title: Roles of Gln81 and Cys80 in catalysis by glycosylphosphatidylinositol-phospholipase C from Trypanosoma brucei.
abstract: Glycosylphosphatidylinositol-specific phospholipase C (GPtdIns-PLC) is found in the protozoan parasite Trypanosoma brucei. A region of protein sequence similarity exists between the protozoan enzyme and eubacterial phosphatidylinositol-phospholipases C. The functional relevance of Cys80 and Gln81 of GPtdIns-PLC, both in this region, was tested with a panel of mutations at each position. Gln81Glu, Gln81Ala, Gln81Gly, Gln81Lys and Gln81Leu mutants were inactive. Cleavage of GPtdIns was detectable in Gln81Asn, although the specific activity decreased 500-fold, and kcat was reduced 50-fold. Thus an amide side-chain at residue 81 is essential for catalysis by GPtdIns-PLC. Sulfhydryl reagents inactivate GPtdIns-PLC, suggesting that a Cys could be close to the enzyme active site. Surprisingly, p-chloromercuriphenyl sulfonate (p-CMPS) is significantly more potent than N-ethylmaleimide, the less bulky compound. This knowledge prompted us to test whether replacement of Cys80 with an amino acid possessing a bulky side-chain would inactivate GPtdIns-PLC: Cys80Ala, Cys80Thr, Cys80Phe, Cys184Ala, and Cys269-270-273Ser were constructed for that purpose. Cys80Phe lacked enzyme activity, while Cys80Ala, Cys80Thr and Cys269-270-273Ser retained 33 to 100% of wild-type activity. Interestingly, the Cys80Ala and Cys80Thr mutants became resistant to p-CMPS, as predicted if the sulfhydryl reagent reacted with Cys80 in the wild-type enzyme to form a cysteinyl mercurylphenylsulfonate moiety, a bulky adduct that inactivated GPtdIns-PLC, similar to the Cys80Phe mutation. We conclude that a bulky side-chain (or adduct) at position 80 of GPtdIns-PLC abolishes enzyme activity. Together, these observations place Cys80 and Gln81 at, or close to, the active site of GPtdIns-PLC from T. brucei.
authors.affiliation: Department of Cellular Biology, The University of Georgia, Athens, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Rashid
authors.forename: M
authors.initials: M
authors.lastname: Russell
authors.forename: K
authors.initials: K
authors.lastname: Mensa-Wilmot
mesh_terms: D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D020134:Catalytic Domain / Q000235:genetics; D003545:Cysteine / Q000737:chemistry; D017931:DNA Primers / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004926:Escherichia coli / Q000235:genetics; D005973:Glutamine / Q000737:chemistry; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014346:Trypanosoma brucei brucei / Q000201:enzymology* / Q000235:genetics; D010738:Type C Phospholipases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D005973:Glutamine; D002731:4-Chloromercuribenzenesulfonate; D010738:Type C Phospholipases; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D003545:Cysteine