Scanning transmission electron microscopic analysis revealed that the 14S fraction of Tetrahymena dynein was of a mixture of two types of particles in approximately equal proportions. The 14S dynein molecules were roughly ellipsoid in shape with approximate axes of 9.5 and 14.5 nm. About half of the particles had tails 20-24-nm long. By the integration of electron scattering intensities, particles with tails had an average mass of 510 kD with a SD of 90 kD. The globular heads of both types of particles had an average mass of 330 kD with a SD of 60 kD. The mass of the tail structure was about 180 kD. By SDS-PAGE, the 14S dynein consisted of two high molecular mass polypeptides above 300 kD that could be distinguished by immunoblot analysis.

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