The Actinobacterial mce Operon: Structure and Functions

The results of studies on mce operons of the causative agent Mycobacterium tuberculosis and actinobacteria are summarized. Mce transporters belong to a group of cell-wall proteins. The genome of M. tuberculosis contains four mce operons that encode complexes, each of which consists of two integral membrane proteins (YrbEB) followed by six other Mce proteins (MceA–F). These proteins are functionally similar to ABC transporters. Despite their significant role in the pathogenesis of M. tuberculosis infection, Mce proteins remain poorly studied due to their complex structure and operon regulation. However, a number of their functions have been characterized; they include the penetration of host cells by M. tuberculosis and its survival, as well as the transport of cholesterol and mycolic acids, which are pathogenicity factors of no small importance. The expression of mce operons depends on many factors, such as the growth phase, the culture medium, and the localization of M. tuberculosis infection. Today, strains of M. tuberculosis and M. bovis with deleted mce operons are considered candidates for the development of new attenuated vaccines that might one day replace the M. bovis BCG vaccine, which is no longer deemed strong enough to hold back the tuberculosis epidemic.