Die Rolle des Proteins "Scaffold Attachment Factor A" (SAF-A) beim Aufbau und der Architektur des Zellkerns

Abstract

The cell nucleus is a highly ordered and complex organelle that can be subdivided into different subcompartiments like chromosome territories, nucleoli or nuclear bodies. An underlying, insoluble structure, called the nuclear matrix, is discussed controversially to be involved in organizing and integrating nuclear function and architecture. The hnRNP-protein SAF-A (Scaffold Attachment Factor A) is a constituent of nuclear matrix preparations. SAF-A is a highly abundant and multifunctional nuclear protein believed to play essential roles for higher order chromatin organization and nuclear architecture. In the present thesis it could be demonstrated that SAF-A is an important factor for the assembly and the chromatin organization of simple nuclei in Xenopus egg extracts. Further on, it was observed that nascent DNA and replication factors like ORC1, RPA or DNA polymerases colocalize with the filamentous-/patch-like SAF-A structures in simple nuclei. Therefore, SAF-A seems to be somehow (more structurally than functionally) involved in the process of DNA replication. This is in line with experiments in complex nuclei of eukaryotic cells. Expression of the truncated SAF-A construct C280 in HEK293 cells interferes with the proper localization of SAF-A and affects chromatin organization, DNA replication and cell cycle progression.Mobility measurements of the fusion construct SAF-A:eGFP in living cells using FRAP revealed that a large fraction of the protein is quite immobile. This is in fact the first time that a component of the inner nuclear matrix shows a very low mobility in vivo.Another interesting observation is the stable accumulation of SAF-A in regions of inactive X-chromosomes (Xi) pointing to an involvement of the protein in the maintenance of the silent Xi-status.