The Cytoskeleton of Primary Astrocytes in Culture Contains Actin, Glial Fibrillary Acidic Protein, and the Fibroblast‐Type Filament Protein, Vimentin

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Abstract
Primary astrocytes were cultured from the forebrains of 1-day-old rats. Immunofluorescence microscopy showed that approximately 80% of the cells were positive for glial fibrillary acidic protein (GFAP) and >80% were stained with an antiserum to the molecular weight 58,000 fibroblast intermediate filament protein (vimentin). Gel electrophoresis of Triton-insoluble cytoskeleton preparations from these cultures revealed three major bands having molecular weights of 58,000, 51,000, and 42,000, together with some prominent lower-molecular-weight species. The protein of molecular weight 51,000 was not present in preparations from fibroblasts. Each of the three major astrocyte proteins was subjected to limited proteolysis, while two of the proteins were cleaved by cyanogen bromide. The electrophoretic peptide patterns of the 58,000 protein were similar to those of vimentin isolated from NIL-8 fibroblasts, and the patterns of the 51,000 protein were similar to those of GFAP isolated from rat spinal cord. The patterns of the protein of molecular weight 42,000 resembled those of muscle actin. Rocket immunoelectrophoresis showed that the 51,000 astrocyte protein reacted with an antiserum to bovine GFAP, but the 58,000 and 42,000 proteins failed to react. We conclude that the major proteins of cytoskeleton preparations from cultured primary astrocytes are vimentin (58,000), GFAP (51,000), and actin (42,000), and that our data show no obvious structural relationship among them.