F Actin Bundles in Drosophila Bristles I. Two Filament Cross-Links Are Involved in Bundling

• Published in: The Journal of cell biology Vol. 130; no. 3; pp. 629 - 638
• Main Authors: Tilney, Lewis G., Tilney, Mary S., Guild, Gregory M.
• Format: Journal Article
• Language: English
• Published: United States Rockefeller University Press 01.08.1995; The Rockefeller University Press
• Subjects: Actins; Actins - ultrastructure; Animals; Biochemistry; Carrier Proteins - genetics; Cell membranes; Cellular biology; Chemoreceptor Cells - ultrastructure; Diptera; Drosophila; Drosophila - anatomy & histology; Drosophila - growth & development; Drosophila - ultrastructure; Drosophila Proteins; Drosophilidae; Genes; Hair cells; Insect Hormones - genetics; Insects; Larva - anatomy & histology; Larva - ultrastructure; Mechanoreceptors - ultrastructure; Membranes; Microfilament Proteins; Microfilaments; Microscopy, Electron, Scanning; Mutation; Neurons; Periodicity; Protein Conformation; Proteins; Pupae; Stereocilia; Thorax
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.130.3.629

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).