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.