Mycobacterium leprae upregulates IRGM expression in monocytes and monocyte-derived macrophages.

Leprosy is caused by the infection of Mycobacterium leprae, which evokes a strong inflammatory response and leads to nerve damage. Immunity-related GTPase family M protein (IRGM) plays critical roles in controlling inflammation. The objective of the study was to investigate whether IRGM is involved in the infection of M. leprae. Levels of IRGM were assessed in M. leprae-infected CD4(+) T cells, monocytes, and monocyte-derived macrophages. Data revealed that both protein and mRNA levels of IRGM were increased in monocytes after M. leprae infection. Interestingly, monocyte-derived macrophages showed more prominent IRGM expression with M. leprae infection, whereas the bacteria did not affect IRGM in CD4(+) T cells. Furthermore, we assessed levels of IRGM in CD4(+) T cells and monocytes from 78 leprosy patients and 40 healthy controls, and observed upregulated protein level of IRGM in the monocytes from leprosy patients. Also, IRGM expression was inversely correlated with the severity of the disease. These findings suggested a close involvement of IRGM in M. leprae infection and indicated a potential mechanism of defending M. leprae infection.

Differential dependence of the D1 and D5 dopamine receptors on the G protein gamma 7 subunit for activation of adenylylcyclase.

The D(1) dopamine receptor, G protein gamma(7) subunit, and adenylylcyclase are selectively expressed in the striatum, suggesting their potential interaction in a common signaling pathway. To evaluate this possibility, a ribozyme strategy was used to suppress the expression of the G protein gamma(7) subunit in HEK 293 cells stably expressing the human D(1) dopamine receptor. Prior in vitro analysis revealed that the gamma(7) ribozyme possessed cleavage activity directed exclusively toward the gamma(7) RNA transcript (Wang, Q., Mullah, B., Hansen, C., Asundi, J., and Robishaw, J. D. (1997) J. Biol. Chem. 272, 26040-26048). In vivo analysis of cells transfected with the gamma(7) ribozyme showed a specific reduction in the expression of the gamma(7) protein. Coincident with the loss of the gamma(7) protein, there was a noticeable reduction in the expression of the beta(1) protein, confirming their interaction in these cells. Finally, functional analysis of ribozyme-mediated suppression of the beta(1) and gamma(7) proteins revealed a significant attenuation of SKF81297-stimulated adenylylcyclase activity in D(1) dopamine receptor-expressing cells. By contrast, ribozyme-mediated suppression of the beta(1) and gamma(7) proteins showed no reduction of SKF81297-stimulated adenylylcyclase activity in D(5) dopamine receptor-expressing cells. Taken together, these data indicate that the structurally related D(1) and D(5) dopamine receptor subtypes utilize G proteins composed of distinct betagamma subunits to stimulate adenylylcyclase in HEK 293 cells. Underscoring the physiological relevance of these findings, single cell reverse transcriptase-polymerase chain reaction analysis revealed that the D(1) dopamine receptor and the G protein gamma(7) subunit are coordinately expressed in substance P containing neurons in rat striatum, suggesting that the G protein gamma(7) subunit may be a new target for drugs to selectively alter dopaminergic signaling within the brain.

Heterogeneous processing of a G protein gamma subunit at a site critical for protein and membrane interactions.

The G protein gamma5 subunit is selectively associated with specific G protein alpha subunits [Wilcox, M. D., et al. (1995) J. Biol. Chem. 270, 4189] and is localized preferentially in focal adhesion plaques [Hansen, C. A., et al. (1996) J. Cell Biol. 126, 811]. What determines the differential association of G proteins and their subunits with specific cellular structures or compartments is not clear, but one factor could be variation in the pattern of processing of the proteins. To study gamma5 subunit diversity and modifications, G protein subunits were fractionated on an HPLC phenyl column and analyzed with a gamma5-specific antiserum. The gamma5 eluted from the column as two peaks of immunoreactivity. Analysis by matrix-assisted laser desorption ionization (MALDI) mass spectrometry and electrospray ionization tandem mass spectrometry revealed that the first immunoreactive peak corresponded to the predicted gamma5 isoform (N-terminally acetylated after removal of methionine, C-terminally geranylgeranylated and carboxymethylated with removal of the last three amino acids), while the second peak of immunoreactivity contained a gamma5 isoform isoprenylated at the C-terminus but retaining its three terminal amino acids. This alternatively processed protein is the predominant gamma5 subunit isoform associated with Go and Gi proteins purified from bovine brain. These results describe a new C-terminal processing pattern for G protein gamma subunits and establish the principle that G protein gamma subunits can be heterogeneously modified at their C-termini. This is a site on the gamma subunit critical for membrane and protein-protein interactions of G proteins. These results open the possibility that one determinant of the localization of G proteins in cells could be the pattern of processing of their gamma subunit constituents.

Mycobacterial phagosome maturation, rab proteins, and intracellular trafficking.

One of the most prominent features of pathogenic mycobacteria, which include the potent human pathogens Mycobacterium tuberculosis and Mycobacterium leprae and their opportunistic relatives Mycobacterium avium and Mycobacterium marinum, is their ability to survive and multiply in phagosomes of mononuclear phagocytic cells. The phagocytosed mycobacteria reside in a vacuolar compartment which is exempted from maturation into the phagolysosome. Recently, the arrest of the maturation of phagosomes containing M. tuberculosis complex organisms (Mycobacterium bovis BCG) has been linked to the accumulation on the phagosomal membrane of the small GTP binding protein rab5, specific for the control of fusion within the early endosomal compartment. Furthermore, M. bovis BCG phagosome is devoid of rab7, a rab protein associated with the late endosome. The selective accumulation of rab5 and exclusion of rab7 defines the check point that has been compromised in mycobacterial phagosome maturation. Here we summarize these observations and relates them to other phenomena in the area of membrane and protein trafficking with the emphasis on phagosomes containing intracellular pathogens.

Purified internal G-domain of translational initiation factor IF-2 displays guanine nucleotide binding properties.

Translational initiation factor IF-2 is involved in a multistep pathway leading to the synthesis of the first peptide bond. IF-2 is a guanine nucleotide binding protein (G-protein) and catalyzes GTP hydrolysis in the presence of ribosomes. According to sequence homologies with other G-proteins, particularly EF-Tu, a theoretical model for the tertiary structure of the putative G-domain of IF-2 has been previously proposed [Cenatiempo, Y., Deville, F., Dondon, J., Grunberg-Manago, M., Hershey, J. W. B., Hansen, H. F., Petersen, H. U., Clark, B. F. C., Kjeldgaard, M., La Cour, T. F. M., Mortensen, K. K., & Nyborg, J. (1987) Biochemistry 26, 5070-5076]. A short fragment of IF-2 encompassing the putative G-domain was purified by limited proteolysis of a chimeric protein, synthesized from a gene fusion, between a segment of the IF-2 gene and lacZ. The N- and C-terminal sequences of this IF-2 peptide were characterized. Its calculated length is 181 amino acids and its molecular mass 19.4 kDa, whereas it migrates at 14 kDa in SDS-polyacrylamide gels. This segment of IF-2 can form binary complexes with GDP and can be cross-linked to GTP, therefore indicating that it really corresponds to the G-domain. However, in contrast to the situation described for the purified G-domain of EF-Tu, the IF-2 fragment did not hydrolyze GTP even in the presence of ribosomes. It is assumed that active centers of IF-2 located outside the G-domain are needed for the latter reaction.(ABSTRACT TRUNCATED AT 250 WORDS)