Molecular analysis of cytoplasmic microtubules in situ: identification of both widespread and specific proteins.

We have analyzed the molecular components of cytoplasmic microtubules in situ in several types of cultured cells. Our approach makes use of preparations of detergent-extracted cytoskeletons that contain the original cellular microtubules. Identification of microtubule components is based on two criteria: their presence in the cytoskeleton preparations is dependent upon the presence of intact microtubules, and they are released from such preparations by buffers that depolymerize microtubules. Proteins that meet these criteria are described. Some of them are restricted to cells of certain species or tissues of origin. Others appear to be of widespread distribution. Among the latter is a protein homologous to one of the tau polypeptides in brain microtubule protein.