SMC-mediated chromosome organization: Does loop extrusion explain it all?

In recent years, loop extrusion has attracted much attention as a general mechanism of chromosome organization mediated by structural maintenance of chromosomes (SMC) protein complexes, such as condensin and cohesin. Despite accumulating evidence in support of this mechanism, it is not fully established whether or how loop extrusion operates under physiological conditions, or whether any alternative or additional SMC-mediated mechanisms operate in the cell. In this review, we summarize non-loop extrusion mechanisms proposed in the literature and clarify unresolved issues to further enrich our understanding of how SMC protein complexes work.

Regulation of condensin II by self-suppression and release mechanisms.

In vertebrates, two distinct condensin complexes, condensin I and condensin II, cooperate to drive mitotic chromosome assembly. It remains largely unknown how the two complexes differentially contribute to this process at a mechanistic level. We have previously dissected the role of individual subunits of condensin II by introducing recombinant complexes into egg extracts.

Elasticity control of entangled chromosomes: Crosstalk between condensin complexes and nucleosomes.

Condensin-mediated loop extrusion is now considered as the main driving force of mitotic chromosome assembly. Recent experiments have shown, however, that a class of mutant condensin complexes deficient in loop extrusion can assemble chromosome-like structures in Xenopus egg extracts, although these structures are somewhat different from those assembled by wild-type condensin complexes. In the absence of topoisomerase II (topo II), the mutant condensin complexes produce an unusual round-shaped structure termed a bean, which consists of a DNA-dense central core surrounded by a DNA-sparse halo.

Cell cycle-specific loading of condensin I is regulated by the N-terminal tail of its kleisin subunit.

Condensin I is a pentameric protein complex that plays an essential role in mitotic chromosome assembly in eukaryotic cells. Although it has been shown that condensin I loading is mitosis specific, it remains poorly understood how the robust cell cycle regulation of condensin I is achieved. Here, we set up a panel of in vitro assays to demonstrate that cell cycle-specific loading of condensin I is regulated by the N-terminal tail (N-tail) of its kleisin subunit CAP-H.

Molecular dissection of condensin II-mediated chromosome assembly using in vitro assays.

In vertebrates, condensin I and condensin II cooperate to assemble rod-shaped chromosomes during mitosis. Although the mechanism of action and regulation of condensin I have been studied extensively, our corresponding knowledge of condensin II remains very limited. By introducing recombinant condensin II complexes into egg extracts, we dissect the roles of its individual subunits in chromosome assembly.

A loop extrusion-independent mechanism contributes to condensin I-mediated chromosome shaping.

Condensin I is a five-subunit protein complex that is central to mitotic chromosome assembly in eukaryotic cells. Despite recent progress, its molecular mechanisms of action remain to be fully elucidated. By using Xenopus egg extracts as a functional assay, we find that condensin I complexes harboring mutations in its kleisin subunit CAP-H produce chromosomes with confined axes in the presence of topoisomerase IIα (topo IIα) and highly compact structures (termed "beans") with condensin-positive central cores in its absence.

Structural basis of HEAT-kleisin interactions in the human condensin I subcomplex.

Condensin I is a multi-protein complex that plays an essential role in mitotic chromosome assembly and segregation in eukaryotes. It is composed of five subunits: two SMC (SMC2 and SMC4), a kleisin (CAP-H), and two HEAT-repeat (CAP-D2 and CAP-G) subunits. Although balancing acts of the two HEAT-repeat subunits have been demonstrated to enable this complex to support the dynamic assembly of chromosomal axes in vertebrate cells, its underlying mechanisms remain poorly understood.

Modeling the functions of condensin in chromosome shaping and segregation.

The mechanistic details underlying the assembly of rod-shaped chromosomes during mitosis and how they segregate from each other to act as individually mobile units remain largely unknown. Here, we construct a coarse-grained physical model of chromosomal DNA and condensins, a class of large protein complexes that plays key roles in these processes. We assume that condensins have two molecular activities: consecutive loop formation in DNA and inter-condensin attractions.

Dynamic organization of mitotic chromosomes.

The assembly of rod-shaped chromosomes during mitosis is an essential prerequisite for faithful segregation of genetic information into daughter cells. Despite the long history of chromosome research, it is only recently that we have acquired powerful approaches and crucial tools that help to unlock the secret of this seemingly complex process. In particular, in vitro assays, mammalian genetics, Hi-C analyses and computer simulations have provided valuable information during the past two years.

One-year intranasal application of growth hormone releasing peptide-2 improves body weight and hypoglycemia in a severely emaciated anorexia nervosa patient.

Background: In Japan, growth hormone releasing peptide-2 (GHRP-2) is clinically used as a diagnostic agent for growth hormone secretion deficiency, but the therapeutic application of GHRP-2 has not been studied in anorexia nervosa. GHRP-2 reportedly exhibits agonistic action for ghrelin receptor and increases food intake.

Methods: We administered GHRP-2 to a patient with a 20-year history of anorexia nervosa to determine whether GHRP-2 treatment increases food intake and body weight.

Balancing acts of two HEAT subunits of condensin I support dynamic assembly of chromosome axes.

