The Mitotic Apparatus

Mitotic apparatuses (MA) isolated from metaphase sea urchin eggs in 12% hexylene glycol at pH 6.4 can be dissolved rapidly in 0.6 M KCl, and more than one-half of the total protein of the MA is soluble under these conditions. In the phase-contrast microscope, the fibrous structure of the MA can be seen to disintegrate in KCl solution, leaving only granular material which, in the electron microscope has been seen to be largely vesicular, with no evidence of microtubules or other fibrous elements. The KCl-soluble material thus must contain the soluble components of the microtubules and consists of one major, homogeneous component with a sedimentation coefficient of 22 Svedbergs, and a much smaller amount of more heterogeneous material sedimenting at 4-5S. A component similar to the 22S component can be identified in extracts of unfertilized eggs, where it forms approximately 8% of the total cell protein. The amount of this protein present in the cell is considerably in excess of that involved in the MA, as can be shown by its presence in the soluble supernate from a mitotic apparatus isolation. This protein must form part of, or be associated with, the fibrous structure of the MA in some fashion that allows its release only upon the dissolution of the mitotic apparatus.