SMC protein complexes and higher-order chromosome dynamics

Separating sister chromatids

Loss of cohesion between sister chromatids triggers their segregation during anaphase. Recent work has identified both a cohesin complex that holds sisters together and a sister-separating protein, separin, that destroys cohesion. Separins are bound by inhibitory proteins whose proteolysis at the metaphase-anaphase transition is mediated by the anaphase-promoting complex and its activator protein CDC20 (APCCDC20). When chromosomes are misaligned, a surveillance mechanism (checkpoint) blocks sister separation by inhibiting APCCDC20. Defects in this apparatus are implicated in causing aneuploidy in human cells.

Sister chromatid cohesion in mitosis

Sister chromatid cohesion is essential for accurate chromosome segregation during the cell cycle. Newly identified structural proteins are required for sister chromatid cohesion and there may be a link in some organisms between the processes of cohesion and condensation. Proteins that induce and regulate the separation of sister chromatids have also been recently identified.

ATP-dependent aggregation of single-stranded DNA by a bacterial SMC homodimer

SMC (structural maintenance of chromosomes) proteins are putative ATPases that are highly conserved among Bacteria, Archaea and Eucarya. Eukaryotic SMC proteins are implicated in a diverse range of chromosome dynamics including chromosome condensation, dosage compensation and recombinational repair. In eukaryotes, two different SMC proteins form a heterodimer, which in turn acts as the core component of a large protein complex. Despite recent progress, no ATP-dependent activity has been found in individual SMC subunits. We report here the first biochemical characterization of a bacterial SMC protein from Bacillus subtilis. Unlike eukaryotic versions, the B.subtilis SMC protein (BsSMC) is a simple homodimer with no associated subunits. It binds preferentially to single-stranded DNA (ssDNA) and has a ssDNA-stimulated ATPase activity. In the presence of ATP, BsSMC forms large nucleoprotein aggregates in a ssDNA-specific manner. Proteolytic cleavage of BsSMC is changed upon binding to ATP and ssDNA. The energy-dependent aggregation of ssDNA might represent a primitive type of chromosome condensation that occurs during segregation of bacterial chromosomes.

Phosphorylation and activation of 13S condensin by Cdc2 in vitro

13S condensin is a multisubunit protein complex essential for mitotic chromosome condensation in Xenopus egg extracts. Purified 13S condensin introduces positive supercoils into DNA in the presence of topoisomerase I and adenosine triphosphate in vitro. The supercoiling activity of 13Scondensin was regulated by mitosis-specific phosphorylation. Immunodepletion, in vitro phosphorylation, and peptide-mapping experiments indicated that Cdc2 is likely to be the kinase that phosphorylates and activates 13S condensin. Multiple Cdc2 phosphorylation sites are clustered in the carboxyl-terminal domain of the XCAP-D2 (Xenopus chromosome-associated polypeptide D2) subunit. These results suggest that phosphorylation of 13Scondensin by Cdc2 may trigger mitotic chromosome condensation in vitro.

Sulfolobus genome: from genomics to biology

Major progress in sequencing the genome of Sulfolobus solfataricus has been closely concerted with the characterization and sequencing of many extrachromosomal genetic elements, including viruses, cryptic plasmids and conjugative plasmids, as well as mobile archaeal introns and transposons. The latter have provided a basis for developing the first generation of vectors that are now being used to study the genetics of Sulfolobus and other Archaea.

Mutations in fission yeast Cut15, an importin alpha homolog, lead to mitotic progression without chromosome condensation

Chromosome condensation is a major mitotic event. Fission yeast mutations in topoisomerase II and condensin subunits produce the characteristic 'cut' phenotypes, in which the septum bisects the nuclear material in the absence of normal condensation and sister chromatid separation. We show here that the same condensation defect is produced in cut15 temperature-sensitive mutants at the restrictive temperature (36 degrees C). The gene product of cut15+ is, surprisingly, very similar to importin alpha, which binds proteins containing a nuclear localization signal (NLS) and forms the heterodimer with importin beta that mediates translocation through the nuclear pore complex. We show that in a nuclear import assay, purified Cut15 protein behaved identically to mammalian importin alpha but mutant Cut15 did not. Mutant Cut15 failed to bind an NLS-containing protein in vitro but could still bind importin beta. Unexpectedly, however, NLS proteins were imported into the nucleus in cut15 mutants. Cut15 is thus essential for mitotic chromosome condensation, but its role in nuclear import might be dispensable. Green fluorescent protein (GFP)-tagged Cut15 was enriched within the nucleus specifically during prometaphase-metaphase, so the interaction of Cut15 with nuclear NLS proteins during mitosis might be important for condensation.