Protein tyrosine phosphorylation is thought to be a unique feature of multicellular animals. Interestingly, the genome of the unicellular protist Monosiga brevicollis reveals a surprisingly high number and diversity of protein tyrosine kinases, protein tyrosine phosphatases (PTPs), and phosphotyrosine-binding domains. Our study focuses on a hypothetical SH2 domain-containing PTP (SHP), which interestingly has a predicted structure that is distinct from SHPs found in animals. In this study, we isolated cDNA of the enzyme and discovered that its actual sequence was different from the predicted sequence as a result of non-consensus RNA splicing. Contrary to the predicted structure with one SH2 domain and a disrupted phosphatase domain, Monosiga brevicollis SHP (MbSHP) contains two SH2 domains and an intact PTP domain, closely resembling SHP enzymes found in animals. We further expressed the full-length and SH2 domain-truncated forms of the enzyme in Escherichia coli cells and characterized their enzymatic activities. The double-SH2 domain-truncated form of the enzyme effectively dephosphorylated a common PTP substrate with a specific activity among the highest in characterized PTPs, while the full-length and the N-terminal SH2 domain-truncated forms of the enzyme showed much lower activity with altered pH dependency and responses to ionic strength and common PTP inhibitors. This indicates that SH2 domains suppress the catalytic activity. SHP represents a highly conserved ancient PTP, and studying MbSHP should provide a better understanding about the evolution of tyrosine phosphorylation. (C) 2014 Elsevier Inc. All rights reserved.