Tuberculosis is the deadly disease caused by Mycobacterium tuberculosis (Mtb). Like most pathogens, iron metabolism is critical for survival. Because of the poor solubility of iron in physiological conditions, bacteria have evolved elaborate iron acquisition systems to steal the essential metal from the host. Ferritins evolved to store iron and keep it bioavailable. The Mtb genome encodes for two ferritin homologs: a heme-containing bacterioferritin (BfrA) and a non-heme ferritin (BfrB). Both are structurally conserved and share nearly identical architectures. The main structural difference is that bacterioferritins contain a heme in a pocket created by the interface between two subunits of a dimer related by a non-crystallographic twofold axis.
Both Mtb BfrA and Mtb BfrB were crystallized and solved by X-ray crystallography, and the typical ferritin fold was observed for each. Additionally, analysis of the Mtb BfrB crystal structure combined with sequence alignments with other structurally characterized bacterial ferritins revealed the presence of an unstructured 17-residue C-terminal extension. By protein pull-down experiments, we showed that BfrB interacts with a newly described protein nanocompartment, encapsulin, and we have shown that this extension is the BfrB anchor peptide for the interaction. However, BfrA has no such extension and does not exhibit an interaction with encapsulin. Hence, in an effort to gain a better understanding of Mtb iron metabolism, we report the structural and functional characterization of the Mtb BfrA and BfrB assemblies, as well as a description of a novel association with encapsulin.
Additionally, we have shown that Mtb has a novel heme uptake system, and have identified the genomic region responsible. Found encoded within this genomic region is a secreted protein that binds heme tightly, which we propose to be a hemophore, and its crystal structure without heme bound has been solved. This protein transfers heme to two predicted transmembrane heme transporters, MmpL11 and MmpL3. We also observed that non-pathogenic Mycobacterium bovis BCG (BCG) has an attenuated system in comparison with Mtb. By sequence alignment, the homologous BCG hemophore is identical to the Mtb hemophore, except for lysine 87 substituted with threonine, and this substitution may contribute to the attenuation seen in the BCG system. In an effort to define the heme binding site and consequences of the K87T mutation, attempts were made to crystallize both proteins with and without heme bound.
|Commitee:||Ribbe, Markus, Tsai, Sheryl|
|School:||University of California, Irvine|
|Department:||Biological Sciences - Ph.D.|
|School Location:||United States -- California|
|Source:||DAI-B 73/04, Dissertation Abstracts International|
|Subjects:||Molecular biology, Microbiology, Biochemistry|
|Keywords:||Bacterioferritin, Encapsulin, Ferritin, Heme, Iron storage proteins, Mycobacterium tuberculosis|
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