Papic, Natasa (2007) Biochemical Analysis of Immunity-Related GTPase Irga6 In Vivo and In Vitro. The Role of the Myristoyl Group. PhD thesis, Universität zu Köln.
Immunity-Related GTPases (IRGs) are implicated in cell-autonomous resistance against intracellular bacterial and protozoan pathogens in the mouse. Members of IRG family are found to be involved in Toxoplasma gondii-vacuole vesiculation and disruption, in acidification of phagosomes containing Mycobacterium tuberculosis, as well as in induction of autophagy. However, the exact mechanism of their function is unclear. As IRG proteins are not redundant in resisting pathogen infection, they might regulate each other�s activity in vivo. Irga6, a member of the IRG family, accumulate at vacuoles containing T. gondii, in a process that requires intact GTP-binding domain. Although Irga6 is lipid modified in vivo, the role of myristoyl group in Irga6 function and ability to target T. gondii parasitophorous vacuole was not known. Biochemical properties of Irga6 have been extensively studied in vitro. Recombinant nonmyristoylated Irga6wt was shown to form GTP-dependent, catalytically active homooligomers, characterized by the increased GTPase activity. However, there was no information about Irga6 self-association in vivo. In addition, the effect of myristoyl group on the enzymatic properties of Irga6 was not characterized. This study was set to analyze In order to understand the mechanism of Irga6 function in cells, the ability of Irga6 to form GTP-dependent homooligomers in vivo was studied in attempt to analyze the nucleotide state of Irga6 in IFNã-induced cells and if the infection induces the change of the Irga6 activity. Irga6 nucleotide state in vivo is regulated by other members of IRG family and by the infectious status of the cell. Irga6 forms GTP-dependent homooligomers in vivo, in the absence of other IFNã-induced factors. In IFNã-induced cells, however, Irga6 is kept at the ER in an inactive GDP-bound form through interactions with members of the GMS family. Infection with T. gondii results in the relocalisation of the activated, GTP-bound Irga6 to the parasitophorous vacuole membrane. GTP binding promotes the conformational change of Irga6, which involves GTP-binding domain, N-terminal 12 amino acids and the myristoyl group. Myristoylation of Irga6 protein, itself, is required for the efficient binding of the enzyme to the vacuoles containing T. gondii. Recombinant myristoylated Irga6wt and nonmyristoylated Irga6-G2A proteins show striking differences in their biochemical properties in vitro. In contrast to Irga6-G2A, which hydrolysis GTP to GDP only, myristoylated Irga6wt hydrolysis GTP to GDP and GMP Thus, both in vivo and in vitro data suggest that the myristoyl group plays an important role in Irga6 enzymatic activity and, therefore, in Irga6 function in cells.
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