Purification of cytoplasmic tubulin and microtubule organizing center proteins functioning in microtubule initiation from the alga Polytomella.

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Abstract
Cytoplasmic tubulin and the microtubule organizing centers (MTOC) for the cytoskeletal microtubule system of the flagellate Polytomella were isolated. The isolated MTOC serve as sites for the in vitro assembly of the purified tubulin protein. The major proteins (4 polypeptides of MW 190,000-210,000) functioning in this assembly have been extracted from the MTOC and purified. Kinetic studies and experiments with in vivo 35S-labeled MTOC proteins (or 35S-labeled tubulin) demonstrate that these proteins function specifically in microtubule initiation and do not contribute to microtubule elongation. Microtubule assembly in vivo is apparently controlled by microtubule initiating proteins associated with the organelles termed MTOCs.