SmLMWPTP, a teleost low molecular weight protein tyrosine phosphatase, inhibits the immune response of peripheral blood leukocytes in a manner that depends on the conserved P-loop

• SmLMWPTP is a low molecular weight protein tyrosine phosphatase from turbot.
• rSmLMWPTP exhibited optimum tyrosine phosphatase activity at pH 5 and 50 °C.
• Peripheral blood leukocytes (PBL) infected by pathogen expressed and secreted SmLMWPTP.
• rSmLMWPTP reduced the biological activity, gene expression, and phagocytosis of PBL.
• Mutation of the P-loop sequence abolished the regulatory effect of SmLMWPTP on PBL.

Protein tyrosine phosphatases (PTPs) are a family of enzymes that play a key role in cellular signal transduction. Low molecular weight PTPs (LMWPTPs) are a subfamily of PTPs that are characterized by the presence of a conserved phosphate-binding loop (P-loop) with the signature sequence of (V/I)CXGNXCRS. To date, very little study on teleost LMWPTPs has been documented, and, as a result, the function of LMWPTPs in fish is essentially unknown. In this study, we identified a LMWPTP from turbot (Scophthalmus maximus) and examined its biological activity and functionality. The turbot LMWPTP (SmLMWPTP) is composed of 158 residues and possesses a typical P-loop sequence in the form of 12VCLGNICRS20. Purified recombinant SmLMWPTP (rSmLMWPTP) exhibited apparent phosphatase activity, which was optimal at pH 5 and 50 °C. The activity of SmLMWPTP was abolished when C13 and, in particular, R19 of the P-loop were mutated. SmLMWPTP expression was detected in a wide range of tissues and upregulated by bacterial and viral infection. Subcellular localization analysis showed that SmLMWPTP was secreted by peripheral blood leukocytes (PBL) into the extracellular milieu. When PBL were treated with rSmLMWPTP, the cells exhibited significant reductions in (i) proliferative and respiratory burst activity, (ii) expression levels of multiple immune relevant genes, and (iii) phagocytic activity. In contrast, the mutant SmLMWPTP bearing R19 mutation had no effect on PBL activity. Taken together, these results indicate that SmLMWPTP is a secreted PTP that exerts a negative regulatory effect on the innate immune response of PBL in a manner that depends on the structural integrity of the P-loop.