title: Functional characterization and ATP-induced dimerization of the isolated ABC-domain of the haemolysin B transporter.
abstract: Nucleotide-binding domains (NBD) are highly conserved constituents of ATP-binding cassette (ABC) transporters. Members of this family couple ATP hydrolysis to the transfer of various molecules across cell membranes. The NBD of the HlyB transporter, HlyB-NBD, was characterized with respect to its uncoupled ATPase activity, oligomeric state, and stability in solution. Experimental data showed that both the nature and pH of an assay buffer influenced the level of protein activity. Comparative analysis of protein stability and ATPase activity in various buffers suggests an inverse relationship between the two. The highest ATPase activity was detected in HEPES, pH 7.0. A kinetic analysis of the ATPase activity in this buffer revealed an enzyme concentration dependence and ATP-induced protein oligomerization. Assuming that the dimer is the active form of enzyme, at least half of the purified HlyB-NBD was estimated to be a dimer at 1.2 microM under the most optimal conditions for ATP hydrolysis. This is about 2 orders of magnitude lower than reported for other canonical ABC-ATPases. The maximum reaction velocity of 0.6 micromol/mg x min at 22 degrees C and the apparent kinetic constant K(app)(0.5) of 0.26 mM for ATP were determined for the dimerized HlyB-NBD. Gel filtration experiments with the wild-type protein and HlyB-NBD mutated in a key catalytic residue, H662A, provided further evidence for ATP-induced protein dimerization. ATPase activity experiments with protein mixtures composed of wild-type and the ATPase-deficient H662A mutant demonstrated that one intact NBD within a dimer is sufficient for ATP hydrolysis. This single site turnover might suggest a sequential mechanism of ATP hydrolysis in the intact HlyB transporter.
authors: Institute of Biochemistry, Biocenter N210, Goethe University Frankfurt, Marie-Curie Str. 9, 60439 Frankfurt, Germany.
authors: Jelena
authors: J
authors: Zaitseva
authors: Stefan
authors: S
authors: Jenewein
authors: Alexander
authors: A
authors: Wiedenmann
authors: Houssain
authors: H
authors: Benabdelhak
authors: I Barry
authors: IB
authors: Holland
authors: Lutz
authors: L
authors: Schmitt
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry* / Q000378:metabolism; D000409:Alanine / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002021:Buffers; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019281:Dimerization; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D006460:Hemolysin Proteins / Q000378:metabolism*; D006639:Histidine / Q000235:genetics; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D010446:Peptide Fragments / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D017434:Protein Structure, Tertiary / Q000235:genetics; D014364:Tryptophan / Q000235:genetics; D014443:Tyrosine / Q000235:genetics
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002021:Buffers; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D006460:Hemolysin Proteins; C050837:Hlyb protein, Bacteria; D010446:Peptide Fragments; D014443:Tyrosine; D006639:Histidine; D000244:Adenosine Diphosphate; D014364:Tryptophan; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; C086257:hemolysin B; D000409:Alanine