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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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Grafting modules are short and in transverse register. (a) Confocal image of a portion of a macrochaete from a 37-h pupa labeled in vivo with GFP actin showing short modules near the tip. Four bundles forming from modules are boxed. Bar, 5 μm. (b and c) Histograms showing actin concentration along forming bundles. Two of the bundles shown in a (arrowheads) were scanned using the ImageJ application and the resulting grayscale values plotted as a function of the bundle length. In each histogram, the distance between minimum grayscale values was taken to represent the maximum length of an individual module (boxed values). Note that module lengths in a single bundle were different and that the linear arrangement of module lengths in adjacent bundles was similar indicating that the modules in the two adjacent bundles were in transverse register. (d) Length distribution histogram of all of the modules within the boxed region shown in a.
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fig3: Grafting modules are short and in transverse register. (a) Confocal image of a portion of a macrochaete from a 37-h pupa labeled in vivo with GFP actin showing short modules near the tip. Four bundles forming from modules are boxed. Bar, 5 μm. (b and c) Histograms showing actin concentration along forming bundles. Two of the bundles shown in a (arrowheads) were scanned using the ImageJ application and the resulting grayscale values plotted as a function of the bundle length. In each histogram, the distance between minimum grayscale values was taken to represent the maximum length of an individual module (boxed values). Note that module lengths in a single bundle were different and that the linear arrangement of module lengths in adjacent bundles was similar indicating that the modules in the two adjacent bundles were in transverse register. (d) Length distribution histogram of all of the modules within the boxed region shown in a.

Mentions: Scans of the fluorescence intensity along newly formed bundles (Fig. 3, a–c) quantified the longitudinal variation in actin filament numbers, with modules 1.82 ± 0.43 (SD) μm in length (Fig. 3 d) separated by short gaps. These are considerably shorter than the 2.8 ± 2.2 μm modules that appear later during bundle breakdown (Tilney et al., 1996; Guild et al., 2002). By comparing module positions in adjacent bundles (Fig. 3, b and c) we note that modules were in transverse register soon after their synthesis. The length of newly synthesized modules can vary widely in length (Fig. 3 d). Another example can be seen in Fig. 1 c, where 22 single modules measured 1.00 ± 0.29 μm.


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)

Grafting modules are short and in transverse register. (a) Confocal image of a portion of a macrochaete from a 37-h pupa labeled in vivo with GFP actin showing short modules near the tip. Four bundles forming from modules are boxed. Bar, 5 μm. (b and c) Histograms showing actin concentration along forming bundles. Two of the bundles shown in a (arrowheads) were scanned using the ImageJ application and the resulting grayscale values plotted as a function of the bundle length. In each histogram, the distance between minimum grayscale values was taken to represent the maximum length of an individual module (boxed values). Note that module lengths in a single bundle were different and that the linear arrangement of module lengths in adjacent bundles was similar indicating that the modules in the two adjacent bundles were in transverse register. (d) Length distribution histogram of all of the modules within the boxed region shown in a.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172841&req=5

fig3: Grafting modules are short and in transverse register. (a) Confocal image of a portion of a macrochaete from a 37-h pupa labeled in vivo with GFP actin showing short modules near the tip. Four bundles forming from modules are boxed. Bar, 5 μm. (b and c) Histograms showing actin concentration along forming bundles. Two of the bundles shown in a (arrowheads) were scanned using the ImageJ application and the resulting grayscale values plotted as a function of the bundle length. In each histogram, the distance between minimum grayscale values was taken to represent the maximum length of an individual module (boxed values). Note that module lengths in a single bundle were different and that the linear arrangement of module lengths in adjacent bundles was similar indicating that the modules in the two adjacent bundles were in transverse register. (d) Length distribution histogram of all of the modules within the boxed region shown in a.
Mentions: Scans of the fluorescence intensity along newly formed bundles (Fig. 3, a–c) quantified the longitudinal variation in actin filament numbers, with modules 1.82 ± 0.43 (SD) μm in length (Fig. 3 d) separated by short gaps. These are considerably shorter than the 2.8 ± 2.2 μm modules that appear later during bundle breakdown (Tilney et al., 1996; Guild et al., 2002). By comparing module positions in adjacent bundles (Fig. 3, b and c) we note that modules were in transverse register soon after their synthesis. The length of newly synthesized modules can vary widely in length (Fig. 3 d). Another example can be seen in Fig. 1 c, where 22 single modules measured 1.00 ± 0.29 μm.

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