ON THE RELATIONSHIP OF BRAIN FILAMENTS TO MICROTUBULES1

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Abstract
Abstract— A method has been developed for the isolation of a previously undescribed fibrous protein from rat brain. The newly isolated material consists of bundles of tightly packed 70‐80 Å diameter filaments. Based on studies employing degenerated rat optic nerve, it is proposed that these filaments correspond to the well‐described astrocyte filaments observed in sectioned preparations of mammalian brain. The purified filaments are stable over a wide temperature range, are not disrupted by colchicine, and exhibit limited solubility in the absence of denaturants or detergents. In neutral SDS‐polyacrylamide gel electrophoresis, the filament protein runs as a single band with an apparent molecular weight of 57,000 daltons. This preparation also migrates as a single band in alkaline urea gels, and as a well‐resolved doublet on discontinuous SDS‐urea gels. In all three electrophoretic systems, the filament subunits co‐migrate with rat brain tubulin. Comparative peptide maps of the filament subunits and tubulin indicate a large degree of homology. Our results suggest that microtubules and astrocyte filaments are composed of the same or very similar protein subunits.