STRUCTURAL GENOMICS STUDIES OF MENAQUINONE BIOSYNTHESIS PROTEINS FROM MYCOBACTERIUM TUBERCULOSIS: INSGHTS FROM THE STRUCTURES OF MENG AND MENB

J. M. Johnston, V. Arcus and E. N. Baker

Laboratory of Structural Biology, School of Biological Sciences, University of Auckland, Auckland, New Zealand (jm.johnson@auckland.ac.nz)

Menaquinone plays an important role in anaerobic electron transport in a range of bacteria. Homologs for the menaquinone biosynthesis pathway genes can be identified in the genome sequence of Mycobacterium tuberculosis H37Rv strain. Due to the significance of this pathway in anaerobic conditions, combined with the fact that the menaquinone biosynthesis pathway is not present in humans, means these genes may be possible drug targets. Six of the seven homologs of menaquinone biosynthesis genes identified in the Mycobacterium tuberculosis H37Rv strain genome sequence were chosen for study. Cloning, expression tests and refolding experiments resulted in three soluble proteins. Two of these proteins have been crystallized and their structures solved. The 1.9 Å trimeric structure of refolded protein Rv3853, annotated as the S-adenosylmethionine (SAM) dependent methyltransferase MenG, revealed no structural similarity to other SAM dependent methyltranferases and posed questions about the validity of its functional annotation. The discovery of two small molecules bound in this structure highlight regions of this protein that may be significant for its function. Unlike Rv3853, the structure of Napthoate synthase (MenB) solved to 2.15 Å, supports the suggested annotation and confirms its place as part of the crotonase superfamily.