Functional and biochemical characterization of a T cell-associated anti-apoptotic protein, GIMAP6.

GTPases of immunity-associated proteins (GIMAPs) are expressed in lymphocytes and regulate survival/death signaling and cell development within the immune system. We found that human GIMAP6 is expressed primarily in T cell lines. By sorting human peripheral blood mononuclear cells and performing quantitative RT-PCR, GIMAP6 was found to be expressed in CD3+ cells. In Jurkat cells that had been knocked down for GIMAP6, treatment with hydrogen peroxide, FasL, or okadaic acid significantly increased cell death/apoptosis. Exogenous expression of GMAP6 protected Huh-7 cells from apoptosis, suggesting that GIMAP6 is an anti-apoptotic protein. Furthermore, knockdown of GIMAP6 not only rendered Jurkat cells sensitive to apoptosis but also accelerated T cell activation under phorbol 12-myristate 13-acetate/ionomycin treatment conditions. Using this experimental system, we also observed a down-regulation of p65 phosphorylation (Ser-536) in GIMAP6 knockdown cells, indicating that GIMAP6 might display anti-apoptotic function through NF-κB activation. The conclusion from the study on cultured T cells was corroborated by the analysis of primary CD3+ T cells, showing that specific knockdown of GIMAP6 led to enhancement of phorbol 12-myristate 13-acetate/ionomycin-mediated activation signals. To characterize the biochemical properties of GIMAP6, we purified the recombinant GIMAP6 to homogeneity and revealed that GIMAP6 had ATPase as well as GTPase activity. We further demonstrated that the hydrolysis activity of GIMAP6 was not essential for its anti-apoptotic function in Huh-7 cells. Combining the expression data, biochemical properties, and cellular features, we conclude that GIMAP6 plays a role in modulating immune function and that it does this by controlling cell death and the activation of T cells.