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Author Notes:

To whom correspondence should be addressed: Nael A. McCarty (Email: namccar@emory.edu)

Edited by Tomoko Ohta, National Institute of Genetics, Mishima, Japan, and approved October 3, 2008.

Author contributions: I.K.J., G.C., C.H.T., and N.A.M. designed research; K.C.K., G.C., C.H.T., and N.A.M. performed research; I.K.J. and K.C.K. analyzed data; and I.K.J., C.H.T., and N.A.M. wrote the paper.

We thank H.C. Hartzell for critically reading the manuscript.

The authors declare no conflict of interest.

Subjects:

Research Funding:

This work was supported by National Institutes of Health Grant DK-056481 (to N.A.M.), Cystic Fibrosis Foundation Grant MCCART07P0 (to N.A.M.), and the School of Biology, Georgia Institute of Technology.

Keywords:

  • ion channel
  • molecular evolution
  • CFTR
  • Type II divergence

Evolutionary and functional divergence between the cystic fibrosis transmembrane conductance regulator and related ATP-binding cassette transporters

Tools:

Journal Title:

Proceedings of the National Academy of Sciences

Volume:

Volume 105, Number 48

Publisher:

, Pages 18865-18870

Type of Work:

Article | Post-print: After Peer Review

Abstract:

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter superfamily, an ancient family of proteins found in all phyla. In nearly all cases, ABC proteins are transporters that couple the hydrolysis of ATP to the transmembrane movement of substrate via an alternating access mechanism. In contrast, CFTR is best known for its activity as an ATP-dependent chloride channel. We asked why CFTR, which shares the domain architecture of ABC proteins that function as transporters, exhibits functional divergence. We compared CFTR protein sequences to those of other ABC transporters, which identified the ABCC4 proteins as the closest mammalian paralogs, and used statistical analysis of the CFTR-ABCC4 multiple sequence alignment to identify the specific domains and residues most likely to be involved in the evolutionary transition from transporter to channel activity. Among the residues identified as being involved in CFTR functional divergence, by virtue of being both CFTR-specific and conserved among all CFTR orthologs, was R352 in the sixth transmembrane helix (TM6). Patch-clamp experiments show that R352 interacts with D993 in TM9 to stabilize the open-channel state; D993 is absolutely conserved between CFTRs and ABCC4s. These data suggest that CFTR channel activity evolved, at least in part, by converting the conformational changes associated with binding and hydrolysis of ATP, as are found in true ABC Transporters, into an open permeation pathway by means of intraprotein interactions that stabilize the open state. This analysis sets the stage for understanding the evolutionary and functional relationships that make CFTR a unique ABC transporter protein.

Copyright information:

© 2008 by The National Academy of Sciences of the USA

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