Actin-filaments localize on the sorting endosomes of 3Y1 fibroblastic cells

By immunofluorescence microscopic observation, monoclonal and polyclonal antibodies against a synthetic actin C-terminal peptide were found to stain too colloguial, ambiguous punctuate structures distributed throughout the cytoplasm of 3Y1 cells, independently of actin stress fibers. Antibody against rab5, a small GTP binding protein of the sorting endosome, and anti-actin antibody co-stained these punctuate structures. On the other hand, transferrin receptor, a well characterized maker of the sorting and recycling endosomes, colocalized with actin on the vesicular structures at the cell peripheral region but not at the perinuclear area where the recycling endosome localized. These observations suggest that actin molecules localize on the sorting endosomes. Tropomyosin, F-actin binding protein, also colocalized with actin on the sorting endosomes. From these results, we proposed that actin-filaments with tropomyosin constitute the membrane skeleton on the sorting endosome surface. This article is the first report to show that actin-filaments localize on the intact endosomes.