The suppressor of T-cell receptor (TCR) signaling (Sts) proteins Sts-1 and Sts-2 suppress receptor-mediated signaling pathways in various immune cells, including the TCR pathway in T cells and the Dectin-1 signaling pathway in phagocytes. As multidomain enzymes, they contain an N-terminal ubiquitin-association domain, a central Src homology 3 domain, and a C-terminal histidine phosphatase domain. Recently, a 2-histidine (2H) phosphoesterase motif was identified within the N-terminal portion of Sts. The 2H phosphoesterase motif defines an evolutionarily ancient protein domain present in several enzymes that hydrolyze cyclic phosphate bonds on different substrates, including cyclic nucleotides. It is characterized by two invariant histidine residues that play a critical role in catalytic activity. Consistent with its assignment as a phosphoesterase, we demonstrate here that the Sts-1 2H phosphoesterase domain displays catalytic, saturable phosphodiesterase activity toward the dinucleotide 29,39-cyclic NADP. The enzyme exhibited a high degree of substrate specificity and selectively generated the 39-nucleotide as the sole product. Sts-1 also had phosphodiesterase catalytic activity toward a 5-mer RNA oligonucleotide containing a 29,39-cyclic phosphate group at its 39 terminus. To investigate the functional significance of Sts-1 2H phosphoesterase activity, we generated His-to-Ala variants and examined their ability to negatively regulate cellular signaling pathways. Substitution of either conserved histidine compromised the ability of Sts-1 to suppress signaling pathways downstream of both the TCR and the Dectin-1 receptor. Our results identify a heretofore unknown cellular enzyme activity associated with Sts-1 and indicate that this catalytic activity is linked to specific cell-signaling outcomes.
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Funding and additional information—This work was supported by NIAID, National Institutes of Health Grants R01AI141592 (to N. C. and J. B. F.), R21AI120859 (to N. C. and J. B. F.), and R21AI133381 (to N. C.), as well as by the NHLBI, National Institutes of Health Grant U01HL127522. In addition, parts of this work were supported by NIGMS, National Institutes of Health Grant R35GM124898 (to J. B. F.). Additional support was provided by Stony Brook University, the Stony Brook University Center for Biotechnology (a New York State Center for Advanced Technology), Cold Spring Harbor Laboratory, Brookhaven National Laboratory, the Feinstein Institute for Medical Research, and the Hormel Foundation. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.