Rita Lange

Regulatory interactions between IRG resistance GTPases in the cellular response to Toxoplasma gondii

Members of the immunity‐related GTPase (IRG) family are interferon‐inducible resistance factors against a broad spectrum of intracellular pathogens including Toxoplasma gondii. The molecular mechanisms governing the function and regulation of the IRG resistance system are largely unknown. We find that IRG proteins function in a system of direct, nucleotide‐dependent regulatory interactions between family members. After interferon induction but before infection, the three members of the GMS subfamily of IRG proteins, Irgm1, Irgm2 and Irgm3, which possess an atypical nucleotide‐binding site, regulate the intracellular positioning of the conventional GKS subfamily members, Irga6 and Irgb6. Following infection, the normal accumulation of Irga6 protein at the parasitophorous vacuole membrane (PVM) is nucleotide dependent and also depends on the presence of all three GMS proteins. We present evidence that an essential role of the GMS proteins in this response is control of the nucleotide‐bound state of the GKS proteins, preventing their GTP‐dependent activation before infection. Accumulation of IRG proteins at the PVM has previously been shown to be associated with a block in pathogen replication: our results relate for the first time the enzymatic properties of IRG proteins to their role in pathogen resistance.

Mechanisms Regulating the Positioning of Mouse p47 Resistance GTPases LRG-47 and IIGP1 on Cellular Membranes: Retargeting to Plasma Membrane Induced by Phagocytosis

The recently identified p47 GTPases are one of the most effective cell-autonomous resistance systems known against intracellular pathogens in the mouse. One member of the family, LRG-47, has been shown to be essential for immune control in vivo of Listeria monocytogenes, Toxoplasma gondii, Mycobacterium tuberculosis, and Mycobacterium avium, possibly by promoting acidification of the phagosome. However, the intracellular localization of LRG-47, and the nature of its association with the phagosomal or any other membrane system is unknown. In this study, we show that LRG-47 is a Golgi-associated protein in the IFN-stimulated cell, which is rapidly recruited to active plasma membrane upon phagocytosis and remains associated with phagosomes as they mature. We show that the Golgi localization of LRG-47 is dependent on the integrity of an amphipathic helix near the C terminus, whereas the plasma membrane localization depends on an unidentified signal associated with the G domain. Unlike LRG-47, but like the published p47 resistance GTPase, IGTP, a further p47 GTPase, IIGP1, is associated with the endoplasmic reticulum. However, unlike IGTP, IIGP1 is associated with the endoplasmic reticulum by an N-terminal myristoylation modification. Thus, the p47 GTPases are a diverse battery of intracellular defense factors dynamically associated with different membrane systems.

Tyrosine and serine phosphorylation of the neural cell adhesion molecule L1 is implicated in its oligomannosidic glycan dependent association with NCAM and neurite outgrowth

We have previously shown that a cis interaction between the cell adhesion molecules L1 and NCAM is mediated by N-linked oligomannosidic glycans carried by L1 and that this L1/NCAM association is involved in basal neurite outgrowth from early postnatal cerebellar neurons of mouse brain [R. Horstkorte et al., J. Cell Biol. 121, 1409-1421 (1993)]. Extending these earlier studies we investigated signal transduction mechanisms elicited by this molecular interaction. We show here that phosphorylation of L1 is reduced concomitant with reduced neurite outgrowth when the L1/NCAM interaction is inhibited by oligomannosidic glycopeptides. Similarly, when a peptide of the 4th immunoglobulin (Ig)-like domain of NCAM - representing part of NCAMs carbohydrate-binding site - was added to the culture medium of the cells, neurite outgrowth and phosphorylation of L1 was strongly reduced. No effect on neurite outgrowth and phosphorylation of L1 was observed when cells were maintained in the presence of a peptide comprising part of the 1st Ig-like domain of NCAM or in the presence of the peptide encoded by the variable alternative spliced exon (VASE), which is also located in the 4th Ig-like domain of NCAM. Furthermore, phosphorylation of tyrosine and serine residues of L1 is reduced when the L1/NCAM interaction at the cell surface of cerebellar neurons is perturbed. Our observations suggest that a signal transduction mechanism is implicated in basal neurite outgrowth in which both tyrosine and serine phosphorylation of L1 represent a possible proximal step. Some of these results were presented at the International Glycoconjugate Symposium in Seattle, USA [P. C. Heiland et al., Glycoconj. J. 12, 521(1995)].

Chromosomal gene transfer of human cytosol thymidine kinase into mouse cells

SummaryIf a chromosomal fragment transferred into recipient cells were integrated or strongly associated with a specific recipient chromosome it should segregate with this chromosome in hybrid cells. In order to corroborate this prediction we studied two independent mouse cell clones (“transferent clones”) which had taken up by chromosomal gene transfer a human chromosomal fragment carrying the gene for cytosol thymidine kinase (TKs, E.C.No.2.7.1.75). The following results were obtained.1.Ten somatic cell hybrids isolated after fusion of transferent mouse clones and chinese hamster cells expressed functional human TKs and mouse galactokinase (GALK, E.C.No.2.7.1.6) activity. Counterselected derivatives of all clones had lost human TKs but still expressed mouse GALK. Recently both mouse genes for TKs and GALK have been assigned to mouse chromosome 11 (Kozak and Ruddle, 1977a, McBreen et al., 1977). Therefore our results argue against integration or association of the human gene for TKs at the site of the homologous defective mouse TKs-GALK-region in the genome of transferent clones.2.In three somatic cell hybrids isolated after fusion of microcells from two different transferent mouse clones with established chinese hamster cells only human TKs but not mouse GALK was expressed. Karyotypic analysis of one hybrid suggested the presence of at least one copy of mouse chromosome 9 per hybrid cell. Chromosome 9 and the mouse isozyme activity of mannose phosphate isomerase which had been previously assigned to a gene locus on mouse chromosome 9 were missing in a subclone counter-selected against the presence of TKs activity. These findings suggested that the transferred human gene for TKs may be integrated or strongly associated with mouse chromosome 9 in this transferent mouse clone.3.Two other somatic cell hybrids were analyzed which were derived from a different phenotypically stable transferent mouse clone by similar microcell fusion with the chinese hamster cells. Although these hybrids expressed human TKs activity, no mouse chromosome could be detected in these clones. Possibly after fusion of microcells derived from certain transferent mouse clones the transferred human chromosomal fragment could become translocated from a mouse chromosome to the chinese hamster genome.