Loop extrusion by structural maintenance of chromosomes (SMC) complexes has been proposed as a mechanism to organize chromatin in interphase and metaphase. However, the requirements for chromatin organization in these cell cycle phases are different, and it is unknown whether loop extrusion dynamics and the complexes that extrude DNA also differ. Here, we used egg extracts to reconstitute and image loop extrusion of single DNA molecules during the cell cycle. We show that loops form in both metaphase and interphase, but with distinct dynamic properties. Condensin extrudes DNA loops non-symmetrically in metaphase, whereas cohesin extrudes loops symmetrically in interphase. Our data show that loop extrusion is a general mechanism underlying DNA organization, with dynamic and structural properties that are biochemically regulated during the cell cycle.
Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA; Division of Hematology, The Children's Hospital of Philadelphia, Philadelphia, PA, USA. Electronic address:
How specific enhancer-promoter pairing is established remains mostly unclear. Besides the CTCF/cohesin machinery, few nuclear factors have been studied for a direct role in physically connecting regulatory elements. Using a murine erythroid cell model, we show via acute degradation experiments that LDB1 directly and broadly promotes connectivity among regulatory elements.
Gladstone Institute for Data Science and Biotechnology, Gladstone Institutes, San Francisco, CA, USA; Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA, USA. Electronic address:
CCCTC-binding factor (CTCF) is a key regulator of 3D genome organization and transcriptional activity. Beyond its well-characterized role in facilitating cohesin-mediated loop extrusion, CTCF exhibits several cohesin-independent activities relevant to chromatin structure and various nuclear processes. These functions include patterning of nucleosome arrangement and chromatin accessibility through interactions with ATP-dependent chromatin remodelers.
Structural maintenance of chromosomes (SMC) protein complexes, including cohesin, condensin, and the Smc5/6 complex, are integral to various processes in chromosome biology. Despite their distinct roles, these complexes share two key properties: the ability to extrude DNA into large loop structures and the capacity to alter the superhelicity of the DNA double helix. In this review, we explore the influence of eukaryotic SMC complexes on DNA topology, debate its potential physiological function, and discuss new structural insights that may explain how these complexes mediate changes in DNA topology.
During mitosis, the condensin I and II complexes compact chromatin into chromosomes. Loss of the chromokinesin, KIF4A, results in reduced condensin I association with chromosomes, but the molecular mechanism behind this phenotype is unknown. In this study, we reveal that KIF4A binds directly to the human condensin I HAWK subunit, NCAPG, via a conserved disordered short linear motif (SLiM) located in its C-terminal tail.
Department of Functional Genomics and Cancer, Institute of Genetics and Molecular and Cellular Biology (IGBMC), UMR7104, Centre National de la Recherche Scientifique, U1258, Institut National de la Santé et de la Recherche Médicale, University of Strasbourg, 6704 Illkirch, France.
The spatiotemporal configuration of genes with distal regulatory elements is believed to be crucial for transcriptional control, but full mechanistic understanding is lacking. We combine simultaneous live tracking of pairs of genomic loci and nascent transcripts with molecular dynamics simulations to assess the gene and its enhancer. We find that both loci exhibit more constrained mobility than control sequences due to stalled cohesin at CCCTC-binding factor sites.
