Yeast cohesin complex embraces 2 micron plasmid sisters in a tri-linked catenane complex

Area of Science:

• Cell biology
• Molecular genetics
• Biochemistry

Background:

• Sister chromatid cohesion is vital for accurate chromosome segregation in eukaryotes.
• Cohesin, a multi-subunit protein complex, mediates this cohesion.
• The 2 micron circle plasmid in Saccharomyces cerevisiae serves as a model for studying cohesion.

Purpose of the Study:

• To investigate the role and mechanism of cohesin in the stable propagation of the 2 micron plasmid.
• To determine the structural model of cohesin responsible for plasmid cohesion.

Main Methods:

• Analysis of cohesin-plasmid interactions in metaphase cell lysates and enriched fractions.
• Methods to release plasmids from cohesin, including DNA linearization and cohesin subunit cleavage.
• Biochemical assays using differentially tagged cohesin molecules to test structural models.

Main Results:

• Plasmid molecules were found topologically trapped by cohesin rings.
• Release of plasmids required DNA linearization or cohesin subunit cleavage.
• Data refuted the hand-cuff and bracelet models, supporting a single monomeric cohesin ring model.

Conclusions:

• A single monomeric cohesin ring model best explains the cohesion of sister 2 micron plasmids.
• This mechanism facilitates sister-to-sister segregation via cohesin disassembly during anaphase.
• Plasmid partitioning loci may share mechanistic similarities with centromeres.