F-ACTIN BUNDLES IN DROSOPHILA BRISTLES .1. 2 FILAMENT CROSS-LINKS AREINVOLVED IN BUNDLING

Transverse sections though Drosophila bristles reveal 7-11 nearly roun d, plasma membrane-associated bundles of actin filaments. These filame nts are hexagonally packed and in a longitudinal section they show a 1 2-nm periodicity in both the 1,1 and 1,0 views. From earlier studies t his periodicity is attributable to cross-links and indicates that the filaments are maximally cross-linked. singed mutants also have 7-11 bu ndles, but the bundles are smaller, flattened, and the filaments withi n the bundles are randomly packed (not hexagonal); no periodicity can be detected in longitudinal sections. Another mutant, forked (f(36a)), 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 peri odicity 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-li nks are necessary can be gleaned by studying bristle formation. Bristl es sprout with only microtubules within them. A little later in develo pment actin filaments appear. At early stages the filaments in the bun dles are randomly packed. Later the filaments in the bundles become he xagonally packed and maximally cross-linked. We consider that the fork ed proteins may be necessary early in development to tie the filaments together in a bundle so that they can be subsequently zippered togeth er by fascin (the singed gene product).