Evidence for conformational change-induced hydrolysis of β-tubulin-GTP

ABSTRACTMicrotubules, protein polymers of α/β-tubulin dimers, form the structural framework for many essential cellular processes including cell shape formation, intracellular transport, and segregation of chromosomes during cell division. It is known that tubulin-GTP hydrolysis is closely associated with microtubule polymerization dynamics. However, the precise roles of GTP hydrolysis in tubulin polymerization and microtubule depolymerization, and how it is initiated are still not clearly defined. We report here that tubulin-GTP hydrolysis can be triggered by conformational change induced by the depolymerizing kinesin-13 proteins or by the stabilizing chemical agent paclitaxel. We provide biochemical evidence that conformational change precedes tubulin-GTP hydrolysis, confirming this process is mechanically driven and structurally directional. Furthermore, we quantitatively measure the average size of the presumptive stabilizing “GTP cap” at growing microtubule ends. Together, our findings provide the molecular basis for tubulin-GTP hydrolysis and its role in microtubule polymerization and depolymerization.

Arf6 determines tissue architecture by stabilizing intercellular adhesion

Correct cell shape is indispensable for tissue architecture, with cell shape being determined by cortical actin and surface adhesion. The role of adhesion in remodelling tissue is to counteract the deformation of cells by force, resulting from actomyosin contractility, and to maintain tissue integrity. The dynamics of this adhesion are critical to the processes of cell shape formation and maintenance. Here, we show that the trafficking molecule Arf6 has a direct impact on cell elongation, by acting to stabilize E-cadherin-based adhesion complexes at the cell surface, in addition to its canonical role in endocytosis. We demonstrate that these functions of Arf6 are dependent on the molecule Flotillin1, which recruits Arf6 to the plasma membrane. Our data suggest that Arf6 and Flotillin1 operate in a pathway distinct from clathrin-mediated endocytosis. Altogether, we demonstrate that Arf6- and Flotillin1-dependent regulation of the dynamics of cell adhesion contribute to moulding tissue in vivo . This article is part of the discussion meeting issue ‘Contemporary morphogenesis’.

Newly Discovered Cell Shape Promoting Protein Complex Involved in the Maintenance of Distinct Helical Shape of Helicobacter Pylori

The distinct helical shape of the bacterium Helicobacter Pylori (H. pylori) assists this organism in colonizing the digestive organs of its target host. It has been discovered that a key determinant of helical cell shape formation in H. pylori is the Csd5 protein, which engages in multiple cell shape promoting interactions with the cell wall and other various proteins. This finding has significant clinical implications, as it outlines Csd5 as a potential drug target for treating H. pylori infection in the future.

Abnormal megakaryocyte morphology and proplatelet formation in mice with megakaryocyte-restricted MYH9 inactivation

Mutations in the MYH9 gene encoding nonmuscle myosin IIA lead to macrothrombocytopenia as observed in MYH9-related disorders. We used mice with megakaryocyte-restricted MYH9 inactivation to explore the role of myosin in thrombopoiesis. In situ, bone marrow MYH9Δ megakaryocytes were irregularly shaped, appearing leaky with poorly defined limits. The demarcation membranes were abnormally organized and poorly developed, pointing to an insufficient reservoir for the future formation of platelets. The cytoskeletal-rich peripheral zone was lacking due to the absence of the myosin filament network that normally surrounds the granular zone in wild-type cells. In vitro studies of cultured cells showed that MYH9Δ megakaryocytes were unable to form stress fibers upon adhesion to collagen, suggesting that the leaky shape results from defects in internal tension and anchorage to the extracellular environment. Surprisingly, the proportion of cells extending proplatelets was increased in MYH9Δ megakaryocytes and the proplatelet buds were larger. Overall, this study provides evidence for a role of myosin in different steps of megakaryocyte development through its participation in the maintenance of cell shape, formation and organization of the demarcation membranes and the peripheral zone, anchorage to the extracellular matrix, and proplatelet formation.

Photosynthesis and Heat Response of the Green Alga Micrasterias denticulata (Desmidiaceae)

Cells of the green alga Micrasterias denticulata cultivated at 15 °C, 20 °C or 25 °C were exposed to heat shocks at different temperatures (30 -40 °C) for varying duration ( 5 - 90 m in). Cell pattern formation, division rate as well as photosynthesis and respiration by measuring oxygen production and consumption have been studied. The degree of cell shape malformations was found dependent on the preceding cultivation temperature along with the mode of the heat shock. Cells cultivated at 15 °C and 20 °C could counteract a 90 min heat shock at 35 °C much better than those cultivated at 25 °C, which was seen by a less reduced young semicell. Cells cultivated at 15 °C and 25 °C reveal a reduced division activity compared to those grown at 20 °C even with a marked retardation when affected by a preceding heat shock. Photosynthesis and the level of plastid pigments (carotenoids, chlorophylls, β-carotene, lutein) of controls determined by HPLC analysis reached a plateau after about 26 days when starting with 22-day old cultures. Photosynthesis and respiration were determined in a range between 15 °C and 40 °C in defined Micrasterias cell cultures of about this age (cultivation temperature 15 °C, 20 °C or 25 °C). Both processes rose steadily with increasing temperature starting with 15 °C and reached peaks between 30 °C and 32 °C, followed by a considerable drop when increasing the incubation temperature up to 40 °C. The experiments reveal that primary processes of energy formation and consumption are much less affected by temperature influences than cell shape formation and division rate

The Structure and Function Of Intermediate Filament Networks In Mammalian Cells

Intermediate filaments (IF) are major cytoskeletal components found in most mammalian cells. One of the most intriguing properties of IF is their high degree of variability with regard to their structural subunit proteins. This ariability is most evident when one compares the protein composition of IF from different cell types. For example, nerve cell IF (neurofilaments) contain the so-called neurofilament triplet proteins, while different epithelial cells contain two or more of the family of IF proteins called keratins. Indeed other cell types appear to have primarily a single subunit species such as in fibroblasts (vimentin, decamin) and muscle cells (desmin).In the specific case of the nervous system, there is very little information available on specific functions of neurofilaments, although they are thought to be involved in nerve cell shape formation and maintenance, as well as in axoplasmic transport and flow.