Cell cycle-dependent regulation of telomere tethering in the nucleus Katrin Paeschke 1 , Stefan Juranek 1 , Daniela Rhodes 2 * & Hans Joachim Lipps 1 * 1 Institute of Cell Biology, University Witten/Herdecke, Stockumer Strasse 10, 58453, Witten, Germany; Tel: +49-(0)2302-926144; Fax: +49-(0)2302-926220; E-mail: lipps@uni-wh.de; 2 Medical Research Council, Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 2QH, UK; E-mail: rhodes@mrc-lmb.cam.ac.uk * Correspondence Received 5 March 2008. Received in revised form and accepted for publication by Dean Jackson 24 March 2008 Key words: cell, ciliate, G-quadruplex, macronucleus, nuclear matrix, telomere Abstract It is well established that telomeres are tethered in the eukaryotic nucleus, but a detailed analysis of the regulation of telomere attachment throughout the cell cycle is still lacking. We show here that the telomeres in the macronucleus of the ciliate Stylonychia lemnae are bound to a sub-nuclear structure by an interaction of the telomere end-binding protein TEBPa with three SNS proteins that are integral parts of this structure. In the course of replication, the interaction of TEBPa with the SNS proteins is resolved and this process is regulated by cell cycle-specific phosphorylation of the SNS proteins. Our data can be incorporated into a mechanistic model for the regulation of telomere conformation and localization throughout the cell cycle. Abbreviations ATP adenosine triphosphate CDK2 cyclin-dependent kinase 2 PVDF polyvinylidene fluoride SDS-PAGE SDSYpolyacrylamide gel electrophoresis SNS sub-nuclear structure TEBPa telomere end-binding protein alpha TEBPb telomere end-binding protein beta TTBS Tween-supplemented Tris-buffered saline Introduction Telomeres are the terminal structures of eukaryotic chromosomes. Besides their important functions in providing genome stability and the correct replica- tion of chromosomal ends, telomeres also contribute to functional nuclear architecture (Cech 2004). It is well established that in eukaryotes the telomeres are attached to the nuclear envelope and co-purify with components of the sub-nuclear structure, the nuclear matrix/scaffold, and such are thought to have an influence on global regulation of gene expression (de Lange 2002, Gasser 2002, Jonsson & Lipps 2002, Zakian 1995). Localization of telomeres to the nuclear periphery is capable of contributing to heterochromatin formation and has been reported for yeast (Gasser et al. 1998, Andrulis et al. 1998, Taddei & Gasser 2004), Drosophila (Mathog et al. 1984, Hochstrasser & Sedat 1987) and the pathogen- ic protozoa Trypanosoma brucei and Plasmodium falciparum (Perez-Morga et al. 2001, Scherf et al. 2001, Marty et al. 2006). In mammalian cells, telomeres are not found at the nuclear periphery but co-purify in a nuclear scaffold preparation (de Lange 1992). In all organisms, nuclear tethering of telomeres presumably requires at least two components, a telomere-bound protein and at least one protein associated with a sub-nuclear structure, such as the nuclear envelope or nuclear matrix/scaffold. In Saccharomyces cerevisiae, telomeres can be anchored in the nucleus either by the Sir or the Ku complex. Telomere-bound Sir4p interacts with Esc1p at the Chromosome Research (2008) 16:721–728 # Springer 2008 DOI: 10.1007/s10577-008-1222-x