Higher plant tyrosine-specific protein phosphatases (PTPs) contain novel amino-terminal domains: expression during embryogenesis

Sequences encoding proteins with homology to protein tyrosine phosphatases have been identified in Arabidopsis, soybean and pea. Each contains a predi cted catalytic domain containing sequence motifs characteristic of tyrosine -specific protein phosphatases (PTPs) which play an important role in signa l transduction in other eukaryotes and are distinct from dual-specificity, cdc25 or low-molecular-weight protein tyrosine phosphatases. Their identity as PTPs was confirmed by characterising the soybean PTP expressed as a rec ombinant His-tagged fusion protein. The enzyme had phosphatase activity tow ards p-nitrophenolphosphate (pNPP) and phosphotyrosine, but did not hydroly se phosphoserine or phosphothreonine at a measureable rate. Phosphotyrosine containing peptides also served as substrates, with K-m values in the micr omolar range. Activity was abolished by inhibitors specific for tyrosine ph osphatases (vanadate, dephostatin) but was unaffected by inhibitors of seri ne/threonine protein phosphatases (fluoride, cantharidin, metal-chelating a gents). Gel filtration chromatography showed that the recombinant enzyme wa s a monomer. The Arabidopsis PTP sequence was isolated both as a genomic cl one and as a partial EST, whereas the pea and soybean sequences were isolat ed as cDNAs. Southern analysis suggested a single gene in Arabidopsis and a small gene family in pea and soybean. In pea, PTP transcripts were present in embryos, and decreased in level with development; transcripts were also detectable in other tissues. The plant PTPs all contain a similar N-termin al domain which shows no similarity to any known protein sequence. This dom ain may be involved in PTP functions unique to plants.