Abstract
Phosphatidylinositol mannosides (PIM) and their multiglycosylated counterparts, lipomannan (LM) and
lipoarabinomannan (LAM), are complex glycolipids and lipoglycans found in the cell envelopes of all
mycobacterial species. They play various essential although poorly defined roles in mycobacterial physiology
and are important immunomodulatory molecules in the course of tuberculosis and leprosy as well as key
ligands promoting the entry of mycobacteria and their survival within phagocytic and non-phagocytic cells.
Although much progress has been made over the last 25 years in elucidating the structures and biosynthesis
of these molecules, fundamental questions remain about the pathways leading to their biosynthesis and
translocation to the cell surface. Furthermore, while the pleiotropic biological activities displayed by purified
PIM, LM and LAM in cellular models suggest that they play important roles in pathogenesis, studies aimed at
validating this assumption and precisely delineating their contribution to host-pathogen interactions when
carried by intact bacilli are still limited by the paucity of mutants deficient in well-defined aspects of the
biosynthesis and export of these molecules that are available.
We propose to pursue structural, genetic and biochemical studies toward a complete definition of the structure
(Aim 1), biosynthesis (Aim 2) and export (Aim 3) of PIM, LM and LAM. Completing our understanding of PIM,
LM and LAM biogenesis, in addition to providing fundamental knowledge about the biochemistry of
Mycobacterium tuberculosis (Mtb), is expected to lead to the discovery of essential enzymes and transporters
which, much like the arabinosyltransferases of the Emb family and the epimerase DprE1, could provide new
opportunities for anti-tuberculosis drug development. The availability of recombinant strains accumulating
structurally defined biosynthetic precursors will facilitate structure-function relationship studies, and that of
defined Mtb mutants deficient in various aspects of PIM, LM and LAM synthesis will allow a precise
assessment of the contribution of these molecules to the immunopathogenesis of tuberculosis.
Abbreviations:
AG, arabinogalactan; AM, arabinomannan; AcylT, acyltransferase; Araf, arabinofuranosyl; AraT, arabinosyltransferase; CZE, capillary
zone electrophoresis; DOC, deoxycholate; GT, glycosyltransferase; Ino, myo-Inositol; LAM, lipoarabinomannan; LM, lipomannan; LPS,
lipopolysaccharide; MALDI-TOF, Matrix-Assisted Laser desorption/ionization time of flight; Manp, mannopyranosyl; ManT,
mannosyltransferase; MPI, mannosylated phosphatidylinositol; MS, mass spectrometry; MTX, methyl-thio-xylose; ORF, open reading
frame; OM, outer membrane; PIM, phosphatidyl-myo-inositol mannosides; TLC, thin-layer chromatography.
Nomenclature:
PIM is used to describe the global family of phosphatidylinositol mannosides that carries one to four fatty acids (attached to the glycerol,
inositol and/or mannose) and one to six mannose residues. In AcXPIMY, x refers to the number of acyl groups esterified to available
hydroxyls on the mannose or myo-inositol residues, y refers to the number of mannose residues; e.g. Ac1PIM1 corresponds to the
phosphatidylinositol mono-mannoside PIM1 carrying two acyl groups attached to the glycerol (the diacylglycerol substituent) and one
acyl group esterified to the mannose residue.