| Research Abstract |
ATP-binding cassette (ABC) proteins comprise a family of structurally related membrane proteins sharing well conserved nucleotide binding domains. They include MDR1, an ATP-dependent efflux pump that extrude toxic xenobiotics, the cystic fibrosis transmembrane conductance regulator (CFTR), an anion channel, and the sulfonylurea receptor (SUR 1), the pancreatic ATP-sensitive K^+ channel regulator.
The objective of the research was
1) to develop a deep understanding of the molecular mechanisms of transporters, channels, and regulators of ABC proteins.
2) to understand the molecular mechanisms of having dual functions.
3) to explore the physiological functions of ABC proteins in cellular signal transduction mechanism.
The K_<ATP> channels in pancreatic beta-cells are critical in the regulation of glucose-induced insulin secretion. Although electrophysiological studies provide clues to the complex control of K_<ATP> channels by ATP, MgADP, and the pharmacological agents, the molecular mechanism
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of K_<ATP> channel regulation remains unclear. The K_<ATP> channel is a hetero-oligomeric complex of SUR 1 subunits of ABC superfamily with two nucleotide- binding folds and the pore-forming Kir6.2 subunits. We revealed that MgATP and MgADP, but not MgATP-gammaS, stabilize binding of prebound 8-azido-[alpha-^<32>P]ATP to SUIR1. Mutation in the Walker A and B motifs of NBF2 of SUIR1 abolished this stabilizing effect of MgADP.These results suggest that SURl binds 8-azido-ATP strongly at NBF1, and that MgADP, either by direct binding to NBF2 or hydrolysis of bound MgATP at NBF2, stabilizes prebound 8-azido-ATP binding at NBF1. The sulfonylurea glibenclamide caused release of prebound 8-azido-[alpha-^<32>P]ATP from SUR1 in the presence of MgADP or MgATP in a concentration-dependent manner. This direct biochemical evidence of cooperative interaction in nucleotide binding of the two NBFs of SUR1 suggests that glibenclamide both blocks this cooperative binding of ATP and MgADP, and, in cooperation with the MgADP bound at NBF2, causes ATP to be released from NBF1. Less
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