Mechanism Of Length Control Research Articles

A frequency-dependent decoding mechanism for axonal length sensing.

We have recently developed a mathematical model of axonal length sensing in which a system of delay differential equations describe a chemical signaling network. We showed that chemical oscillations emerge due to delayed negative feedback via a Hopf bifurcation, resulting in a frequency that is a monotonically decreasing function of axonal length. In this paper, we explore how frequency-encoding of axonal length can be decoded by a frequency-modulated gene network. If the protein output were thresholded, then this could provide a mechanism for axonal length control. We analyze the robustness of such a mechanism in the presence of intrinsic noise due to finite copy numbers within the gene network.

Antenna Mechanism of Length Control of Actin Cables.

Actin cables are linear cytoskeletal structures that serve as tracks for myosin-based intracellular transport of vesicles and organelles in both yeast and mammalian cells. In a yeast cell undergoing budding, cables are in constant dynamic turnover yet some cables grow from the bud neck toward the back of the mother cell until their length roughly equals the diameter of the mother cell. This raises the question: how is the length of these cables controlled? Here we describe a novel molecular mechanism for cable length control inspired by recent experimental observations in cells. This “antenna mechanism” involves three key proteins: formins, which polymerize actin, Smy1 proteins, which bind formins and inhibit actin polymerization, and myosin motors, which deliver Smy1 to formins, leading to a length-dependent actin polymerization rate. We compute the probability distribution of cable lengths as a function of several experimentally tuneable parameters such as the formin-binding affinity of Smy1 and the concentration of myosin motors delivering Smy1. These results provide testable predictions of the antenna mechanism of actin-cable length control.

TGF-β Signaling Regulates the Differentiation of Motile Cilia

The cilium is a small cellular organelle with motility- and/or sensory-related functions that plays a crucial role during developmental and homeostatic processes. Although many molecules or signal transduction pathways that control cilia assembly have been reported, the mechanisms of ciliary length control have remained enigmatic. Here, we report that Smad2-dependent transforming growth factor β (TGF-β) signaling impacts the length of motile cilia at the Xenopus left-right (LR) organizer, the gastrocoel roof plate (GRP), as well as at the neural tube and the epidermis. Blocking TGF-β signaling resulted in the absence of the transition zone protein B9D1/MSKR-1 from cilia in multi-ciliated cells (MCCs) of the epidermis. Interestingly, this TGF-β activity is not mediated by Mcidas, Foxj1, and RFX2, the known major regulators of ciliogenesis. These data indicate that TGF-β signaling is crucial for the function of the transition zone, which in turn may affect the regulation of cilia length.

Molecular ruler determines needle length for the Salmonella Spi-1 injectisome

The type-III secretion (T3S) systems of bacteria are part of self-assembling nanomachines: the bacterial flagellum that enables cells to propel themselves through liquid and across hydrated surfaces, and the injectisome that delivers pathogenic effector proteins into eukaryotic host cells. Although the flagellum and injectisome serve different purposes, they are evolutionarily related and share many structural similarities. Core features to these T3S systems are intrinsic length control mechanisms for external cellular projections: the hook of the flagellum and the injectisome needle. We present evidence that the Spi-1 injectisome, like the Salmonella flagellar hook, uses a secreted molecular ruler, InvJ, to determine needle length. This result supports a universal length control mechanism using molecular rulers for T3S systems.

Transcriptomic analysis reveals the roles of microtubule-related genes and transcription factors in fruit length regulation in cucumber (Cucumis sativus L.).

Cucumber (Cucumis sativus L.) fruit is a type of fleshy fruit that is harvested immaturely. Early fruit development directly determines the final fruit length and diameter, and consequently the fruit yield and quality. Different cucumber varieties display huge variations of fruit length, but how fruit length is determined at the molecular level remains poorly understood. To understand the genes and gene networks that regulate fruit length in cucumber, high throughout RNA-Seq data were used to compare the transcriptomes of early fruit from two near isogenic lines with different fruit lengths. 3955 genes were found to be differentially expressed, among which 2368 genes were significantly up-regulated and 1587 down-regulated in the line with long fruit. Microtubule and cell cycle related genes were dramatically activated in the long fruit, and transcription factors were implicated in the fruit length regulation in cucumber. Thus, our results built a foundation for dissecting the molecular mechanism of fruit length control in cucumber, a key agricultural trait of significant economic importance.

