Lg. Tilney et al., F-ACTIN BUNDLES IN DROSOPHILA BRISTLES .1. 2 FILAMENT CROSS-LINKS AREINVOLVED IN BUNDLING, The Journal of cell biology, 130(3), 1995, pp. 629-638
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).