Christopher D. Bahl

Research Summary

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that causes chronic infection in the lungs of cystic fibrosis (CF) patients. P. aeruginosa has been shown to secrete a soluble protein, termed Cif (CFTR inhibitory factor), which inhibits the endocytic recycling of cystic fibrosis transmembrane conductance regulator (CFTR). The result of Cif treatment on airway epithelial cells is a loss of CFTR mediated chloride efflux across the apical membranes. This effect is due to mislocalization of the transporter to endosomes, and by preventing trafficking back to the plasma membrane, CFTR is shunted to the lysosomes for degradation. Since the majority of CF patients are colonized by P. aeruginosa, Cif represents a potential roadblock for the effectiveness of CFTR targeted therapies in a clinical setting. Additionally, elucidation of the Cif mechanism may illuminate novel pathways regulating CFTR trafficking.

We have determined the structure of Cif by X-ray crystallography and demonstrated it to be an epoxide hydrolase (EH) with unique substrate selectivity. Mammalian EHs can function as regulators of blood pressure and inflammation, while bacterial EHs have mostly been characterized for their ability to detoxify xenobiotic compounds. Cif is the first example of an EH serving as a virulence factor. Based on structural comparison, it appears that the enzyme utilizes a catalytic triad of residues Asp129, Glu153 and His297, with accessory residues His177 and Tyr239 coordinating the epoxide oxygen during ring opening. Additionally, His207 sits along the substrate access tunnel to the active site. Cif is also the first example of an EH utilizing a His-Tyr pair to coordinate an epoxide substrate, rather than the canonical Tyr-Tyr pair. In the proposed enzyme mechanism, Asp129 nucleophilically attacks a carbon of the epoxide moiety of a substrate, forming an ester linked enzyme-acyl intermediate. The preference for which carbon is attacked varies depending upon the substrate. In the second step of the reaction, a water molecule is activated by the charge-relay His297-Glu153 pair, and undergoes nucleophilic attack on the Cγ of Asp129. This hydrolyzes the ester group, liberating the hydrolysis product as a vicinal diol.

Mutation of any active site residue results in the disruption of enzymatic hydrolysis of epoxide compounds. Furthermore, these mutations completely abolish the CFTR inhibitory effect seen on airway epithelial cells. Thus, it appears that EH activity is strictly required for Cif mediated mislocalization of CFTR, suggesting that Cif likely acts on a physiological epoxide with a role in regulating CFTR trafficking. It is possible that this effect could be due to either the depletion of an epoxide target or the generation of a vicinal-diol hydrolysis product. Current work is aimed at elucidation of the endogenous human epoxide/diol pair that is responsible for mediating this effect.

For more information, feel free to visit the Cif Wikipedia page.

Publications

A.E. Ballok, C.D. Bahl, E.L. Dolben, A.K. Lindsay, J.D. St. Laurent, D.A. Hogan, D.R. Madden, G.A. O'Toole (2012) Epoxide-mediated CifR repression of cif gene expression utilizes two binding sites in Pseudomonas aeruginosa. J Bacteriol. 194, 5315-5324.

C.D. Bahl (2012) Searching For a Hijacked Epoxide: Structural, Mechanistic, and Functional Characterization of Cif, a Virulence Factor Secreted by Pseudomonas aeruginosa. (Ph.D. thesis) Dartmouth College, Hanover, NH link

C.D. Bahl, D.R. Madden (2012) Pseudomonas aeruginosa Cif defines a distinct class of α/β epoxide hydrolases utilizing a His/Tyr ring-opening pair. Protein Pept Lett. 19, 186-193.

C.D. Bahl, C. Morisseau, J.M. Bomberger, B.A. Stanton, B.D. Hammock, G.A. O'Toole, D.R. Madden (2010) Crystal structure of the cystic fibrosis transmembrane conductance regulator inhibitory factor Cif reveals novel active-site features of an epoxide hydrolase virulence factor. J Bacteriol. 192, 1785-1795.

C.D. Bahl, D.P. MacEachran, G.A. O'Toole, D.R. Madden. (2010) Purification, crystallization and preliminary X-ray diffraction analysis of Cif, a virulence factor secreted by Pseudomonas aeruginosa. Acta Crystallogr. F66, 26-28.

C.D. Bahl (2006) Characterization of a Putative Palmitoyltransferase in Dictyostelium discoideum. (M.S. thesis) The University of Maine, Orono, ME link

Structures

A full list of structures can be found at the Protein Data Bank

Published Poster Abstracts

C.D. Bahl, S.A. Gerber, D.R. Madden (2010) The Pseudomonas virulence factor Cif alters human ABC transporter trafficking and stability through epoxide hydrolase enzyme activity. A Special Symposium Celebrating the 40th Anniversary of the Protein Data Bank Abstract Book. Cold Spring Harbor Laboratory, New York. link

C.D. Bahl, S.A. Gerber, D.R. Madden (2010) The Pseudomonas virulence factor Cif requires epoxide hydrolase activity to mediate CFTR degradation. Pediatr Pulmonol. 45, 349. link

C.D. Bahl, C. Morisseau, J.M. Bomberger, D.P. MacEachran, G.A. O'Toole, B.A. Stanton, B.D. Hammock, D.R. Madden (2009) Epoxide hydrolase activity of CFTR inhibitory factor, a virulence factor on airway epithelial cells. Pediatr Pulmonol. 44, 318. link

Invited Talks

December 2011	Dr. David Baker, Department of Biochemistry, University of Washington
November 2011	Biochemistry Department Annual Retreat
February 2011	Molecular and Cellular Biology Program Recruitment Weekend Symposium
February 2010	Microbiology and Molecular Pathogenesis Program Retreat
February 2009	Microbiology and Molecular Pathogenesis Program Retreat

Fellowships and Awards

April 2012	E. Lucile Smith Award for Scientific Excellence in Biochemistry
2010-2012	NRSA Institutional Research Training Grant Fellow T32-DK007301 Renal function and disease
March 2010	Dartmouth-Fogarty International Fellowship in Global Health Aravind Eye Hospital, Madurai, India
2008-2010	NRSA Institutional Research Training Grant Fellow T32-AI007519 Host-microbe interactions

Teaching Experience

Fall 2011	Molecular Pathogenesis Journal Club Dartmouth Medical School
Fall 2007	Biochemistry Dartmouth College
Spring 2006	Elementary Physiological Chemistry Laboratory University of Maine, Orono
Fall 2005	Fundamentals of Chemistry Laboratory University of Maine, Orono
Spring 2005	Introduction to Molecular and Cellular Biology University of Maine, Orono

Professional Memberships

Biophysical Society
Student Spotlight in the June 2011 Newsletter

American Society for Biochemistry and Molecular Biology