Characterisation of the essential role of the Smc5/6 complex in chromosome metabolism during the eukaryotic cell cycle

Projektinformationen

SMC5_6 ROLE

ID Finanzhilfevereinbarung: 16049

Startdatum 30 September 2005

Enddatum 29 September 2006

Finanziert unter

• Human resources and Mobility in the specific programme for research, technological development and demonstration "Structuring the European Research Area" under the Sixth Framework Programme 2002-2006

Gesamtkosten € 0,00

EU-Beitrag € 40 000,00

Koordiniert durch

UNIVERSITAT DE LLEIDA
Spain

Final Activity Report Summary - SMC5_6 ROLE (Characterisation of the essential role of the Smc5/6 complex in chromosome metabolism during the eukaryotic cell cycle)

Eukaryotic organisms have a cell cycle machinery that orchestrates the sequential ordering of all processes required for cell division. The most fundamental process regulated by the cell cycle is the faithful replication and segregation of all chromosomes. Co-ordination of these events is essential for normal development and for proper inheritance of the genetic material, and ultimately to avoid tumourigenesis and the death of the organism.

Most of these processes are monitored by mechanisms known as checkpoints, which prevent clocking of the cell cycle timer until a certain condition has been met. For example, it has been proposed that checkpoints ensure that DNA replication is completed before mitosis.

Our molecular understanding of chromosome architecture and dynamics has been recently boosted by the characterisation of two protein complexes, cohesin and condensin, and their regulation by the cell cycle. At the core of these two complexes lie members of a family of chromosomal ATPases, the structural maintenance of chromosomes (SMC). There are three different SMC complexes in all eukaryotic cells: cohesin, condensin and the Smc5/6 complex. In concert or shortly before replication, cohesion must be established between sister chromatids and maintained until the metaphase to anaphase transition. Besides, chromosomes must be condensed along their longitudinal axis to yield two condensed sister chromatids tightly paired along their length.

The aim of this research proposal was to determine the ill-defined essential function of the Smc5/6 complex and its relation to chromosome replication, segregation and repair. Using smc5/6 thermosensitive mutants and tagged versions of the Smc5/6 proteins, we have shown that Smc5/6 mutants locate to and aid segregation of specific chromosomal regions, such as the rDNA locus. Importantly, we have shown that in the absence of Smc5-Smc6, chromosome nondisjunction occurs as a consequence of two main problems: (i) defects in recombination-mediated repair and (ii) mitotic entry with unfinished replication despite intact checkpoint responses.

Smc5/6 mutants display abnormally elevated levels of recombination events, which can prevent sister chromatid segregation if left unresolved. Smc5/6 mutants also present a delayed replication pattern, most significantly at natural replication-impeding loci like the ribosomal DNA gene cluster. Eliminating processes that obstruct replication fork progression restores the temporal uncoupling between replication and segregation in smc5-smc6 mutants.

This findings have let us propose, contrary to previously hypothesised, that the completion of replication is not under the surveillance of known checkpoints.