| Abstract |
Transcription factors are subject to various post-translational modifications, which regulate their activity at multiple levels. Posttranslational modification by SUMO elicits a repressive effect on many transcription factors. In principle, sumoylation may either influence transcription factor activity on promoters, or it may act indirectly by targeting the modified factors to specific cellular compartments. To provide direct experimental evidence for the above, not necessarily mutually exclusive models, we analysed the role of SUMO modification on the localization and the activity of the orphan nuclear receptor LRH-1. The results demonstrate that sumoylated LRH-1 is exclusively localized in promyelocytic leukemia protein (PML) nuclear bodies and that this association is a dynamic process. Release of LRH-1 from nuclear bodies correlated with its desumoylation, pointing to the pivotal role of SUMO conjugation in keeping LRH-1 in these locations. SUMO-dependent shuttling of LRH-1 into PML bodies defines two spatially separated pools of the protein, of which only the soluble, unmodified one is associated with actively transcribed target genes. The results suggest that SUMO-PML nuclear bodies may primarily function as dynamic molecular reservoirs, controlling the availability of certain transcription factors to active chromatin domains. Mutations in the HNF1β gene, encoding the dimeric POUhomeodomain transcription factor HNF1β; cause various phenotypes including maturity onset diabetes of the young 5 (MODY5). In order to gain insight into the molecular mechanisms underlying this phenotype, in the second part of the thesis four disease-causing mutations of HNF1β were functionally characterized. The transcriptional impairment of those mutants, whose DNA-binding activity was weakly or not affected, correlated with the loss of association with one of the histone acetyltransferases CBP or PCAF. In contrast to wild-type HNF1β, whose transactivation potential depends on the synergistic action of CBP and PCAF, the activity of these mutants was not increased by the synergistic action of these two coactivators or by treatment with the specific histone deacetylase inhibitor TSA. Our findings suggest that the complex syndrome associated with these MODY5 mutations arise from defective transactivation function.
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Μελέτη της ρύθμισης ηπατικών μεταγραφικών παραγόντων: α. Σουμοϋλίωση του LRH-1 β. Μεταλλαγές MODY του HNF1β
