Characterization of a membrane-specific tubulin isoform by peptide mapping

Subcellular localization of iodinated thyroid tubulin

Subcellular fractions enriched in mitochondria, plasma membranes, microsomes and Golgi apparatus were obtained from thyroid glands of rats injected with I125. Autoradiography of SDS-polyacrylamide gels revealed the presence of a number of radiolabelled proteins in each membrane fraction. One polypeptide, with the same electrophoretic mobility as brain tubulin, was found in all fractions except the plasma membranes and was immunoprecipitated with commercial anti-tubulin monoclonal antibodies. Hydrolysis of Asp-Pro linkages of I125 labelled tubulin with formic acid indicated that there were iodination sites in both the carboxy terminal one third and the amino terminal two thirds of the molecule. These results, together with the absence of iodinated tubulin from the cytosolic fraction, are consistent with the idea that a population of thyroid membrane tubulin is iodinated at multiple sites either just before or after insertion into intracellular membranes where it may act as an anchorage point for microtubule-membrane interactions.

The characterization of phospholipids associated with microtubules, purified tubulin and microtubule associated proteins in vitro

1. Significant levels of total phospholipid phosphate were detected in highly purified microtubule protein preparations. 2. While the phospholipid profiles of total microtubule proteins and microtubule-associated proteins showed both similarities and differences to that of a whole brain homogenate, purified tubulin was associated only with phospholipids that were not detectable in the latter. 3. Phosphatidyl ethanolamine, found exclusively in a fraction of microtubule associated proteins, stimulated microtubule assembly in vitro.