Following extraction of actomyosin and tubulin from cultured cells treated with Triton X-100, a cytoskeleton remains which is composed predominantly of the cell nucleus encompassed by a network of 10-nm filaments. After negative staining the dense perinuclear region appears as a densely woven filament net punctuated by patches of high electron density. Enucleation of 3T3 cells with cytochalasin B gives rise to karyoplasts surronunded by 10-nm filaments and cytoplasts in which 10-nm filaments remain situated in the central region of the cytoplasm. While the 10-nm filaments occurred mainly as single filaments in human skin fibroblasts and 3T3 cells, in epithelioid PtK1 and PtK2 cells they were commonly associated in prominent meandering bundles. In addition, in these latter cells after Triton extraction the remaining ribosomes were bound specifically to the 10-nm-filament net. After exposure of 3T3 cells to cytochalasin B the 10-nm filaments formed branches that radiated from the perinuclear region into the immobile cell extensions. Concavalin A had no marked effect on the distribution of the 10-nm-filament net. The results suggest that the 10-nm filaments act primarily as structural elements, serving, in particular, to support and constrain the nucleus in its position in the cell.

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