In the previous report, we showed that lysosomal movements and transformations were greatly affected by actin filament destabilizers and antimicrotubular drugs (3). To confirm this circumstantial evidence suggesting an important role of cytoskeletal elements in the regulation of lysosomal movements, the direct morphological interaction of actin filaments with lysosomes in rat alveolar macrophages were examined by several electron microscopic techniques. In the observation of ultrathin sections of alveolar macrophages, some filamentous elements appeared to be closely associated with lysosomes in the cytoplasm. The three-dimensional observations in the whole mount macrophages and the cells cleaved by the dry-blowing method also demonstrated that the filamentous elements were in contact with lysosomes in situ. The organization of filamentous elements closely associated with lysosomes was mainly composed of actin filaments which could be decorated with heavy meromyosin (HMM), although a few intermediate filaments existed among them.
In a cell-free experimentation in vitro, when G-actin was mixed with the lysosomal fraction isolated from alveolar macrophages and then polymerized to Factin, it was found that F-actin filaments appeared to be in contact with the membrane of lysosomes by negative-stained electron microscopy. In vitro interaction of actin filaments with isolated lysosomes implies that the lysosomal membrane has the ability to bind to actin filaments. This finding of in vitro experimentation coincides with those of in situ observation.
These results provide substantial evidence for the interaction of actin filaments with the lysosomal membrane, which suggest that actin filaments participate in regulating intracellular lysosomal movements.