Lipopolysaccharide O antigen size distribution is determined by a chain extension complex of variable stoichiometry in Escherichia coli O9a.

The lengths of bacterial polysaccharides can be critical for their biological function. Unlike DNA or protein synthesis, where polymer length is implicit in the nucleic acid template, the molecular mechanisms for regulating polysaccharide length are poorly understood. Two models are commonly cited: a "molecular clock" regulates length by controlling the duration of the polymer extension process, whereas a "molecular ruler" determines length by measurement against a physical structure in the biosynthetic complex. Escherichia coli O9a is a prototype for the biosynthesis of O polysaccharides by ATP-binding cassette transporter-dependent processes. The length of the O9a polysaccharide is determined by two proteins: an extension enzyme, WbdA, and a termination enzyme, WbdD. WbdD is known to self-oligomerize and also to interact with WbdA. Changing either enzyme's concentration can alter the polysaccharide length. We quantified the O9a polysaccharide length distribution and the enzyme concentration dependence in vivo, then made mathematical models to predict the polymer length distributions resulting from hypothetical length-regulation mechanisms. Our data show qualitative features that cannot be explained by either a molecular clock or a molecular ruler model. Therefore, we propose a "variable geometry" model, in which a postulated biosynthetic WbdA-WbdD complex assembles with variable stoichiometry dependent on relative enzyme concentration. Each stoichiometry produces polymers with a distinct, geometrically determined, modal length. This model reproduces the enzyme concentration dependence and modality of the observed polysaccharide length distributions. Our work highlights limitations of previous models and provides new insight into the mechanisms of length control in polysaccharide biosynthesis.

Inter-telomeric recombination is present in telomerase-positive human cells

Immortal cells require a mechanism of telomere length control in order to divide infinitely. One mechanism is telomerase, an enzyme that compensates the loss of telomeric DNA. The second mechanism is the alternative lengthening of telomeres (ALT) pathway. In ALT pathway cells, homologous recombination between telomeric DNA is the mechanism by which telomere homeostasis is achieved. We developed a novel homologous recombination reporter system that is able to measure inter-telomeric recombination in a sensitive manner. We asked the fundamental question if homologous recombination between different telomeres is present in telomerase-positive cells. In this in vitro study, we showed that homologous recombination between telomeres is detectable in ALT cells with the same frequency as in cells that utilize the telomerase pathway. We further described an ALT cell clone that showed peaks of recombination which were not detected in telomerase-positive clones. In telomerase-positive cells the frequency of inter-telomeric recombination was not increased by shortened telomeres or by a fragile telomere phenotype induced with aphidicolin. ALT cells, in contrast, responded to aphidicolin with an increase in the frequency of recombination. Our results indicate that inter-telomeric recombination is present in both pathways of telomere length control, but the factors that increase recombination are different in ALT and telomerase-positive cells.

KIF19A Is a Microtubule-Depolymerizing Kinesin for Ciliary Length Control

Cilia control homeostasis of the mammalian body by generating fluid flow. It has long been assumed that ciliary length-control mechanisms are essential for proper flow generation, because fluid flow generation is a function of ciliary length. However, the molecular mechanisms of ciliary length control in mammals remain elusive. Here, we suggest that KIF19A, a member of the kinesin superfamily, regulates ciliary length by depolymerizing microtubules at the tips of cilia. Kif19a(-/-) mice displayed hydrocephalus and female infertility phenotypes due to abnormally elongated cilia that cannot generate proper fluid flow. KIF19A localized to cilia tips, and recombinant KIF19A controlled the length of microtubules polymerized from axonemes in vitro. KIF19A had ATP-dependent microtubule-depolymerizing activity mainly at the plus end of microtubules. Our results indicated a molecular mechanism of ciliary length regulation in mammals, which plays an important role in the maintenance of the mammalian body.

