From Proteopediaproteopedia link
Crystal structure of mouse Sts-1 PGM domain in complex with phosphate
[UBS3B_MOUSE] Interferes with CBL-mediated down-regulation and degradation of receptor-type tyrosine kinases. Promotes accumulation of activated target receptors, such as T-cell receptors and EGFR, on the cell surface. Exhibits tyrosine phosphatase activity toward several substrates including EGFR, FAK, SYK, and ZAP70. Down-regulates proteins that are dually modified by both protein tyrosine phosphorylation and ubiquitination.   
Publication Abstract from PubMed
Precise signaling by the T cell receptor (TCR) is crucial for a proper immune response. To ensure that T cells respond appropriately to antigenic stimuli, TCR signaling pathways are subject to multiple levels of regulation. Sts-1 negatively regulates signaling pathways downstream of the TCR by an unknown mechanism(s). Here, we demonstrate that Sts-1 is a phosphatase that can target the tyrosine kinase Zap-70 among other proteins. The X-ray structure of the Sts-1 C terminus reveals that it has homology to members of the phosphoglycerate mutase/acid phosphatase (PGM/AcP) family of enzymes, with residues known to be important for PGM/AcP catalytic activity conserved in nature and position in Sts-1. Point mutations that impair Sts-1 phosphatase activity in vitro also impair the ability of Sts-1 to regulate TCR signaling in T cells. These observations reveal a PGM/AcP-like enzyme activity involved in the control of antigen receptor signaling.
A phosphatase activity of Sts-1 contributes to the suppression of TCR signaling.,Mikhailik A, Ford B, Keller J, Chen Y, Nassar N, Carpino N Mol Cell. 2007 Aug 3;27(3):486-97. PMID:17679096
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.