The mechanochemical origins of the microtubule sliding motility within the kinesin-5 domain organization

The conserved kinesin-5 bipolar tetrameric motors slide apart microtubules during mitotic spindle assembly and elongation. Kinesin-5 bipolar organization originates from its conserved tetrameric helical minifilament, which position the C-terminal tail domains of two subunits near the N-terminal motor domains of two anti-parallel subunits (Scholey et al, 2014). This unique tetrameric structure enables kinesin-5 to simultaneously engage two microtubules and transmit forces between them, and for multiple kinesin-5 motors to organize via tail to motor interactions during microtubule sliding (Bodrug et al, 2020). Here, we show how these two structural adaptations, the kinesin-5 tail-motor domain interactions and the length of the tetrameric minifilament, determine critical aspects of kinesin-5 motility and sliding mechanisms. An x-ray structure of the 34-nm kinesin-5 minifilament reveals how the dual dimeric N-terminal coiled-coils emerge from the tetrameric central bundle. Using this structure, we generated active bipolar mini-tetrameric motors from Drosophila and human orthologs, which are half the length of native kinesin-5. Using single-molecule motility assays, we show that kinesin-5 tail domains promote mini-tetramers static pauses that punctuate processive motility. During such pauses, kinesin-5 mini-tetramers form multi-motor clusters mediated via tail to motor domain cross-interactions. These clusters undergo slow and highly processive motility and accumulate at microtubule plus-ends. In contrast to native kinesin-5, mini-tetramers require tail domains to initiate microtubule crosslinking. Although mini-tetramers are highly strained in initially aligning microtubules, they slide microtubules more efficiently than native kinesin-5, due to their decreased minifilament flexibility. Our studies reveal that the conserved kinesin-5 motor-tail mediated clustering and the length of the tetrameric minifilament are key features for sliding motility and are critical in organizing microtubules during mitotic spindle assembly and elongation.

Epidermal and anatomical studies on Chrysophyllum L. (Sapotaceae) species of South-Eastern Nigeria

The epidermal characters of leaves and anatomical characters of petioles and young stems of some members of the genus Chrysophyllum, viz. C. albidum, C. perpulchrum, C. cainito, and C. delevoyi were analyzed to determine their diagnostic features. Paracytic stomata, crystal sand, prismatic crystals, secretory canals occurred in all the species. All the species are hypostomatic except C. perpulchrum. The midribs of all species have an open semi-circular vascular system except in C. cainito with a closed system. Midrib and petiole of C. albidum and C. cainito have non-glandular T-shaped trichomes. Accessory bundles are only seen in the petioles of C. delevoyi, and in the midribs of C. albidum and C. cainito. The midrib and petiole of C. delevoyi and C. albidum have a central bundle. Laticifers, prismatic and sand crystals occur mainly in the cortical cells, pith cells, xylem, phloem, and mesophyll. Lamina of all species studied showed uniseriate epidermis except C. cainito which has two layers of the adaxial epidermis. The stomatal index, the ratio of the spongy to palisade mesophyll thickness, the ratio of cortex thickness, outline, number, and arrangement of the vascular bundles in the midrib, petiole, and young stem differ and are valuable diagnostic features in Chrysophyllum. Bangladesh J. Plant Taxon. 28(1): 217-231, 2021 (June)

Comparative study of the leaf’s anatomical structure of some endemic species Umbelliferae family in Middle Asia

This article presents the results of comparative study of leaf structure in species Komarovia anisosperma,Sphaerosciadium denaense, Kamelinia tianschanica, Korshinskya olgae и Autumnalia innopinata of the Apiaceae Lindlfamily. For the studied species, a close relationship between the anatomical structure of the leaf and environmental conditions has been revealed. The diagnostic leaf-blade features are: for Komarovia anisosperma, single layer of the palisadeparenchyma with tightly closed or in places loosely located rather elongated cells and the presence of 3 secretory ducts onthe abaxial side (under the central and marginal lateral vascular bundles); for Sphaerosciadium denaense, marginal partswrapped to adaxial side, 2(3) secretory ducts above the main bundle at the abaxial side, one larger at the adaxial side,above and below lateral bundles they locate one by one, and a single duct at the adaxial part of the leaf top; for Kamelinia tianschanica, two and above the central bundle three-layer palisades and the presence of a single secretory duct underthe central bundle; Korshinskya olgae, two-layer palisade and the presence on the abaxial side of 3 (4) secretory ducts (1or 2 large under the central and median lateral vascular bundles); Autumnalia innopinata, isopalisade mesophyll and thepresence of a 2–3-layer aquifer parenchyma.

An anatomical and biomechanical assessment of the interosseous membrane of the cadaveric forearm

Ten cadaveric specimens underwent biomechanical assessment on a motorized jig with an in-built torque sensor. A differential variable reluctance transducer was placed on the central bundle of the interosseous membrane to detect changes in strain. Torque was measured with an intact interosseous membrane and a sectioned central bundle of the interosseous membrane. Changes in strain and torque were plotted against the degree of rotation of the cadaveric forearms. We found that the overall magnitude of strain to be greatest in pronation and smallest in supination. However, the relative displacement of the interosseous membrane between pronation and supination was minimal in absolute terms. There was no difference in torque between an intact and cut central bundle. We conclude that the interosseous membrane acts as a static longitudinal stabilizer of the forearm and less so as a rotational stabilizer.

