Identification of a Cysteine Residue Conferring Sensitivity of a Maize â-Glukosidase to Silver Ion and Alkylation Treatment.
|Druh||Článek ve sborníku|
|Konference||V. Pracovní setkání biochemiků a molekulárních biologů, Sborník příspěvků, Brno 2001|
|Fakulta / Pracoviště MU|
|Obor||Genetika a molekulární biologie|
|Klíčová slova||Maize b-glucosidase; cysteine residues; thioredoxin fusion|
|Popis||The maize Zm-p60.1 gene encodes a b-glucosidase that can release active cytokinins from their storage forms, cytokinin-O-glucosides. Mature catalytically active Zm-p60.1 is a homodimer containing five cysteine residues per a subunit. High sensitivity of the enzyme to inhibition by Ag+ or Hg2+ ions together with loss of enzyme activity upon treatment with alkylating agents indicates that a free sulfhydryl group is required for enzyme activity. Site directed mutagenesis followed by enzyme kinetic analysis suggested that cysteine residue in position 479 (C479) provides the sulfhydryl group required for enzyme activity (1). Here we report data confirming that C479 is the target for Ag+ and alkylation agent action. When the wild type enzyme was incubated in the presence of 0.5 to 128 mM silver nitrate, almost complete loss of activity was observed at 32 mM AgNO3. Mutations leading to substitutions of C479 by alanine (A), serine (S), arginine (R) or aspartic acid (D) residues resulted in a dramatic drop in sensitivity of the mutated enzyme to Ag+ inhibition. Further, treatment of the wild type enzyme with an alkylating agent iodoacetic acid resulted in a loss of enzyme activity while enzyme activity in C479A mutant was not affected by the alkylation treatment. Interestingly, as revealed by native PAGE followed by western blot analysis, both Ag+ and alkylation treatment led to dimer dissociation. No dimer dissociation was detected in the mutants under identical conditions. Thus, inhibition of Zm-p60.1 enzyme activity by Ag+ and/or alkylation treatment might reflect dissociation of active dimer into inactive subunits triggered by binding of a bulky group to C479.|