An unexpected 2-histidine phosphoesterase activity of suppressor of T-cell receptor signaling protein 1 contributes to the suppression of cell signaling

Roles of TULA-family proteins in T cells and autoimmune diseases

The T cell Ubiquitin Ligand (TULA) protein family contains two members, UBASH3A and UBASH3B, that display similarities in protein sequence and domain structure. Both TULA proteins act to repress T cell activation via a combination of overlapping and nonredundant functions. UBASH3B acts mainly as a phosphatase that suppresses proximal T cell receptor (TCR) signaling. In contrast, UBASH3A acts primarily as an adaptor protein, interacting with other proteins (including UBASH3B) in T cells upon TCR stimulation and resulting in downregulation of TCR signaling and NF-κB signaling. Human genetic and functional studies have revealed another notable distinction between UBASH3A and UBASH3B: numerous genome-wide association studies have identified statistically significant associations between genetic variants in and around the UBASH3A gene and at least seven different autoimmune diseases, suggesting a key role of UBASH3A in autoimmunity. However, the evidence for an independent role of UBASH3B in autoimmune disease is limited. This review summarizes key findings regarding the roles of TULA proteins in T cell biology and autoimmunity, highlights the commonalities and differences between UBASH3A and UBASH3B, and speculates on the individual and joint effects of TULA proteins on T cell signaling.

Role of Tula-Family Proteins in Cell Signaling and Activation: Advances and Challenges

This Special Issue entitled "Role of Tula-Family Proteins in Cell Signaling and Activation: Advances and Challenges" is focused on a relatively novel vertebrate gene/protein family termed alternatively TULA, UBASH3, or STS [...].

Rebamipide and Derivatives are Potent, Selective Inhibitors of Histidine Phosphatase Activity of the Suppressor of T Cell Receptor Signaling Proteins

The suppressor of T cell receptor signaling (Sts) proteins are negative regulators of immune signaling. Genetic inactivation of these proteins leads to significant resistance to infection. From a 590,000 compound high-throughput screen, we identified the 2-(1H)-quinolinone derivative, rebamipide, as a putative inhibitor of Sts phosphatase activity. Rebamipide, and a small library of derivatives, are competitive, selective inhibitors of Sts-1 with IC50 values from low to submicromolar. SAR analysis indicates that the quinolinone, the acid, and the amide moieties are all essential for activity. A crystal structure confirmed the SAR and reveals key interactions between this class of compound and the protein. Although rebamipide has poor cell permeability, we demonstrated that a liposomal preparation can inactivate the phosphatase activity of Sts-1 in cells. These studies demonstrate that Sts-1 enzyme activity can be pharmacologically inactivated and provide foundational tools and insights for the development of immune-enhancing therapies that target the Sts proteins.

Inactivation of the Sts enzymes promotes resistance to lethal Staphylococcus aureus infection

Staphylococcus aureus is a highly infective Gram-positive bacterial pathogen that causes a wide range of diseases in both healthy and immunocompromised individuals. It can evade host immune defenses by expressing numerous virulence factors and toxins. Coupled with the inability of the human host to develop protective immunity against S. aureus, the emergence of antibiotic-resistant strains complicates treatment options. The non-canonical Sts phosphatases negatively regulate signaling pathways in varied immune cell types. To determine the role of the Sts proteins in regulating host responses to a Gram-positive microorganism, we investigated the response of mice lacking Sts expression to S. aureus infection. Herein, we demonstrate that Sts -/- animals are significantly resistant to lethal intravenous doses of S. aureus strain USA300. Resistance is characterized by significantly enhanced survival and accelerated bacterial clearance in multiple peripheral organs. Infected Sts -/- animals do not display increased levels of cytokines TNFα, IFNγ, and IL-6 in the spleen, liver, and kidney during the early stages of the infection, suggesting that a heightened pro-inflammatory response does not underlie the resistance phenotype. In vivo ablation of mononuclear phagocytes compromises the Sts -/- enhanced CFU clearance phenotype. Additionally, Sts -/- bone marrow-derived macrophages demonstrate significantly enhanced restriction of intracellular S. aureus following ex vivo infection. These results reveal the Sts enzymes to be critical regulators of host immunity to a virulent Gram-positive pathogen and identify them as therapeutic targets for optimizing host anti-microbial responses.

TULA Proteins in Men, Mice, Hens, and Lice: Welcome to the Family

The two members of the UBASH3/STS/TULA protein family have been shown to critically regulate key biological functions, including immunity and hemostasis, in mammalian biological systems. Negative regulation of signaling through immune receptor tyrosine-based activation motif (ITAM)- and hemITAM-bearing receptors mediated by Syk-family protein tyrosine kinases appears to be a major molecular mechanism of the down-regulatory effect of TULA-family proteins, which possess protein tyrosine phosphatase (PTP) activity. However, these proteins are likely to carry out some PTP-independent functions as well. Whereas the effects of TULA-family proteins overlap, their characteristics and their individual contributions to cellular regulation also demonstrate clearly distinct features. Protein structure, enzymatic activity, molecular mechanisms of regulation, and biological functions of TULA-family proteins are discussed in this review. In particular, the usefulness of the comparative analysis of TULA proteins in various metazoan taxa, for identifying potential roles of TULA-family proteins outside of their functions already established in mammalian systems, is examined.

The Sts Proteins: Modulators of Host Immunity

The suppressor of TCR signaling (Sts) proteins, Sts-1 and Sts-2, are a pair of closely related signaling molecules that belong to the histidine phosphatase (HP) family of enzymes by virtue of an evolutionarily conserved C-terminal phosphatase domain. HPs derive their name from a conserved histidine that is important for catalytic activity and the current evidence indicates that the Sts HP domain plays a critical functional role. Sts-1HP has been shown to possess a readily measurable protein tyrosine phosphatase activity that regulates a number of important tyrosine-kinase-mediated signaling pathways. The in vitro catalytic activity of Sts-2HP is significantly lower than that of Sts-1HP, and its signaling role is less characterized. The highly conserved unique structure of the Sts proteins, in which additional domains, including one that exhibits a novel phosphodiesterase activity, are juxtaposed together with the phosphatase domain, suggesting that Sts-1 and -2 occupy a specialized intracellular signaling niche. To date, the analysis of Sts function has centered predominately around the role of Sts-1 and -2 in regulating host immunity and other responses associated with cells of hematopoietic origin. This includes their negative regulatory role in T cells, platelets, mast cells and other cell types, as well as their less defined roles in regulating host responses to microbial infection. Regarding the latter, the use of a mouse model lacking Sts expression has been used to demonstrate that Sts contributes non-redundantly to the regulation of host immunity toward a fungal pathogen (C. albicans) and a Gram-negative bacterial pathogen (F. tularensis). In particular, Sts-/- animals demonstrate significant resistance to lethal infections of both pathogens, a phenotype that is correlated with some heightened anti-microbial responses of phagocytes derived from mutant mice. Altogether, the past several years have seen steady progress in our understanding of Sts biology.

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.