Condensin I is a five-subunit protein complex that plays a central role in mitotic chromosome assembly and segregation in eukaryotes. To dissect its mechanism of action, we reconstituted wild-type and mutant complexes from recombinant subunits and tested their abilities to assemble chromosomes in Xenopus egg cell-free extracts depleted of endogenous condensins. We find that ATP binding and hydrolysis by SMC subunits have distinct contributions to the action of condensin I and that continuous ATP hydrolysis is required for structural maintenance of chromosomes.

Meiotic regulation of TPX2 protein levels governs cell cycle progression in mouse oocytes.

Formation of female gametes requires acentriolar spindle assembly during meiosis. Mitotic spindles organize from centrosomes and via local activation of the RanGTPase on chromosomes. Vertebrate oocytes present a RanGTP gradient centred on chromatin at all stages of meiotic maturation.

XMAP215 is a processive microtubule polymerase.

Fast growth of microtubules is essential for rapid assembly of the microtubule cytoskeleton during cell proliferation and differentiation. XMAP215 belongs to a conserved family of proteins that promote microtubule growth. To determine how XMAP215 accelerates growth, we developed a single-molecule assay to visualize directly XMAP215-GFP interacting with dynamic microtubules.

NDEL1 phosphorylation by Aurora-A kinase is essential for centrosomal maturation, separation, and TACC3 recruitment.

NDEL1 is a binding partner of LIS1 that participates in the regulation of cytoplasmic dynein function and microtubule organization during mitotic cell division and neuronal migration. NDEL1 preferentially localizes to the centrosome and is a likely target for cell cycle-activated kinases, including CDK1. In particular, NDEL1 phosphorylation by CDK1 facilitates katanin p60 recruitment to the centrosome and triggers microtubule remodeling.

CDC2 phosphorylation of the fission yeast dis1 ensures accurate chromosome segregation.

Shortened kinetochore microtubules take separated chromatids to the opposing spindle poles in anaphase. Fission yeast Dis1 belongs to the Dis1/XMAP215/TOG family that is required for proper microtubule dynamics. Here, we report that Dis1is regulated by Cdc2 phosphorylation and that this mitotic phosphorylation ensures the fidelity of chromosome segregation.

Global and local control of microtubule destabilization promoted by a catastrophe kinesin MCAK/XKCM1.

Traditionally, kinesins have been identified as proteins that use the energy of ATP to translocate along microtubules. However, in the last decade some kinesin-like proteins were found to destabilize microtubule ends. The kinesins that destabilize microtubules are known as "catastrophe kinesins".

Aurora A activates D-TACC-Msps complexes exclusively at centrosomes to stabilize centrosomal microtubules.

Centrosomes are the dominant sites of microtubule (MT) assembly during mitosis in animal cells, but it is unclear how this is achieved. Transforming acidic coiled coil (TACC) proteins stabilize MTs during mitosis by recruiting Minispindles (Msps)/XMAP215 proteins to centrosomes. TACC proteins can be phosphorylated in vitro by Aurora A kinases, but the significance of this remains unclear.

Aurora A phosphorylation of TACC3/maskin is required for centrosome-dependent microtubule assembly in mitosis.

Centrosomes act as sites of microtubule growth, but little is known about how the number and stability of microtubules emanating from a centrosome are controlled during the cell cycle. We studied the role of the TACC3-XMAP215 complex in this process by using purified proteins and Xenopus laevis egg extracts. We show that TACC3 forms a one-to-one complex with and enhances the microtubule-stabilizing activity of XMAP215 in vitro.

An essential function of the C. elegans ortholog of TPX2 is to localize activated aurora A kinase to mitotic spindles.

In vertebrates, the microtubule binding protein TPX2 is required for meiotic and mitotic spindle assembly. TPX2 is also known to bind to and activate Aurora A kinase and target it to the spindle. However, the relationship between the TPX2-Aurora A interaction and the role of TPX2 in spindle assembly is unclear.

A comparison of the ability of XMAP215 and tau to inhibit the microtubule destabilizing activity of XKCM1.

During mitosis, microtubules not only grow fast, but also have a high rate of catastrophe. This is achieved in part by the activity of the MAP, XMAP215, which can stimulate the growth rate of microtubules without fully inhibiting the function of the catastrophe-kinesin XKCM1. We do not know whether this activity is particular to XMAP215, or is a general property of all MAPs.

Doublecortin microtubule affinity is regulated by a balance of kinase and phosphatase activity at the leading edge of migrating neurons.

Doublecortin (Dcx) is a microtubule-associated protein that is mutated in X-linked lissencephaly (X-LIS), a neuronal migration disorder associated with epilepsy and mental retardation. Although Dcx can bind ubiquitously to microtubules in nonneuronal cells, Dcx is highly enriched in the leading processes of migrating neurons and the growth cone region of differentiating neurons. We present evidence that Dcx/microtubule interactions are negatively controlled by Protein Kinase A (PKA) and the MARK/PAR-1 family of protein kinases.

XMAP215: a key component of the dynamic microtubule cytoskeleton.

Microtubules are essential for various cellular processes including cell division and intracellular organization. Their function depends on their ability to rearrange their distribution at different times and places. Microtubules are dynamic polymers and their behaviour is described as dynamic instability.