F-Actin Bundles Are Derivatives of Microvilli

Fruit fly bristles have us tearing out our few remaining hairs. The puzzle is, in forming long bristles, how and why do flies assemble long actin bundles by gluing shorter bundles end-to-end? Fruit fly bristles extend posteriorly in a gentle curve over the fly's back like the hair on a well-groomed individual from the 60's. These bristles, which are cellular extensions of 70 ␮ m in the microchaete and 400 ␮ m in the macrochaete, are supported by a ring of 7–11 cross-linked, membrane-attached actin bundles that run the length of the extension. To generate such an extension and yet to allow curvature of the bristle, the bundles are composed of units or modules, which bend at their junction points (Tilney et al., 1996, 1998). Fig. 1, a light mi-crograph of a macrochaete, reveals the segmentation of the long actin bundles into modules. The average length of the modules is 3 ␮ m but there is quite a range of module lengths (1–5 ␮ m). What is peculiar and more puzzling is that the modules in adjacent bundles in the same bristle, all of which are formed at the same time during bristle elongation, tend to be the same length. The result of this is that the modules in adjacent bundles tend to be in transverse register (see Fig. 1 and Tilney et al., 1996). What are the steps used by the bristle cell in the assembly of bundles, as they may provide insights into the puz-zle? Bristle elongation occurs by the assembly of modules at the elongating bristle tip. Modules form in three stages as indicated in Fig. 2. In stage 1, which occurs near the tip of the bristle, tiny bundles composed of 10–12 filaments appear attached to the plasma membrane both at their tips and laterally. In three dimensions each tiny bundle appears to extend basally from a small patch of dense material similar to that found at the tip of a microvillus. In fact, these tiny bundles look like tiny microvilli. The filaments in the bundle are polar with their barbed ends facing into a small patch of dense material. In stage 2, which occurs farther down the bristle, a number of these tiny bundles aggregate laterally in a polar manner into 7–11 larger bundles. In stage 3, the bundles are zippered up into hexagonally packed arrays by the addition of fascin. In both stage 1 …