Chromosome Scaffold is a Double-Stranded Assembly of Scaffold Proteins

Abstract

Chromosome higher order structure has been an enigma for over a century. The most important structural finding has been the presence of a chromosome scaffold composed of non-histone proteins; so-called scaffold proteins. However, the organization and function of the scaffold are still controversial. Here, we use three dimensional-structured illumination microscopy (3D-SIM) and focused ion beam/scanning electron microscopy (FIB/SEM) to reveal the axial distributions of scaffold proteins in metaphase chromosomes comprising two strands. We also find that scaffold protein can adaptably recover its original localization after chromosome reversion in the presence of cations. This reversion to the original morphology underscores the role of the scaffold for intrinsic structural integrity of chromosomes. We therefore propose a new structural model of the chromosome scaffold that includes twisted double strands, consistent with the physical properties of chromosomal bending flexibility and rigidity. Our model provides new insights into chromosome higher order structure.

Calcium ions function as a booster of chromosome condensation

“…Major scaffold proteins involved in chromosome organization include condensins, topoisomerase IIα (Topo IIα), and kinesin family 4 (KIF4)1113. Although depletion of these proteins disrupts chromosome structure, X-shaped chromosomes are still formed1314. This suggests that scaffold proteins are not the only factors contributing to chromosome condensation.…”

Structural Basis for Dimer Formation of Human Condensin Structural Maintenance of Chromosome Proteins and Its Implications for Single-stranded DNA Recognition

“…The SMC subunits form pairs at the hinge region to form three different heterodimers that bind three non-SMC subunits; these in turn lead to the establishment of the large, five-subunit condensin, cohesin, and Smc5-6 protein complexes. The Smc1-3 heterodimer is a component of cohesin, which is essential for chromosome cohesion; the Smc2-4 heterodimer is a component of condensin I and condensin II, which are localized to the axis of metaphase chromosomes in a double-stranded manner and contribute to the condensed chromosome structure (3)(4)(5). In addition, as functional studies of condensin, introduction of supercoiling to DNA and reannealing activity that convert single-stranded DNA (ssDNA) into double strand DNA (dsDNA) have been investigated (6 -8), which might be responsible for diverse DNA metabolism mediated by condensin.…”

Condensin in Chromatid Cohesion and Segregation

“…In vertebrate cells, enrichment of hCAP-E in the centromeric regions of mitotic chromosomes, which contain large amounts of repetitive DNAs, was observed [Poonperm et al, 2015], and condensin II is enriched in the kinetochores during mitosis as its phosphorylation level increases in an aurora B and/or hMis6/CENP-I-dependent manner [Ono et al, 2004;Nakazawa et al, 2008]. Enrichment of condensin in pericentromeric regions was also reported in a number of organisms including budding yeast [Eckert et al, 2007;D'Ambrosio et al, 2008;Kim et al, 2013].…”

“…Our previous proteome analysis of human metaphase chromosomes indicated that the molar ratio of hCAP-C to 100 molecules of histone H4 is 0.32 at the most, corresponding to condensin binding to the chromatin fiber every 33 kb on average [Uchiyama et al, 2005]. Our recent study using superresolution microscopy revealed that the chromosome scaffold appears at the axis of the sister chromatids, which was first discovered by Paulson and Laemmli [1977] and was later identified as barber pole model [Maeshima and Laemmli, 2003], and is constructed in a twisted doublestranded fashion in each chromatid [Poonperm et al, 2015] ( fig. 2 ).…”