Dual-specific protein phosphatases in the Archaea
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Three distinct families of PTPs, the conventional (cPTPs), low molecular weight (LMW PTPs), and Cdc25 PTPs, have converged upon a common catalytic mechanism and active site sequence, mainly, the phosphate-binding loop encompassing the PTP signature motif (H/V)C(X)5R(S/T) and an essential Asp residue on a surface loop. There is little sequence similarity among the three families of phosphatases. All known LMW PTP remove phosphoryl groups esterified to the hydroxyl amino acid: tyrosine, whereas all members of the Cdc25 family are dual-specificity protein phosphatases that dephosphorylate all the hydroxyl amino acids: tyrosine, serine and threonine. The cPTP family primarily functions as tyrosine phosphatases, but it also includes dual-specific members.
ORFs encoding potential cPTPs have been identified in five archaeal species: Methanobacterium thermoautotrophicum, Methanococcus jannaschii, Thermococcus kodakaraensis, Pyrococcus horikoshii, and S. solfataricus. Only one has been partially characterized, Tk-PTP from T. kodakaraensis. Hence, our current body of knowledge concerning the functional properties and physiological roles of these enzymes remains fragmented.
The genome of S. solfataricus encodes a single conventional protein tyrosine phosphatase, SsoPTP. SsoPTP is the smallest known archaeal PTP (18.3 kDa) with a primary amino acid sequence that conforms to the cPTP protein tyrosine phosphatase paradigm, HCX5R(S/T).
Relatively little is known about its mode of action â whether it follows the conventional PTP mechanism or employs a different route for catalysis â or its physiological role.
ORF sso2453 from the genome of Sulfolobus solfataricus, encoding a protein tyrosine phosphatase, was cloned and its recombinant protein product, SsoPTP, was expressed in E. coli and purified by immobilized metal affinity chromatography. SsoPTP displayed the ability to dephosphorylate protein-bound phosphotyrosine as well as protein-bound phosphoserine/phosphothreonine. SsoPTP hydrolyzed both isomers of naphthyl phosphate, an indication of dual specificity. The four conserved residues within the presumed active site sequence: Asp69, His95, Cys96, and Arg102, and the invariant Gln139 residue were essential for catalysis, as it was predicted for the established members of the PTP family in both bacteria and eukaryotes. A substrate trapping protein variant, SsoPTP-C96S/D69A, was constructed to isolate possible SsoPTP substrates present in S. solfataricus cell lysates. Several potential substrates were isolated and identified by mass spectroscopy.
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