Mammalian telomere biology

Review is devoted to detailed consideration of the functioning in normal and immortal cells of one of the main chromosomal regions, telomeres, being dynamic nucleoprotein structures that cap the ends of eukaryotic chromosomes, protecting them from degradation and end-to-end fusion. The role of telomeres in maintenance of genome stability and cell division was also analyzed. Telomere function depends on many interrelated parameters such as telomerase activity, status of the telomere safety complex shelterin and telomere associated proteins (factors of replication, recombination, and reparation of DNA breaks, and so on). We have focused on mechanisms of telomere length control in normal and immortal cells as well as in cells containing active telomerase and cells wherein it is absent. We have analyzed the features attributed to alternative telomere lengthening, namely in view of recently discovered additional mechanism of telomere shortening by trimming of t-cycles. We have viewed a possibility of expression in normal mammalian cells of both telomerase dependent and recombinational ways of telomere length control and the role of shelterin proteins in choice of the one of them as the dominant way. The role oftelomeres in spatial organization of nucleus, in mitosis and meiosis has been also considered. Diversity of telomere organization in mammalians including unusual telomeres in Iberian shrews has been discussed.

Microtubule Length Regulation by Molecular Motors

Length regulation of microtubules (MTs) is essential for many cellular processes. Molecular motors like kinesin-8, which move along MTs and also act as depolymerases, are known as key players in MT dynamics. However, the regulatory mechanisms of length control remain elusive. Here, we investigate a stochastic model accounting for the interplay between polymerization kinetics and motor-induced depolymerization. We determine the dependence of MT length and variance on rate constants and motor concentration. Moreover, our analyses reveal how collective phenomena lead to a well-defined MT length.

TERRA Promotes Telomere Shortening through Exonuclease 1–Mediated Resection of Chromosome Ends

The long noncoding telomeric repeat containing RNA (TERRA) is expressed at chromosome ends. TERRA upregulation upon experimental manipulation or in ICF (immunodeficiency, centromeric instability, facial anomalies) patients correlates with short telomeres. To study the mechanism of telomere length control by TERRA in Saccharomyces cerevisiae, we mapped the transcriptional start site of TERRA at telomere 1L and inserted a doxycycline regulatable promoter upstream. Induction of TERRA transcription led to telomere shortening of 1L but not of other chromosome ends. TERRA interacts with the Exo1-inhibiting Ku70/80 complex, and deletion of EXO1 but not MRE11 fully suppressed the TERRA–mediated short telomere phenotype in presence and absence of telomerase. Thus TERRA transcription facilitates the 5′-3′ nuclease activity of Exo1 at chromosome ends, providing a means to regulate the telomere shortening rate. Thereby, telomere transcription can regulate cellular lifespan through modulation of chromosome end processing activities.

Control of poly(A) tail length

Poly(A) tails have long been known as stable 3' modifications of eukaryotic mRNAs, added during nuclear pre-mRNA processing. It is now appreciated that this modification is much more diverse: A whole new family of poly(A) polymerases has been discovered, and poly(A) tails occur as transient destabilizing additions to a wide range of different RNA substrates. We review the field from the perspective of poly(A) tail length. Length control is important because (1) poly(A) tail shortening from a defined starting point acts as a timer of mRNA stability, (2) changes in poly(A) tail length are used for the purpose of translational regulation, and (3) length may be the key feature distinguishing between the stabilizing poly(A) tails of mRNAs and the destabilizing oligo(A) tails of different unstable RNAs. The mechanism of length control during nuclear processing of pre-mRNAs is relatively well understood and is based on the changes in the processivity of poly(A) polymerase induced by two RNA-binding proteins. Developmentally regulated poly(A) tail extension also generates defined tails; however, although many of the proteins responsible are known, the reaction is not understood mechanistically. Finally, destabilizing oligoadenylation does not appear to have inherent length control. Rather, average tail length results from the balance between polyadenylation and deadenylation.

Control of Mitotic Spindle Length

The mitotic spindle accurately segregates genetic instructions by moving chromosomes to spindle poles (anaphase A) and separating the poles (anaphase B) so that, in general, the chromosomes and poles are positioned near the centers of the nascent daughter cell products of each cell division. Because the size of different types of dividing cells, and thus the spacing of their daughter cell centers, can vary significantly, the length of the metaphase or postanaphase B spindle often scales with cell size. However, significant exceptions to this scaling rule occur, revealing the existence of cell size–independent, spindle-associated mechanisms of spindle length control. The control of spindle length reflects the action of mitotic force-generating mechanisms, and its study may illuminate general principles by which cells regulate the size of internal structures. Here we review molecules and mechanisms that control spindle length, how these mechanisms are deployed in different systems, and some quantitative models that describe the control of spindle length.

