The centrosome of the slime mould Dictyostelium discoideum displays a morphology markedly different from centriolar centrosomes or yeast spindle pole bodies, while fulfilling the same conserved functions in the organization of the microtubule cytoskeleton. Recent advances suggest that the Dictyostelium centrosome may offer an interesting model system, usefully complementing other well-studied centrosome models. The establishment of an isolation procedure and the generation of a range of monoclonal antibodies have been achieved, which are important pre-requisites for biochemical investigation. Furthermore, the role of the centrosome in cell motility and centrosome duplication process have been investigated using cells with GFP-labelled centrosomes.

Animals↗

PubMedDate not supplied

Kinesin and dynein mutants provide novel insights into the roles of vesicle traffic during cell morphogenesis in Neurospora.

BACKGROUND: Kinesin and cytoplasmic dynein are force-generating molecules that move in opposite directions along microtubules. They have been implicated in the directed transport of a wide variety of cellular organelles, but it is unclear whether they have overlapping or largely independent functions. RESULTS: We analyzed organelle transport in kinesin and dynein single mutants, and in a kinesin and dynein double mutant of Neurospora crassa. Remarkably, the simultaneous mutation of kinesin and dynein was not lethal and resulted in an additive phenotype that combined the features of the single mutants. The mutation of kinesin and dynein had opposite effects on the apical and retrograde transport, respectively, of vesicular organelles. In the kinesin mutant, apical movement of submicroscopic, secretory vesicles to the Spitzenkörper - an organelle in the hyphal apex - was defective, whereas the predominantly retrograde movement of microscopic organelles was only slightly reduced. In contrast, the dynein mutant still had a prominent Spitzenkörper, demonstrating that apical transport was intact, but retrograde transport was essentially inhibited completely. A major defect in vacuole formation and dynamics was also evident. In agreement with the observations on apical transport, protein secretion into the medium was markedly inhibited in the kinesin mutant but not in the dynein mutant. CONCLUSIONS: Transport of secretory vesicles is necessary but not sufficient for normal apical extension. A component of retrograde transport, presumably precursors of the vacuole system, is also essential. Our findings provide new information on the role microtubule motors play in cell morphogenesis and suggest that kinesin and cytoplasmic dynein have largely independent functions within separate pathways.

Biological Transport, Active↗

PubMedDate not supplied

Persistent, directional motility of cells and cytoplasmic fragments in the absence of microtubules.

Directional cell locomotion is displayed by many cell types both in vivo and in vitro. In many instances, persistency and directionality are imposed by external stimuli such as chemical attractants or substrate properties. Some cell types, such as fibroblasts or leukocytes, are capable of migrating in the absence of known stimuli in a pattern known as persistent random walk, where the direction of movement is maintained for at least one cell diameter before the cell performs a sudden directional change. In many examples of persistent motility, microtubules are believed to have a key role as elements that stabilize or even determine a cell's direction of movement. If disassembled, persistency is reduced or impaired. Despite some reports to the contrary, these and other observations have led to the widely accepted view that microtubules may be the overall organizers of cell geometry, polarity and motile activity. Here we report that rapid, directional locomotion of fish epidermal keratocytes is independent of the presence of microtubules. Moreover, small cytoplasmic fragments derived from the anterior lamella of these cells are capable of locomoting in a pattern indistinguishable from that of intact cells. Since these fragments contain no nucleus, microtubules or centrioles, the persistency-determining component must be sought in some other component(s) of the cytoplasm, possibly the motile machinery of the lamella itself.

Animals↗

PubMedDate not supplied

Active sliding between cytoplasmic microtubules.

Microtubules are versatile cellular polymers that play a role in cell shape determination and mediate various motile processes such as ciliary and flagellar bending, chromosome movements and organelle transport. That a sliding microtubule mechanism can generate force has been demonstrated in highly ordered structures such as axonemes, and microtubule-based force generation almost certainly contributes to the function of mitotic and meiotic spindles. Most cytoplasmic microtubule arrays, however, do not exhibit the structural regularity of axonemes and some spindles, and often appear disorganized. Yet many cellular activities (such as shape changes during morphogenesis, axonal extension and spindle assembly) involve highly coordinated microtubule behaviour and possibly require force generated by an intermicrotubule sliding mechanism, or perhaps use sliding to move microtubules rapidly into a protrusion for stabilization. Here we show that active sliding between cytoplasmic microtubules can occur in microtubule bundles of the amoeba Reticulomyxa. A force-producing mechanism of this sort could be used by this organism to facilitate the extension of cell processes and to generate the dynamic movements of the cytoplasmic network.

Adenine↗

PubMedDate not supplied

Universal and unique features of kinesin motors: insights from a comparison of fungal and animal conventional kinesins.

Kinesins are microtubule motors that use the energy derived from the hydrolysis of ATP to move unidirectionally along microtubules. The founding member of this still growing superfamily is conventional kinesin, a dimeric motor that moves processively towards the plus end of microtubules. Within the family of conventional kinesins, two groups can be distinguished to date, one derived from animal species, and one originating from filamentous fungi. So far no conventional kinesin has been reported from plant cells. Fungal and animal conventional kinesins differ in several respects, both in terms of their primary sequence and their physiological properties. Thus all fungal conventional kinesins move at velocities that are 4-5 times higher than those of animal conventional kinesins, and all of them appear to lack associated light chains. Both groups of motors are characterized by a number of group-specific sequence features which are considered here with respect to their functional importance. Animal and fungal conventional kinesins also share a number of sequence characteristics which point to common principles of motor function. The overall domain organization is remarkably similar. A C-terminal sequence motif common to all kinesins, which constitutes the only region of high homology outside the motor domain, suggests common principles of cargo association in both groups of motors. Consideration of the differences of, and similarities between, fungal and animal kinesins offers novel possibilities for experimentation (e. g., by constructing chimeras) that can be expected to contribute to our understanding of motor function.

Amino Acid Sequence↗