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.
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.
The condensin protein complex plays a key role in the structural organization of genomes. How the ATPase activity of its SMC subunits drives large-scale changes in chromosome topology has remained ...unknown. Here we reconstruct, at near-atomic resolution, the sequence of events that take place during the condensin ATPase cycle. We show that ATP binding induces a conformational switch in the Smc4 head domain that releases its hitherto undescribed interaction with the Ycs4 HEAT-repeat subunit and promotes its engagement with the Smc2 head into an asymmetric heterodimer. SMC head dimerization subsequently enables nucleotide binding at the second active site and disengages the Brn1 kleisin subunit from the Smc2 coiled coil to open the condensin ring. These large-scale transitions in the condensin architecture lay out a mechanistic path for its ability to extrude DNA helices into large loop structures.
Display omitted
•Smc4 and Smc2 ATPase head structures reorganize upon ATP binding and dimerization•A Q-loop-mediated switch releases the Ycs4 HEAT-repeat subunit from the Smc4 head•The Smc2 head engages with the ATP-bound Smc4 head into an asymmetric heterodimer•Head dimerization releases the Brn1 kleisin from Smc2 via coiled-coil rotation
Hassler et al. report structural and functional insights into the enzymatic core of the condensin protein complex that reveal large-scale conformational changes upon ATP binding by and subsequent dimerization of its catalytic SMC head domains. These movements presumably power the condensin-mediated extrusion of DNA loops during mitotic chromosome formation.
Condensin is a conserved SMC complex that uses its ATPase machinery to structure genomes, but how it does so is largely unknown. We show that condensin’s ATPase has a dual role in chromosome ...condensation. Mutation of one ATPase site impairs condensation, while mutating the second site results in hyperactive condensin that compacts DNA faster than wild-type, both in vivo and in vitro. Whereas one site drives loop formation, the second site is involved in the formation of more stable higher-order Z loop structures. Using hyperactive condensin I, we reveal that condensin II is not intrinsically needed for the shortening of mitotic chromosomes. Condensin II rather is required for a straight chromosomal axis and enables faithful chromosome segregation by counteracting the formation of ultrafine DNA bridges. SMC complexes with distinct roles for each ATPase site likely reflect a universal principle that enables these molecular machines to intricately control chromosome architecture.
Display omitted
•Condensin’s two ATPase sites have distinct roles in structuring chromosomes•One ATPase site drives loop formation and the second promotes higher-order Z loops•Hyperactive condensin I shortens mitotic chromosomes in the absence of condensin II•Condensin II is key to decatenation and forms a straight chromosomal axis
Chromosome condensation is essential for accurate cell division. Elbatsh et al. find that condensin’s two ATPase sites control condensation through an asymmetric division of tasks. The balanced activity of these sites allows condensin to correctly structure chromosomes. This likely reflects a universal mechanism through which SMC complexes shape genomes in all domains of life.
Maternal safety through pertussis vaccination and subsequent maternal-fetal-antibody transfer are well documented, but information on infant protection from pertussis by such antibodies and by ...subsequent vaccinations is scarce. Since mice are used extensively for maternal-vaccination studies, we adopted that model to narrow those gaps in our understanding of maternal pertussis immunization. Accordingly, we vaccinated female mice with commercial acellular pertussis (aP) vaccine and measured offspring protection against
challenge and specific-antibody levels with or without revaccination. Maternal immunization protected the offspring against pertussis, with that immune protection transferred to the offspring lasting for several weeks, as evidenced by a reduction (4-5 logs,
< 0.001) in the colony-forming-units recovered from the lungs of 16-week-old offspring. Moreover, maternal-vaccination-acquired immunity from the first pregnancy still conferred protection to offspring up to the fourth pregnancy. Under the conditions of our experimental protocol, protection to offspring from the aP-induced immunity is transferred both transplacentally and through breastfeeding. Adoptive-transfer experiments demonstrated that transferred antibodies were more responsible for the protection detected in offspring than transferred whole spleen cells. In contrast to reported findings, the protection transferred was not lost after the vaccination of infant mice with the same or other vaccine preparations, and conversely, the immunity transferred from mothers did not interfere with the protection conferred by infant vaccination with the same or different vaccines. These results indicated that aP-vaccine immunization of pregnant female mice conferred protective immunity that is transferred both transplacentally and
offspring breastfeeding without compromising the protection boostered by subsequent infant vaccination. These results-though admittedly not necessarily immediately extrapolatable to humans-nevertheless enabled us to test hypotheses under controlled conditions through detailed sampling and data collection. These findings will hopefully refine hypotheses that can then be validated in subsequent human studies.
PDF