Synthesis of galactofuranose-based acceptor substrates for the study of the carbohydrate polymerase GlfT2

Despite the prevalence and importance of carbohydrate polymers, the molecular details of their biosynthesis remain elusive. Many enzymes responsible for the synthesis of carbohydrate polymers require a ‘primer’ or ‘initiator’ carbohydrate sequence. One example of such an enzyme is the mycobacterial galactofuranosyltransferase GlfT2 (Rv3808c), which generates an essential cell wall building block. We recently demonstrated that recombinant GlfT2 is capable of producing a polymer composed of alternating β-(1,5) and β-(1,6)-linked galactofuranose (Gal f) residues. Intriguingly, the length of the polymers produced from a synthetic glycosyl acceptor is consistent with those found in the cell wall. To probe the mechanism by which polymer length is controlled, a collection of initiator substrates has been assembled. The central feature of the synthetic route is a ruthenium-catalyzed cross-metathesis as the penultimate transformation. Access to synthetic substrates has led us to postulate a new mechanism for length control in this template-independent polymerization. Moreover, our investigations indicate that lipids possessing but a single galactofuranose residue can act as substrates for GlfT2.

A molecular ruler mechanism for length control of extended protein structures in bacteria

The lengths of the hook structure of flagellar motors and of the needle of the injectosome are both carefully controlled, by apparently similar mechanisms. In this paper we propose a novel mechanism for this length control and develop a mathematical model of this process which shows excellent agreement with published data on hook lengths. The proposed mechanism for length control (described using biochemical nomenclature appropriate for hooks) is as follows: Hook growth is terminated when the C-terminus of the length control molecule FliK interacts with FlhB, the secretion gatekeeper. The probability of this interaction is an increasing function of the length of the hook for two reasons. First, FliK is secreted through the hook intermittently during hook growth. Second, the probability of interaction with FlhB is a function of the amount of time the C-terminus of a secreted FliK spends in the vicinity of FlhB. This time is short when the hook is short because the folding of FliK exiting the distal end of the hook acts to pull the FliK molecule through the hook rapidly. In contrast, this time is much longer when the hook is longer than the unfolded FliK polymer since movement through the tube is not enhanced by folding. Thus, it is much more likely that interaction will occur when the hook is long than when the hook is short.

A tethering mechanism for length control in a processive carbohydrate polymerization

Carbohydrate polymers are the most abundant organic substances on earth. Their degrees of polymerization range from tens to thousands of units, yet polymerases generate the relevant lengths without the aid of a template. To gain insight into template-independent length control, we investigated how the mycobacterial galactofuranosyl-transferase GlfT2 mediates formation of the galactan, a polymer of galactofuranose residues that is an integral part of the cell wall. We show that isolated recombinant GlfT2 can catalyze the synthesis of polymers with degrees of polymerization that are commensurate with values observed in mycobacteria, indicating that length control by GlfT2 is intrinsic. Investigations using synthetic substrates reveal that GlfT2 is processive. The data indicate that GlfT2 controls length by using a substrate tether, which is distal from the site of elongation. The strength of interaction of that tether with the polymerase influences the length of the resultant polymer. Thus, our data identify a mechanism for length control by a template-independent polymerase.

Corporate Governance and M&A in Australia: An Overview for Assessing Japan and the 'Americanisation' Thesis

