditionally, this phenomenon was thought to be sequence specific; however, recent research suggests that PTMs can also occur in a non-sequence-specific manner confined to a specific location in a cell. In this Opinion, we compile the accumulated evidence of spray-type PTMs and propose a mechanism for this phenomenon based on the exposure level of reactive electrophilic substrate molecules at the active site of the PTM writers. Overall, a spray-type PTM conceptual framework is useful for comprehending the promiscuous PTM writer events that cannot be adequately explained by the traditional concept of sequence-dependent PTM events.

Monoclonal antibody-based systematic identification of SUMO1-modification sites reveals TFII-I SUMOylation is involved in tumor growth

SUMOylation plays an essential role in diverse physiological and pathological processes. Identification of wild-type SUMO1-modification sites by mass spectrometry is still challenging. In this study, we produced a monoclonal SUMO1C-K antibody recognizing SUMOylated peptides and proposed an efficient streamline for identification of SUMOylation sites. We identified 471 SUMOylation sites in 325 proteins from five raw data. These identified sites exhibit a high positive rate when evaluated by mutation-verified SUMOylation sites. We identified many SUMOylated proteins involved in mitochondrial metabolism and non-membrane-bounded organelles formation. We proposed a SUMOylation motif, ΨKXD/EP, where proline is required for efficient SUMOylation. We further revealed SUMOylation of TFII-I was stimulated by growth signals and was required for nucleus-localization of p-ERK1/2. Mutation of SUMOylation sites of TFII-I suppressed tumor cell growth in vitro and in vivo. Taken together, we provided a strategy for personalized identification of wild-type SUMO1-modification sites and revealed the physiological significance of TFII-I SUMOylation in this study.

Protein SUMOylation promotes cAMP-independent EPAC1 activation

Exchange protein directly activated by cAMP (EPAC1) mediates the intracellular functions of a critical stress-response second messenger, cAMP. Herein, we report that EPAC1 is a cellular substrate of protein SUMOylation, a prevalent stress-response posttranslational modification. Site-specific mapping of SUMOylation by mass spectrometer leads to identifying K561 as a primary SUMOylation site in EPAC1. Sequence and site-directed mutagenesis analyses reveal a functional SUMO-interacting motif required for cellular SUMOylation of EPAC1. SUMO modification of EPAC1 mediates its heat shock-induced Rap1/2 activation in a cAMP-independent manner. Structural modeling and molecular dynamics simulation studies demonstrate that SUMO substituent on K561 of EPAC1 promotes Rap1 interaction by increasing the buried surface area between the SUMOylated receptor and its effector. Our studies identify a functional SUMOylation site in EPAC1 and unveil a novel mechanism in which SUMOylation of EPAC1 leads to its autonomous activation. The findings of SUMOylation-mediated activation of EPAC1 not only provide new insights into our understanding of cellular regulation of EPAC1 but also will open up a new field of experimentation concerning the cross-talk between cAMP/EPAC1 signaling and protein SUMOylation, two major cellular stress response pathways, during cellular homeostasis.

Biomolecular Condensates: Structure, Functions, Methods of Research

The term "biomolecular condensates" is used to describe membraneless compartments in eukaryotic cells, accumulating proteins and nucleic acids. Biomolecular condensates are formed as a result of liquid-liquid phase separation (LLPS). Often, they demonstrate properties of liquid-like droplets or gel-like aggregates; however, some of them may appear to have a more complex structure and high-order organization. Membraneless microcompartments are involved in diverse processes both in cytoplasm and in nucleus, among them ribosome biogenesis, regulation of gene expression, cell signaling, and stress response. Condensates properties and structure could be highly dynamic and are affected by various internal and external factors, e.g., concentration and interactions of components, solution temperature, pH, osmolarity, etc. In this review, we discuss variety of biomolecular condensates and their functions in live cells, describe their structure variants, highlight domain and primary sequence organization of the constituent proteins and nucleic acids. Finally, we describe current advances in methods that characterize structure, properties, morphology, and dynamics of biomolecular condensates in vitro and in vivo.

Dynamics of Hydrogen Bonds between Water and Intrinsically Disordered and Structured Regions of Proteins

Recent studies indicate more restricted dynamics of water around intrinsically disordered proteins (IDPs) than structured proteins. We examine here the dynamics of hydrogen bonds between water molecules and two proteins, small ubiquitin-related modifier-1 (SUMO-1) and ubiquitin-conjugating enzyme E2I (UBC9), which we compare around intrinsically disordered regions (IDRs) and structured regions of these proteins. It has been recognized since some time that excluded volume effects, which influence access of water molecules to hydrogen-bonding sites, and the strength of hydrogen bonds between water and protein affect hydrogen bond lifetimes. While we find those two properties to mediate lifetimes of hydrogen bonds between water and protein residues in this study, we also find that the lifetimes are affected by the concentration of charged groups on other nearby residues. These factors are more important in determining the hydrogen bond lifetimes than whether a residue hydrogen bonding with water belongs to an IDR or to a structured region.

Phase separation-based visualization of protein-protein interactions and kinase activities in plants

Protein activities depend heavily on protein complex formation and dynamic posttranslational modifications, such as phosphorylation. The dynamic nature of protein complex formation and posttranslational modifications is notoriously difficult to monitor in planta at cellular resolution, often requiring extensive optimization. Here, we generated and exploited the SYnthetic Multivalency in PLants (SYMPL)-vector set to assay protein-protein interactions (PPIs) (separation of phases-based protein interaction reporter) and kinase activities (separation of phases-based activity reporter of kinase) in planta, based on phase separation. This technology enabled easy detection of inducible, binary and ternary PPIs among cytoplasmic and nuclear proteins in plant cells via a robust image-based readout. Moreover, we applied the SYMPL toolbox to develop an in vivo reporter for SNF1-related kinase 1 activity, allowing us to visualize tissue-specific, dynamic SnRK1 activity in stable transgenic Arabidopsis (Arabidopsis thaliana) plants. The SYMPL cloning toolbox provides a means to explore PPIs, phosphorylation, and other posttranslational modifications with unprecedented ease and sensitivity.

Paradoxes of Cellular SUMOylation Regulation: A Role of Biomolecular Condensates?

Protein SUMOylation is a major post-translational modification essential for maintaining cellular homeostasis. SUMOylation has long been associated with stress responses as a diverse array of cellular stress signals are known to trigger rapid alternations in global protein SUMOylation. In addition, while there are large families of ubiquitination enzymes, all small ubiquitin-like modifiers (SUMOs) are conjugated by a set of enzymatic machinery comprising one heterodimeric SUMO-activating enzyme, a single SUMO-conjugating enzyme, and a small number of SUMO protein ligases and SUMO-specific proteases. How a few SUMOylation enzymes specifically modify thousands of functional targets in response to diverse cellular stresses remains an enigma. Here we review recent progress toward understanding the mechanisms of SUMO regulation, particularly the potential roles of liquid-liquid phase separation/biomolecular condensates in regulating cellular SUMOylation during cellular stresses. In addition, we discuss the role of protein SUMOylation in pathogenesis and the development of novel therapeutics targeting SUMOylation. SIGNIFICANCE STATEMENT: Protein SUMOylation is one of the most prevalent post-translational modifications and plays a vital role in maintaining cellular homeostasis in response to stresses. Protein SUMOylation has been implicated in human pathogenesis, such as cancer, cardiovascular diseases, neurodegeneration, and infection. After more than a quarter century of extensive research, intriguing enigmas remain regarding the mechanism of cellular SUMOylation regulation and the therapeutic potential of targeting SUMOylation.