An inhibitor of exported Mycobacterium tuberculosis glutamine synthetase selectively blocks the growth of pathogenic mycobacteria in axenic culture and in human monocytes: Extracellular proteins as potential novel drug targets
G. Harth et Ma. Horwitz, An inhibitor of exported Mycobacterium tuberculosis glutamine synthetase selectively blocks the growth of pathogenic mycobacteria in axenic culture and in human monocytes: Extracellular proteins as potential novel drug targets, J EXP MED, 189(9), 1999, pp. 1425-1435
Mycobacterium tuberculosis and other pathogenic mycobacteria export abundan
t quantities of proteins into their extracellular milieu when growing eithe
r axenically or within phagosomes of host cells. One major extracellular pr
otein, the enzyme glutamine synthetase, is of particular interest because o
f its Link to pathogenicity. Pathogenic mycobacteria, but not nonpathogenic
mycobacteria, export large amounts of this protein. Interestingly, export
of the enzyme is associated with the presence of a poly-L-glutamate/glutami
ne structure in the mycobacterial cell wall. In this study, we investigated
the influence of glutamine synthetase inhibitors on the growth of pathogen
ic and nonpathogenic mycobacteria and on the poly-L-glutamate/glutamine cel
l wall structure.
The inhibitor L-methionine-S-sulfoximine rapidly inactivated purified M. tu
berculosis glutamine synthetase, which was 100-fold more sensitive to this
inhibitor than a representative mammalian glutamine synthetase. Added to cu
ltures of pathogenic mycobacteria, L-methionine-S-sulfoximine rapidly inhib
ited extracellular glutamine synthetase in a concentration-dependent manner
but had only a minimal effect on cellular glutamine synthetase, a finding
consistent with failure of the drug to cross the mycobacterial cell wall. R
emarkably, the inhibitor selectively blocked the growth of pathogenic mycob
acteria, all of which release glutamine synthetase extracellularly, but had
no effect on nonpathogenic mycobacteria or nonmycobacterial microorganisms
, none of which release glutamine synthetase extracellularly. The inhibitor
was also bacteriostatic for M, tuberculosis in human mononuclear phagocyte
s (THP-1 cells), the pathogen's primary host cells. Paralleling and perhaps
underlying its bacteriostatic effect, the inhibitor markedly reduced the a
mount of poly-L-glutamate/glutamine cell wall structure in M. tuberculosis.
Although it is possible that glutamine synthetase inhibitors interact with
additional extracellular proteins or structures, our findings support the c
oncept that extracellular proteins of M. tuberculosis and other pathogenic
mycobacteria are worthy targets for new antibiotics. Such proteins constitu
te readily accessible targets of these relatively impermeable organisms, wh
ich are rapidly developing resistance to conventional antibiotics.