First Structural Characterization of a Bon-Domain in a Protein from Mycobacterium Tuberculosis: OmpATb Tracks toward an Oligomerization Process to form a Cell Wall Pore

Abstract

The etiological agent of tuberculosis (TB), Mycobacterium tuberculosis, causing nearly two millions deaths per year is presently one of the greatest infectious agents of mortality worldwide. One of the major problems in TB therapy is the slow uptake of drugs across the thick mycobacterial cell wall made of unique lipid and glycolipid moities . Water-filled protein channels, called porins, are considered as the main pathway for hydrophilic drugs through the mycobacterial cell wall . In this study, we investigated the 3D structure of the porin-like protein OmpATb (326 residues) from Mycobacterium tuberculosis by NMR in solution. We have found that the N-terminal domain of OmpATb (73-204), sufficient to form channels in planar lipid bilayers, forms an α/βsandwich composed of 6 β-strands and 3 α-helices. It appears that a sub-domain of this structure is relied to the BON (bacterial OsmY and nodulation) domain fold which was initially identified in bacterial proteins as a conserved ∼ 60 residues module supposed to associate with phospholipids. Thus our study gives rise for the first time to the 3D fold of a BON domain member. Other bacterial proteins belonging to macromolecular complexes of the type III secretion systems have BON-like domain structures and form multi-subunits membrane-associated rings at the basal body of the secretion machinery. By analogy with these superstructures, we have built a model of an oligomeric ring assembly of the OmpATb protein to support the formation of functional pores in the mycobacterial cell wall. The numbers of units involved in the pore structure is discussed regarding the biochemical and channel properties of OmpATb.