Structural basis for HCMV Pentamer receptor recognition and antibody neutralization

Human cytomegalovirus (HCMV) represents the viral leading cause of congenital birth defects and uses the gH/ gL/UL128-130-131A complex (Pentamer) to enter different cell types, including epithelial and endothelial cells. Upon infection, Pentamer elicits the most potent neutralizing response against HCMV, representing a key vaccine candidate. Despite its relevance, the structural basis for Pentamer receptor recognition and antibody neutralization is largely unknown. Here, we determine the structures of Pentamer bound to neuropilin 2 (NRP2) and a set of potent neutralizing antibodies against HCMV. Moreover, we identify thrombomodulin (THBD) as a functional HCMV receptor and determine the structures of the Pentamer-THBD complex. Unexpectedly, both NRP2 and THBD also promote dimerization of Pentamer. Our results provide a framework for understanding HCMV receptor engagement, cell entry, antibody neutralization, and outline strategies for antiviral therapies against HCMV

Cryo-EM structures of holo condensin reveal a subunit flip-flop mechanism

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

Condensin Smc2-Smc4 Dimers Are Flexible and Dynamic

Structural maintenance of chromosomes (SMC) protein complexes, including cohesin and condensin, play key roles in the regulation of higher-order chromosome organization. Even though SMC proteins are thought to mechanistically determine the function of the complexes, their native conformations and dynamics have remained unclear. Here, we probe the topology of Smc2-Smc4 dimers of the S. cerevisiae condensin complex with high-speed atomic force microscopy (AFM) in liquid. We show that the Smc2-Smc4 coiled coils are highly flexible polymers with a persistence length of only ~4 nm. Moreover, we demonstrate that the SMC dimers can adopt various architectures that interconvert dynamically over time, and we find that the SMC head domains engage not only with each other, but also with the hinge domain situated at the other end of the ~45-nm-long coiled coil. Our findings reveal structural properties that provide insights into the molecular mechanics of condensin complexes.BN/Cees Dekker La

A quantitative map of human Condensins provides new insights into mitotic chromosome architecture

The two Condensin complexes in human cells are essential for mitotic chromosome structure. We used homozygous genome editing to fluorescently tag Condensin I and II subunits and mapped their absolute abundance, spacing, and dynamic localization during mitosis by fluorescence correlation spectroscopy (FSC)–calibrated live-cell imaging and superresolution microscopy. Although ∼35,000 Condensin II complexes are stably bound to chromosomes throughout mitosis, ∼195,000 Condensin I complexes dynamically bind in two steps: prometaphase and early anaphase. The two Condensins rarely colocalize at the chromatid axis, where Condensin II is centrally confined, but Condensin I reaches ∼50% of the chromatid diameter from its center. Based on our comprehensive quantitative data, we propose a three-step hierarchical loop model of mitotic chromosome compaction: Condensin II initially fixes loops of a maximum size of ∼450 kb at the chromatid axis, whose size is then reduced by Condensin I binding to ∼90 kb in prometaphase and ∼70 kb in anaphase, achieving maximum chromosome compaction upon sister chromatid segregation