During asymmetric mitosis, both in male Drosophila germline stem cells and in mouse embryo neural progenitors, the mother centrosome is retained by the self-renewed cell; hence suggesting that mother centrosome inheritance might contribute to stemness. We test this hypothesis in Drosophila neuroblasts (NBs) tracing photo converted centrioles and a daughter-centriole-specific marker generated by cloning the Drosophila homologue of human Centrobin. Here we show that upon asymmetric mitosis, the mother centrosome is inherited by the differentiating daughter cell. Our results demonstrate maturation-dependent centrosome fate in Drosophila NBs and that the stemness properties of these cells are not linked to mother centrosome inheritance.
During interphase in Drosophila neuroblasts, the Centrobin (CNB)-positive daughter centriole retains pericentriolar material (PCM) and organizes an aster that is a key determinant of the orientation of cell division. Here we show that daughter centrioles depleted of CNB cannot fulfil this function whereas mother centrioles that carry ectopic CNB can. CNB co-precipitates with a set of centrosomal proteins that include γ-TUB, ANA2, CNN, SAS-4, ASL, DGRIP71, POLO and SAS-6. Following chemical inhibition of POLO or removal of three POLO phosphorylation sites present in CNB, the interphase microtubule aster is lost. These results demonstrate that centriolar CNB localization is both necessary and sufficient to enable centrioles to retain PCM and organize the interphase aster in Drosophila neuroblasts. They also reveal an interphase function for POLO in this process that seems to have co-opted part of the protein network involved in mitotic centrosome maturation.
The cloning of asl offers new insight into the molecular composition of Drosophila centrioles and a possible model for the role of its human homolog. In addition, the phenotype of asl-deficient flies reveals that a functional centrosome is required for Drosophila embryo development.
Previous data suggested that anastral spindles, morphologically similar to those found in oocytes, can assemble in a centrosome-independent manner in cells that contain centrosomes. It is assumed that the microtubules that build these acentrosomal spindles originate over the chromatin. However, the actual processes of centrosome-independent microtubule nucleation, polymerisation, and sorting have not been documented in centrosome-containing cells. We have identified two experimental conditions in which centrosomes are kept close to the plasma membrane, away from the nuclear region, throughout meiosis I in Drosophila spermatocytes. Time-lapse confocal microscopy of these cells labelled with fluorescent chimeras reveals centrosome-independent microtubule nucleation, growth, and sorting into a bipolar spindle array over the nuclear region, away from the asters. The onset of noncentrosomal microtubule nucleation is significantly delayed with respect to nuclear envelope breakdown and coincides with the end of chromosome condensation. It takes place in foci that are close to the membranes that ensheath the nuclear region, not over the condensed chromosomes. Metaphase plates are formed in these spindles, and, in a fraction of them, some degree of polewards chromosome segregation takes place. In these cells that contain both membrane-bound asters and an anastral spindle, the orientation of the cytokinesis furrow correlates with the position of the asters and is independent of the orientation of the spindle. We conclude that the fenestrated nuclear envelope may significantly contribute to the normal process of spindle assembly in Drosophila spermatocytes. We also conclude that the anastral spindles that we have observed are not likely to provide a robust back-up able to ensure successful cell division. We propose that these anastral microtubule arrays could be a constitutive component of wild-type spindles, normally masked by the abundance of centrosome-derived microtubules and revealed when asters are kept away. These observations are consistent with a model in which centrosomal and noncentrosomal microtubules contribute to the assembly and are required for the robustness of the cell division spindle in cells that contain centrosomes.
PDZ domains are small globular domains that recognize the last 4-7 amino acids at the C-terminus of target proteins. The specificity of the PDZ-ligand recognition is due to side chain-side chain interactions, as well as the positioning of an alpha-helix involved in ligand binding. We have used computer-aided protein design to produce mutant versions of a Class I PDZ domain that bind to novel Class I and Class II target sequences both in vitro and in vivo, thus providing an alternative to primary antibodies in western blotting, affinity chromatography and pull-down experiments. Our results suggest that by combining different backbone templates with computer-aided protein design, PDZ domains could be engineered to specifically recognize a large number of proteins.
Sensory cilia are organelles that convey information to the cell from the extracellular environment. In vertebrates, ciliary dysfunction results in ciliopathies that in humans comprise a wide spectrum of developmental disorders. In Drosophila, sensory cilia are found only in the neurons of type I sensory organs, but ciliary dysfunction also has dramatic consequences in this organism because it impairs the mechanosensory properties of bristles and chaetae and leads to uncoordination, a crippling condition that causes lethality shortly after eclosion. The cilium is defined by the ciliary membrane, a protrusion of the cell membrane that envelops the core structure known as the axoneme, a microtubule array that extends along the cilium from the basal body. In vertebrates, basal body function requires centriolar distal and subdistal appendages and satellites. Because these structures are acquired through centriole maturation, only mother centrioles can serve as basal bodies. Here, we show that although centriole maturity traits are lacking in Drosophila, basal body fate is reserved to mother centrioles in Drosophila type I neurons. Moreover, we show that depletion of the daughter-centriole-specific protein Centrobin (CNB) enables daughter centrioles to dock on the cell membrane and to template an ectopic axoneme that, although structurally defective, protrudes out of the cell and is enveloped by a ciliary membrane. Conversely, basal body capability is inhibited in mother centrioles modified to carry CNB. These results reveal the crucial role of CNB in regulating basal body function in Drosophila ciliated sensory organs.
Authors have been listed in alphabetical order. Sandra J. van Vliet and Barbara Richichi are co-last and co-corresponding authors.
GDSL and SGNH hydrolases are lipases involved in a wide range of functions, behaving in many cases as bifunctional enzymes. In this work, the isolation and characterization of AgaSGNH, a cDNA encoding a member of the SGNH-hydrolase superfamily from young leaf epidermis of the monocot Agave americana L., is reported. The protein possesses a typical signal peptide at its N-terminus that allows its secretion to the epidermis cell wall, as verified by immunolocalization experiments. In addition, the AgaSGNH sequence contains a His-Leu-Gly-Ala-Glu (HLGAE) motif which is similar to that observed in other plant acyltransferases. Expression levels by northern blot and in situ localization of the corresponding mRNA, as well as the immunolocalization of the protein in Agave young leaves indicate that the protein is specifically present in the epidermal cells. The detailed study performed in different parts of the Agave leaf confirms two aspects: first, the expression of AgaSGNH is limited to the epidermis, and second, the maximum mRNA levels are found in the epidermis of the youngest zones of the leaf which are especially active in cutin biosynthesis. These levels dramatically decrease in the oldest zone of the leaf, where the presence of AgaSGNH mRNA is undetectable, and the biosynthesis of different cuticle components is severely reduced. These data could be compatible with the hypothesis that AgaSGNH could carry out both the hydrolysis and the transfer, from an activated acyl-CoA to a crescent cutin in Agave americana leaves and, therefore, be involved in the still unknown mechanism of plant cutin biosynthesis.
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