Abstract
The subunit protein has been isolated from the central-pair and outer-doublet microtubules of sea urchin sperm tails. Both proteins have a sedimentation constant of 6S and a molecular weight of 120,000. Both are converted to a 60,000 molecular weight species by denaturation in 6 M guanidine hydrochloride and reduction with mercaptoethanol. The reduced-alkylated proteins have the same R(f) on disc electrophoresis, and the same amino acid composition, which is very similar to that of muscle actin. The central-pair protein has one binding site for colchicine per 120,000 g. Both proteins appear to have a guanine nucleotide binding site, but the ability to bind GTP in solution has been demonstrated only for the central-pair protein. Although 1 mole of guanine nucleotide is bound per 60,000 g to outer-doublet tubules, the protein obtained by dissolving the doublets at pH 10.5 has lost the guanine nucleotide-binding site and also shows little or no colchicine-binding activity. Comparison of the properties of the isolated protein with electron microscopic evidence on structure of microtubules suggests that the chemical subunit (M = 120,000) consists of two of the 40 A morphological subunits.
Highlights
The central pair and outer-nine doublet tubules of cilia and flagella are regarded as members of the general class of structures referred to as microtubules
Previous work showed the central pair to consist of a protein subunit, of sedimentation constant 6S, with a specific binding site for colchicine [1]
The present work describes the isolation and characterization of the protein subunits of centralpair and outer-nine tubules of sea urchin sperm tails. In their physical properties and amino acid compositions, both proteins are very similar and correspond closely to the protein obtained from mammalian brain.' A characteristic feature of tubule protein obtained from four different phyla is the binding of guanine nucleotides
Summary
The central pair and outer-nine doublet tubules of cilia and flagella are regarded as members of the general class of structures referred to as microtubules. Previous work showed the central pair to consist of a protein subunit, of sedimentation constant 6S, with a specific binding site for colchicine [1]. A colchicine-binding protein with similar properties was found in mitotic apparatus, in the cytoplasm of dividing cells, and in nervous tissue [2, 3]. The present work describes the isolation and characterization of the protein subunits of centralpair and outer-nine tubules of sea urchin sperm tails. In their physical properties and amino acid compositions, both proteins are very similar and correspond closely to the protein obtained from mammalian brain.' A characteristic feature of tubule protein obtained from four different phyla is the binding of guanine nucleotides. The properties of the tubule proteins reported here are very similar to those recently described by Renaud et al [4] for Tetrahymena cilia
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