Condensin and cohesin display different arm conformations with characteristic hinge angles

Abstract

Structural maintenance of chromosomes (SMC) proteins play central roles in higher-order chromosome dynamics from bacteria to humans. In eukaryotes, two different SMC protein complexes, condensin and cohesin, regulate chromosome condensation and sister chromatid cohesion, respectively. Each of the complexes consists of a heterodimeric pair of SMC subunits and two or three non-SMC subunits. Previous studies have shown that a bacterial SMC homodimer has a symmetrical structure in which two long coiled-coil arms are connected by a flexible hinge. A catalytic domain with DNA- and ATP-binding activities is located at the distal end of each arm. We report here the visualization of vertebrate condensin and cohesin by electron microscopy. Both complexes display the two-armed structure characteristic of SMC proteins, but their conformations are remarkably different. The hinge of condensin is closed and the coiled-coil arms are placed close together. In contrast, the hinge of cohesin is wide open and the coiled-coils are spread apart from each other. The non-SMC subunits of both condensin and cohesin form a globular complex bound to the catalytic domains of the SMC heterodimers. We propose that the “closed” conformation of condensin and the “open” conformation of cohesin are important structural properties that contribute to their specialized biochemical and physiological functions.

Molecular and Genetic Analysis of Condensin Function in Vertebrate Cells

“…Electron microscopy studies reveal that the cohesin arms seem to form an open loop, and they are thus topologically in a position to encircle DNA, whereas condensin predominately forms a "lollipop" like structure with the arms tightly apposed to one another (Anderson et al, 2002). Our observations that the condensin complex remains largely intact despite the cleavage of SMC2 are consistent with the notion that condensin arms are apposed in a lollipop structure.…”

Association of cohesin and Nipped-B with transcriptionally active regions of the Drosophila melanogaster genome

“…One is that, in Drosophila, transcription might be needed in many cases to provide a 10-nm chromatin fiber that fits into the 35-nm internal diameter of the cohesin ring (Anderson et al 2002), while in yeast, much of the chromosome already has an accessible structure. For instance, the H1 linker histone that helps form higher order chromatin structures is likely present at most nucleosomes in metazoan organisms, while in yeast, the related Hho1 linker histone is present at low levels and does not globally regulate chromatin structure or gene expression (Freidkin and Katcoff 2001).…”

Roles of the sister chromatid cohesion apparatus in gene expression, development, and human syndromes

“…The N and C termini of Rad21 interact with the head domains of Smc3, and Smc1, respectively, to form a ring-like structure. The Smc3 arm has a kink, which forms a more open structure than seen in other SMC complexes (Anderson et al 2002).…”