A bent actin cable reveals overlapping bundles.

An actin cable is the sum of its parts, according to Guild et al. (page 1069), who find that short bundles of actin filaments are grafted together to form extra long actin cables in fly bristle cells.

Bristle cells, which can reach lengths of up to 400 μm, are initially supported by an assembly of multiple short stretches of polarized actin bundles. By looking closely at these bundles as they break, the authors see that individual modules of short bundles are assembled by a grafting-like mechanism. In bent cables, the modules separated slightly to reveal the tapered ends of overlapping bundles, suggesting that the modules are not connected by simple end joining.

The group found that two initially unconnected bundles are joined as one extends over the end of the adjacent bundle. Overlapping bundles are then grafted together by fascin and forked, cross-linking proteins that also bundle individual actin filaments. The grafts are hidden by the addition of more filaments to the original bundles, so that the entire cable appears to be one continous, smooth entity.The total length of short cellular extensions, such as microvilli in intenstinal cells or stereocilia in ear hair cells, may be determined by the length of a single actin cable. But overlap and grafting probably create a long yet flexible cable that can either curve, as do bristle cells, or contract by sliding, as do actin cables in fly nurse cells. ▪