This paper sketches a roadmap of key features and directions in Australian corporate governance, generally (Part 1) and specifically regarding takeovers law and practice (Part 2). In particular, it introduces highlights from a growing corpus of empirical studies, and identifies points that may be of special interest to those familiar with major changes or debates underway in many parts of the Asia-Pacific, particularly in Japan. Part 1.A begins with the insight that Australia may not be so Anglo-American after all, because traditionally it has more blockholders (concentrated shareholdings) and less active institutional investors. This may help explain still limited changes to law and/or practice regarding derivative shareholder suits, as well as more arm's-length market-based monitoring mechanisms such as disclosure, independent directors, and executive remuneration packages (Part 1.B). On the other hand, Australia is experiencing a new boom in M&A (Part 2.A), and its levels of hostile takeovers and success rates even in the 1990s appear close to those in the UK (2.B), although success rates in both countries are much higher than the US. This arm's-length control mechanism may have become more prevalent due to broader economic changes, such as liberalisation of financial markets to allow easier financing of takeovers. But another factor seems to have been the historical legacy of following the law in England, despite the latter's less concentrated shareholding patterns (2.C). The environment is likely to have become even more conducive to hostile takeovers from 2000, when disputes were largely diverted from the courts to a full-scale Takeovers Panel (2.D), similar to that introduced along with the UK City Code on Takeovers in 1968. However, Australia's version is somewhat more formalised, and was recently challenged for unconstitutionally usurping judicial power. Overall (Part 3), this analysis suggests a more complex picture of corporate governance transformations. They do not necessarily pull all in the same direction (as the takeovers market shows), and there remain some important differences from developments in the UK and especially the US.

Development of Ultra Compact Orbital Welding Machine for Thermal Power Generating Boiler

An ultra compact orbital welding machine for butt welding of water-wall tubes of power generating boilers on erection sites has been developed. The minimum radial clearance of the welding head is 20 mm, and this value is below the half of the existing orbital welding machines. To achieve the 20 mm radial clearance, the new arc length control mechanism and the flat welding torch of 7 mm thickness have been developed. The high quality of welds was achieved by installing arc length control function to adopt new structural differential motion mechanism. To make the flow of the shielding gas a uniform flow, the metal fiber filter was set up at the gas blow exit of the thin torch.Using the developed orbital welding machines, several thousand joints of water-wall tubes of power generating boiler have been successfully welded on erection site.

Extreme ionization of Xe clusters driven by ultraintense laser fields

We applied theoretical models and molecular dynamics simulations to explore extreme multielectron ionization in Xe(n) clusters (n=2-2171, initial cluster radius R(0)=2.16-31.0 A) driven by ultraintense infrared Gaussian laser fields (peak intensity I(M)=10(15)-10(20) W cm(-2), temporal pulse length tau=10-100 fs, and frequency nu=0.35 fs(-1)). Cluster compound ionization was described by three processes of inner ionization, nanoplasma formation, and outer ionization. Inner ionization gives rise to high ionization levels (with the formation of [Xe(q+)](n) with q=2-36), which are amenable to experimental observation. The cluster size and laser intensity dependence of the inner ionization levels are induced by a superposition of barrier suppression ionization (BSI) and electron impact ionization (EII). The BSI was induced by a composite field involving the laser field and an inner field of the ions and electrons, which manifests ignition enhancement and screening retardation effects. EII was treated using experimental cross sections, with a proper account of sequential impact ionization. At the highest intensities (I(M)=10(18)-10(20) W cm(-2)) inner ionization is dominated by BSI. At lower intensities (I(M)=10(15)-10(16) W cm(-2)), where the nanoplasma is persistent, the EII contribution to the inner ionization yield is substantial. It increases with increasing the cluster size, exerts a marked effect on the increase of the [Xe(q+)](n) ionization level, is most pronounced in the cluster center, and manifests a marked increase with increasing the pulse length (i.e., becoming the dominant ionization channel (56%) for Xe(2171) at tau=100 fs). The EII yield and the ionization level enhancement decrease with increasing the laser intensity. The pulse length dependence of the EII yield at I(M)=10(15)-10(16) W cm(-2) establishes an ultraintense laser pulse length control mechanism of extreme ionization products.

Tension applied through the Dam1 complex promotes microtubule elongation providing a direct mechanism for length control in mitosis

In dividing cells, kinetochores couple chromosomes to the tips of growing and shortening microtubule fibres and tension at the kinetochore-microtubule interface promotes fibre elongation. Tension-dependent microtubule fibre elongation is thought to be essential for coordinating chromosome alignment and separation, but the mechanism underlying this effect is unknown. Using optical tweezers, we applied tension to a model of the kinetochore-microtubule interface composed of the yeast Dam1 complex bound to individual dynamic microtubule tips. Higher tension decreased the likelihood that growing tips would begin to shorten, slowed shortening, and increased the likelihood that shortening tips would resume growth. These effects are similar to the effects of tension on kinetochore-attached microtubule fibres in many cell types, suggesting that we have reconstituted a direct mechanism for microtubule-length control in mitosis.