F actin bundles in Drosophila bristles. I. Two filament cross-links are involved in bundling.
Transverse sections though Drosophila bristles reveal 7-11 nearly round, plasma membrane-associated bundles of actin filaments. These filaments are hexagonally packed and in a longitudinal section they show a 12-nm periodicity in both the 1.1 and 1.0 views. From earlier studies this periodicity is attributable to cross-links and indicates that the filaments are maximally cross-linked, singed mutants also have 7-11 bundles, but the bundles are smaller, flattened, and the filaments within the bundles are randomly packed (not hexagonal); no periodicity can be detected in longitudinal sections. Another mutant, forked (f36a), also has 7-11 bundles but even though the bundles are very small, the filaments within them are hexagonally packed and display a 12-nm periodicity in longitudinal section. The singed-forked double mutant lacks filament bundles. Thus there are at least two species of cross-links between adjacent actin filaments. Hints of why two species of cross-links are necessary can be gleaned by studying bristle formation. Bristles sprout with only microtubules within them. A little later in development actin filaments appear. At early stages the filaments in the bundles are randomly packed. Later the filaments in the bundles become hexagonally packed and maximally cross-linked. We consider that the forked proteins may be necessary early in development to tie the filaments together in a bundle so that they can be subsequently zippered together by fascin (the singed gene product).
Tilney
L G
LG
Department of Biology, University of Pennsylvania, Philadelphia 19104, USA.
Tilney
M S
MS
Guild
G M
GM
D000199:Actins / Q000648:ultrastructure*; D000818:Animals; D002352:Carrier Proteins / Q000235:genetics; D002628:Chemoreceptor Cells / Q000648:ultrastructure*; D004330:Drosophila / Q000033:anatomy & histology* / Q000254:growth & development / Q000648:ultrastructure*; D029721:Drosophila Proteins; D007301:Insect Hormones / Q000235:genetics; D007814:Larva / Q000033:anatomy & histology / Q000648:ultrastructure; D008465:Mechanoreceptors / Q000648:ultrastructure*; D008840:Microfilament Proteins; D008855:Microscopy, Electron, Scanning; D009154:Mutation; D011487:Protein Conformation
D000199:Actins; D002352:Carrier Proteins; D029721:Drosophila Proteins; D007301:Insect Hormones; D008840:Microfilament Proteins; C084628:f protein, Drosophila; C020324:fascin