Weak* and entropy approximation of nonhyperbolic measures: a geometrical approach

We study C1-robustly transitive and nonhyperbolic diffeomorphisms having a partially hyperbolic splitting with one-dimensional central bundle whose strong un-/stable foliations are both minimal. In dimension 3, an important class of examples of such systems is given by those with a simple closed periodic curve tangent to the central bundle. We prove that there is a C1-open and dense subset of such diffeomorphisms such that every nonhyperbolic ergodic measure (i.e. with zero central exponent) can be approximated in the weak* topology and in entropy by measures supported in basic sets with positive (negative) central Lyapunov exponent. Our method also allows to show how entropy changes across measures with central Lyapunov exponent close to zero. We also prove that any nonhyperbolic ergodic measure is in the intersection of the convex hulls of the measures with positive central exponent and with negative central exponent.

Structural basis for the assembly of the mitotic motor Kinesin-5 into bipolar tetramers

Chromosome segregation during mitosis depends upon Kinesin-5 motors, which display a conserved, bipolar homotetrameric organization consisting of two motor dimers at opposite ends of a central rod. Kinesin-5 motors crosslink adjacent microtubules to drive or constrain their sliding apart, but the structural basis of their organization is unknown. In this study, we report the atomic structure of the bipolar assembly (BASS) domain that directs four Kinesin-5 subunits to form a bipolar minifilament. BASS is a novel 26-nm four-helix bundle, consisting of two anti-parallel coiled-coils at its center, stabilized by alternating hydrophobic and ionic four-helical interfaces, which based on mutagenesis experiments, are critical for tetramerization. Strikingly, N-terminal BASS helices bend as they emerge from the central bundle, swapping partner helices, to form dimeric parallel coiled-coils at both ends, which are offset by 90°. We propose that BASS is a mechanically stable, plectonemically-coiled junction, transmitting forces between Kinesin-5 motor dimers during microtubule sliding.

The tissue structure of the vegetative organs of strawberry (Fragaria moschata Duch®)

The tissue structure of the vegetative organs of strawberry (root, rhizome, stolon, leaf) is discussed in this paper. The authors stated that the root structure described by Muromcev (1969) and Naumann-Seip (1989) develops further from the primary structure. It grows secondarily and the transport tissue becomes continuous having ring shape. In the primary cortex of the rhizome periderm like tissue differentiates, but according to the examinations up to now, it does not take over the role of the exodermis. The exodermis is phloboran filled primary cortex tissue with 3-4 cell rows under the rhizodermis. The development of the transport tissue of the petiole is also a new recognition. In the lower third of the petiole the transport tissue consists of 3 collaterally compound vascular bundles. In the middle third there are 5 bundles because of the separation of the central bundle and in the upper third of the petiole 7 bundles can be observed because of the ramification of the outside bundles. Therefore attention must be taken also in the case of other plants at making sections. There might be confusions in the results of the examinations if the number of bundles increases in the petiole. The tissue structure might vary depending on the origin of the tissue segment. The palisade parenchyma of the leaf blade has two layers and it is wider than the spongy parenchyma. Among the 5-6-angular cells of the upper epidermis do not develop stomata while in the lower epidermis there are a fairly lot of them.

Supercontracted state of vertebrate smooth muscle cell fragments reveals myofilament lengths.

Isolated cell preparations from chicken gizzard smooth muscle typically contain a mixture of cell fragments and whole cells. Both species are spontaneously permeable and may be preloaded with externally applied phalloidin and antibodies and then induced to contract with Mg ATP. Labeling with antibodies revealed that the cell fragments specifically lacked certain cytoskeletal proteins (vinculin, filamin) and were depleted to various degrees in others (desmin, alpha-actinin). The cell fragments showed a unique mode of supercontraction that involved the protrusion of actin filaments through the cell surface during the terminal phase of shortening. In the presence of dextran, to minimize protein loss, the supercontracted products were star-like in form, comprising long actin bundles radiating in all directions from a central core containing myosin, desmin, and alpha-actinin. It is concluded that supercontraction is facilitated by an effective uncoupling of the contractile apparatus from the cytoskeleton, due to partial degradation of the latter, which allows unhindered sliding of actin over myosin. Homogenization of the cell fragments before or after supercontraction produced linear bipolar dimer structures composed of two oppositely polarized bundles of actin flanking a central bundle of myosin filaments. Actin filaments were shown to extend the whole length of the bundles and their length averaged integral to 4.5 microns. Myosin filaments in the supercontracted dimers averaged 1.6 microns in length. The results, showing for the first time the high actin to myosin filament length ratio in smooth muscle are readily consistent with the slow speed of shortening of this tissue. Other implications of the results are also discussed.