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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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Actin bundles become smooth by actin filament addition shortly after module grafting. A bristle tip expressing GFP actin was visualized by time-lapse confocal microscopy at 15-min intervals (displayed left to right). Each image represents a 3.5-μm optical section containing five actin bundles. Images were aligned using fiduciary marks provided by several module–module junctions that were bent in this preparation. Grafting areas characterized by increased actin fluorescence are indicated by asterisks. These grafts are subsequently the site of baseward actin addition (arrows) that result in a smoother and thicker bundle. Bar, 2 μm.
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fig6: Actin bundles become smooth by actin filament addition shortly after module grafting. A bristle tip expressing GFP actin was visualized by time-lapse confocal microscopy at 15-min intervals (displayed left to right). Each image represents a 3.5-μm optical section containing five actin bundles. Images were aligned using fiduciary marks provided by several module–module junctions that were bent in this preparation. Grafting areas characterized by increased actin fluorescence are indicated by asterisks. These grafts are subsequently the site of baseward actin addition (arrows) that result in a smoother and thicker bundle. Bar, 2 μm.

Mentions: We examined actively elongating macrochaetes labeled with GFP actin by time-lapse confocal microscopy to catch smoothing in action. By concentrating on bristles with bent tips we could use module–module junctions as fiduciary marks. We found that tipward module ends show an increase in fluorescence indicating the position of module–module grafting (Fig. 6, a and b, asterisks). These fluorescent regions elongate in a baseward direction within 15 min (Fig. 6, b and c, arrows). Thus, new filament formation, or at least accumulation, seems to begin at the module knuckles and “paint” the module from the apical end toward the module below merging modules into a smooth bundle.


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)

Actin bundles become smooth by actin filament addition shortly after module grafting. A bristle tip expressing GFP actin was visualized by time-lapse confocal microscopy at 15-min intervals (displayed left to right). Each image represents a 3.5-μm optical section containing five actin bundles. Images were aligned using fiduciary marks provided by several module–module junctions that were bent in this preparation. Grafting areas characterized by increased actin fluorescence are indicated by asterisks. These grafts are subsequently the site of baseward actin addition (arrows) that result in a smoother and thicker bundle. Bar, 2 μm.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172841&req=5

fig6: Actin bundles become smooth by actin filament addition shortly after module grafting. A bristle tip expressing GFP actin was visualized by time-lapse confocal microscopy at 15-min intervals (displayed left to right). Each image represents a 3.5-μm optical section containing five actin bundles. Images were aligned using fiduciary marks provided by several module–module junctions that were bent in this preparation. Grafting areas characterized by increased actin fluorescence are indicated by asterisks. These grafts are subsequently the site of baseward actin addition (arrows) that result in a smoother and thicker bundle. Bar, 2 μm.
Mentions: We examined actively elongating macrochaetes labeled with GFP actin by time-lapse confocal microscopy to catch smoothing in action. By concentrating on bristles with bent tips we could use module–module junctions as fiduciary marks. We found that tipward module ends show an increase in fluorescence indicating the position of module–module grafting (Fig. 6, a and b, asterisks). These fluorescent regions elongate in a baseward direction within 15 min (Fig. 6, b and c, arrows). Thus, new filament formation, or at least accumulation, seems to begin at the module knuckles and “paint” the module from the apical end toward the module below merging modules into a smooth bundle.

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