The immunomodulatory lipoglycans, lipoarabinomannan and lipomannan, are exposed at the mycobacterial cell surface.

By labeling surface carbohydrates, we found that a pool of lipoglycans, cell wall associated, is exposed at the cell surface of mycobacteria and thus, most probably, inserted in the outer leaflet of the outer membrane. In contrast, plasma membrane anchored lipoglycans are not accessible to surface labeling. This result supports the role of lipoglycans as key immunomodulatory molecules but raises the question of their transport from the plasma membrane, where they are synthesized, to the outermost layers of the envelope, where they can act as modulins. The data are discussed in terms of consequences for cell envelope organization.