BEAR, C.E., E; LI, C., ; KIDD, J.F., ; KOGAN, I., ; HUAN, L.-J., ; RAMJEESINGH, M., ; Hospital for Sick Children, Toronto, Canada: Quaternary structure of CFTR and putative intramolecular interactions may mediate CFTR function

CFTR is a member of the ABC superfamily of transport ATPases. Recent high resolution structures of intact procaryotic transporters from this family permit predictions regarding the functional quaternary structure and interdomain interactions in CFTR. The solved structures of the lipid flippase; MsbA and the vitamin B12 transporter; BtuCD proteins show that they dimeric. As the CFTR molecule possesses an internal duplication of the domains found in MsbA and BtuCD, we would predict the functional unit of CFTR is a monomer. Our recent biochemical studies support this prediction and show that purified, reconstituted CFTR monomers are fully functional. Functional and biochemical studies of various intact ABC transporters, reveal that activation requires long range conformational changes to transmit signals between domains or subunits. In our work with purified, reconstituted CFTR, we have shown that ligands that interact specifically with the membrane domains of CFTR; ie. large anions such as glutathione and DPC, induce changes in ATP catalysis by the nucleotide binding folds (NBDs). In turn, intrinsic ATP hydrolysis by the NBDs is important for optimal gating (opening and closing) of the CFTR chloride channel. Our work suggests that, optimal hydrolysis by CFTR requires functional interaction between the two NBDs. These findings are consistent with other biochemical studies of CFTR and certain crystal structures of prokaryotic ABC proteins which suggest that the physical interaction between the two NBDs is an important part of the mechanism underlying ATP binding and hydrolysis. Our approach to studying the physical basis for interaction between the NBDs of CFTR in the intact protein will be discussed further in this symposium.