Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Marcelo C. Sousa

Second Advisor

Joseph J. Falke

Third Advisor

James Goodrich

Fourth Advisor

Charles S. McHenry

Fifth Advisor

Michael H.B. Stowell

Abstract

Gram-negative bacteria are characterized by the presence of an outer membrane (OM), which is a highly selective permeability barrier that protects the bacteria from the environment and allows for exchange of nutrients and waste products. The OM consists of phospholipids located in the inner leaflet and lipopolysaccharide located in the outer leaflet of the membrane, as well as embedded β-barrel proteins and lipoproteins facing the periplasmic side. Integral outer membrane protein (OMP) biogenesis is essential making it an ideal antibiotic target. The molecular mechanisms that mediate OMP biogenesis are still poorly understood. It has been established that the machinery responsible for β-barrel protein assembly into the OM consists of the OMP BamA and four lipoproteins (BamB-E). It is not clear yet how the β-Barrel Assembly Machinery (BAM) complex folds OMPs and/or inserts them in the OM.

We approached this problem through the structure-function analysis of this system. Here, I present the crystal structure of BamB from Pseudomonas aeruginosa and a structure of BamA-BamB chimera protein from Escherichia coli. Using these structures we were guided to test some plausible functions of the proteins on the path to uncover the BAM mechanism during the folding and insertion of OMPs into the OM. BamB is an eight-bladed β-propeller with conserved two loops protruding from the bottom of the propeller. The loop cluster residues previously shown to mediate BamB-BamA interaction are located at POTRA3 near the flexible hinge between POTRA2 and 3 region of BamA. These structures are a valuable guide to improve our understanding of BAM complex mechanism.

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