Biophysical Insights into the Spatial Organization and Dynamics of Structurally Complex Mycobacterial Membrane Lipids

Abstract

The remarkably complex cell wall of mycobacterium tuberculosis (Mtb) forms a formidable barrier to many host stresses and therapeutics. It houses distinct lipids with long hydrocarbon chains, and chemical groups such as trehalose, methyl branches and cyclopropane rings; not found in eukaryotic membranes. The current holistic picture of the Mtb cell envelope from cryo-TEM comprises of peptidoglycan (PG) covalently modified with arabinogalactan polymers that are in turn elaborated with mycolic acids (C60-C90). These mycolic acids form the inner leaflet of the mycomembrane (MM); MM also includes an outer leaflet constituted by a variety of non-covalently associated lipids such as phthiocerol dimycocerosate (PDIM), dimycolyltrehalose (TDM), sulfoglycolipids (SL), phosphatidylinositol mannosides (PIMs), lipoarabinomannan (man-LAM) and DAGs along with phospholipids. The vital role of each Mtb membrane component in forming a spatially complex barrier, and how mycobacteria uses its membrane platforms to impact infection biology remain understudied. in our current work we have characterized the outer membrane (OM) and inner membrane (IM) from Mycobacterium smegmatis by isolating and reconstituting lipids from these regions to form model membranes mimicking the natural cell envelope. Using methods such as atomic force microscopy and spectroscopy, infrared spectroscopy, time-resolved fluorescence spectroscopy, two-photon and confocal microscopy, we provide crucial insights into the lateral organization, fluidity and domain architecture of mycobacterial membrane lipids and its correlation to lipid structure. Our findings may facilitate identification of compounds with membrane perturbing activity against specific membranous regions of mycobacterial envelope. We also foresee our characterized mycobacterial model membrane system to serve as molecularly complete membrane scaffolds for anti-tubercular drug membrane screening in future.