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Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments.

Guild GM, Connelly PS, Ruggiero L, Vranich KA, Tilney LG - J. Cell Biol. (2003)

Bottom Line: These long bundles are built from much shorter modules that graft together.Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing.These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018, USA. gguild@sas.upenn.edu

ABSTRACT
The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation. Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing. These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.

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There is no preferred surface for module grafting. Confocal image through a portion of a 48-h pupal bristle stained with fluorescent phalloidin. The left-most bundle (along the membrane) exhibits two breaks. The orientation of module ends relative to one another is indicated. Bristle tip is up (arrowhead). 38 examples of module–module juxtaposition along the plasma membrane in 12 bristles were analyzed. In about half the cases (47%) a module was grafted to the inside (cytoplasmic side) of the adjacent tip-wise module (top example). Approximately half the grafting (53%) occurred on the outside (nearest the plasma membrane) of a module above it (lower example). Bar, 5 μm.
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fig8: There is no preferred surface for module grafting. Confocal image through a portion of a 48-h pupal bristle stained with fluorescent phalloidin. The left-most bundle (along the membrane) exhibits two breaks. The orientation of module ends relative to one another is indicated. Bristle tip is up (arrowhead). 38 examples of module–module juxtaposition along the plasma membrane in 12 bristles were analyzed. In about half the cases (47%) a module was grafted to the inside (cytoplasmic side) of the adjacent tip-wise module (top example). Approximately half the grafting (53%) occurred on the outside (nearest the plasma membrane) of a module above it (lower example). Bar, 5 μm.

Mentions: Because the actin bundles are attached to the plasma membrane, we tried to determine if module overlap, as revealed by fracture, was always on the same side of the bundle relative to the plasma membrane. In other words, can grafting take place at any position around the circumference of the module. Here we examined thin optical sections from fixed bristles stained with fluorescent phalloidin. We selected “center cut” sections of the bristle to be sure that there was a lateral association with the plasma membrane in the image. We examined 38 module–module fractures and found that the point of overlap is not fixed, but occurred with similar frequency on either the cytoplasmic (47%) or the membrane surface (53%) of the upper module (Fig. 8). In most cases, the overlap zone appeared as a graft of two tapered ends, not as an end to end abutment.


Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments.

Guild GM, Connelly PS, Ruggiero L, Vranich KA, Tilney LG - J. Cell Biol. (2003)

There is no preferred surface for module grafting. Confocal image through a portion of a 48-h pupal bristle stained with fluorescent phalloidin. The left-most bundle (along the membrane) exhibits two breaks. The orientation of module ends relative to one another is indicated. Bristle tip is up (arrowhead). 38 examples of module–module juxtaposition along the plasma membrane in 12 bristles were analyzed. In about half the cases (47%) a module was grafted to the inside (cytoplasmic side) of the adjacent tip-wise module (top example). Approximately half the grafting (53%) occurred on the outside (nearest the plasma membrane) of a module above it (lower example). Bar, 5 μm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172841&req=5

fig8: There is no preferred surface for module grafting. Confocal image through a portion of a 48-h pupal bristle stained with fluorescent phalloidin. The left-most bundle (along the membrane) exhibits two breaks. The orientation of module ends relative to one another is indicated. Bristle tip is up (arrowhead). 38 examples of module–module juxtaposition along the plasma membrane in 12 bristles were analyzed. In about half the cases (47%) a module was grafted to the inside (cytoplasmic side) of the adjacent tip-wise module (top example). Approximately half the grafting (53%) occurred on the outside (nearest the plasma membrane) of a module above it (lower example). Bar, 5 μm.
Mentions: Because the actin bundles are attached to the plasma membrane, we tried to determine if module overlap, as revealed by fracture, was always on the same side of the bundle relative to the plasma membrane. In other words, can grafting take place at any position around the circumference of the module. Here we examined thin optical sections from fixed bristles stained with fluorescent phalloidin. We selected “center cut” sections of the bristle to be sure that there was a lateral association with the plasma membrane in the image. We examined 38 module–module fractures and found that the point of overlap is not fixed, but occurred with similar frequency on either the cytoplasmic (47%) or the membrane surface (53%) of the upper module (Fig. 8). In most cases, the overlap zone appeared as a graft of two tapered ends, not as an end to end abutment.

Bottom Line: These long bundles are built from much shorter modules that graft together.Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing.These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of Pennsylvania, Philadelphia, PA 19104-6018, USA. gguild@sas.upenn.edu

ABSTRACT
The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation. Thus, bundle morphogenesis has several components: module formation, elongation, grafting, and bundle smoothing. These actin bundles are much like a rope or cable, made by overlapping elements that run a small fraction of the overall length, and stiffened by cross-linking.

Show MeSH
Related in: MedlinePlus