ABSTRACT
Structural maintenance of chromosomes (SMC) protein complexes structure genomes by extruding DNA loops, but the molecular mechanism that underlies their activity has remained unknown. We show that the active condensin complex entraps the bases of a DNA loop transiently in two separate chambers. Single-molecule imaging and cryo-electron microscopy suggest a putative power-stroke movement at the first chamber that feeds DNA into the SMC-kleisin ring upon adenosine triphosphate binding, whereas the second chamber holds on upstream of the same DNA double helix. Unlocking the strict separation of "motor" and "anchor" chambers turns condensin from a one-sided into a bidirectional DNA loop extruder. We conclude that the orientation of two topologically bound DNA segments during the SMC reaction cycle determines the directionality of DNA loop extrusion.
Subject(s)
Adenosine Triphosphatases , DNA-Binding Proteins , DNA , Multiprotein Complexes , Adenosine Triphosphatases/chemistry , Cryoelectron Microscopy , DNA/chemistry , DNA-Binding Proteins/chemistry , Multiprotein Complexes/chemistry , Nucleic Acid Conformation , Single Molecule ImagingABSTRACT
Complexes containing a pair of structural maintenance of chromosomes (SMC) family proteins are fundamental for the three-dimensional (3D) organization of genomes in all domains of life. The eukaryotic SMC complexes cohesin and condensin are thought to fold interphase and mitotic chromosomes, respectively, into large loop domains, although the underlying molecular mechanisms have remained unknown. We used cryo-EM to investigate the nucleotide-driven reaction cycle of condensin from the budding yeast Saccharomyces cerevisiae. Our structures of the five-subunit condensin holo complex at different functional stages suggest that ATP binding induces the transition of the SMC coiled coils from a folded-rod conformation into a more open architecture. ATP binding simultaneously triggers the exchange of the two HEAT-repeat subunits bound to the SMC ATPase head domains. We propose that these steps result in the interconversion of DNA-binding sites in the catalytic core of condensin, forming the basis of the DNA translocation and loop-extrusion activities.
Subject(s)
Carrier Proteins/chemistry , Chromosomal Proteins, Non-Histone/chemistry , Nuclear Proteins/chemistry , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae/chemistry , Adenosine Triphosphatases/chemistry , Adenosine Triphosphatases/metabolism , Adenosine Triphosphatases/ultrastructure , Adenosine Triphosphate/metabolism , Carrier Proteins/metabolism , Carrier Proteins/ultrastructure , Cell Cycle Proteins , Chromosomal Proteins, Non-Histone/metabolism , Chromosomal Proteins, Non-Histone/ultrastructure , Cryoelectron Microscopy , DNA-Binding Proteins/chemistry , DNA-Binding Proteins/metabolism , DNA-Binding Proteins/ultrastructure , Models, Molecular , Multiprotein Complexes/chemistry , Multiprotein Complexes/metabolism , Multiprotein Complexes/ultrastructure , Nuclear Proteins/metabolism , Nuclear Proteins/ultrastructure , Protein Conformation , Protein Folding , Protein Multimerization , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae Proteins/ultrastructureABSTRACT
Structural Maintenance of Chromosomes (SMC) protein complexes play key roles in the three-dimensional organization of genomes in all kingdoms of life. Recent insights from chromosome contact mapping experiments and single-molecule imaging assays suggest that these complexes achieve distinct cellular functions by extruding large loops of DNA while they move along the chromatin fiber. In this short review, we summarize recent insights into the molecular architecture of these unconventional DNA motor complexes, their interaction with their DNA substrates, and the remarkable dynamic changes they can undergo during their ATPase reaction cycle.
