| Summary: | 博士 === 臺灣大學 === 分子醫學研究所 === 95 === The cytoplasmic domain of common β chain (βc) of human granulocyte- macrophage colony-stimulating factor (GM-CSF)/ Interleukin-3 (IL-3)/ Interleukin-5 (IL-5) receptor contains a region, proximal to the transmembrane domain, which is sufficient to mediate ligand- dependent mitogenic activity. This membrane proximal region has homology with members of the cytokine receptor superfamily and is designated as box 1 and box 2 motifs. Whereas box 1 motif is required for the recruitment and phosphorylation of Janus kinase 2 (JAK2) kinase, the function of box 2 motif remains largely unknown. Here I report the identification of the common beta chain associating protein (CBAP), a putative transmembrane protein, by yeast two-hybrid selection. GST pull-down experiments show that CBAP directly associates with βc via the box 2 motif. Association of CBAP with βc increases in the absence of GM-CSF in vivo and requires the region containing the putative transmembrane domain. Ectopic overexpression of CBAP in hematopoietic cells triggered apoptosis via mitochondria dysfunction, which could be negated by Bcl-2 overexpression. In addition, reduced expression of endogenous CBAP by siRNA significantly inhibited apoptosis induced by GM-CSF deprivation, but did not inhibit other death stimuli or proliferation signals. These findings suggest that the mitochondrial-dependent βc/CBAP pathway is involved in and modulates GM-CSF-deprivation induced apoptosis. Lastly, the physiology role of CBAP was investigated by knocking out mouse cbap gene expression. Cbap deficient mice were viable, fertile and normal appearance with no significant phenotype in my observation.
At present, cytokine receptors mediated survival signals were well established, while the signaling transduced by cytokine receptors during cytokine deprivation has not yet been identified. In this study, we identified a novel apoptogenic protein CBAP which interacts with GM-CSF receptor βc and modulates cellular apoptosis in the absence of GM-CSF. These discoveries not only let us to know the function of the novel protein CBAP but also shed light on the pleiotropism of receptor βc which transduce both positive and negative signaling upon presence and absence of cytokine-a new study field of the cytokine receptors.
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