ACTIN-FILAMENTS IN HONEYBEE-FLIGHT MUSCLE MOVE COLLECTIVELY

To investigate the pattern of actin-filament translation in the intact myofibrillar matrix, we carried out electron micrographic experiments on the ''rigor-stretch'' model of insect-flight muscle. In this model , thin filaments are mechanically severed from their connections to th e Z-line and may then slide freely over the myosin filament when activ ated. The model is similar to the in vitro motility assay in that unte thered actin filaments slide over myosin, but here the natural filamen t lattice is retained: sliding takes place through the lattice of thic k filaments. We find, in this model, that while the extent of thin fil ament translation is variable from sarcomere to sarcomere, filaments n ever translate far enough to enter the opposite I-band, Unlike the in vitro motility assay, where the actin filament translates over the ent ire thick filament even with ''incorrectly'' polarized crossbridges as the sole driver, in this intact filament-lattice model, cross-bridges are apparently unable to move filaments in both directions. We also f ind that the pattern of filament translation is collective. Although t he extent of translation may vary among sarcomeres, in any given half- sarcomere all actin filaments translate by the same degree. Further, t he extent of translation is the same in both halves of a given sarcome re. In rare instances where the extent of translation exhibited a tran sverse gradient across the myofibrillar half-sarcomere, the gradient w as similar on both sides of the sarcomere. Filament translation within the sarcomere is thus collective. Some mechanism ensures that nearby but distinctly separated actin filaments move together and that cooper ative-like behavior therefore extends to the supramolecular level. (C) 1995 Wiley-Liss, Inc.