Microtubules, mitochondria, and molecular chaperones: a new hypothesis for in vivo assembly of microtubules

In Chinese hamster ovary cells, a number of independent mutants selected for resistance to antimitotic drugs have been found to be specifically altered in two major cellular proteins, designated P1 (relative mass (Mr) approximately 60-63 kilodaltons (kDa] and P2 (Mr approximately 69-70 kDa), which appeared microtubule related by a number of genetic and biochemical criteria. Antibodies to P1 have been found to bind specifically to mitochondria that showed specific association with microtubules in interphase cells. Biochemical and cDNA sequence studies on P1 showed that this protein, which is localized in the matrix compartment, is the mammalian homolog of the highly conserved chaperonin family of proteins (other members include the GroEL protein of Escherichia coli, the 60-kDa heat-shock protein of yeast, and the rubisco subunit binding protein of plant chloroplasts). The chaperonin proteins in various systems play a transient but essential molecular chaperone role in the proper folding of polypeptide chains and their assembly into oligomeric protein complexes. Our studies on P2 protein established that it corresponds to the constitutive form of the major 70-kDa heat-shock protein of mammalian cells (i.e., hsc70), which also acts as a molecular chaperone in the intracellular transport of nascent proteins to organelles such as mitochondria and endoplasmic reticulum. To account for the above, as well as a number of other observations (e.g., binding of fluorescent-labeled antimitotic drugs to mitochondria, association of tubulin with mitochondria as well as other membranes, and high affinity binding of antimitotic drugs to free tubulin but not to assembled microtubules), a new model for the in vivo assembly of interphase microtubules is proposed. The model ascribes a central role to the mitochondrially localized chaperonin (i.e., P1) protein in the intracellular formation of tubulin dimers and in their addition to the growth sites in microtubules. The proposed model also explains a number of other observations related to microtubule assembly in the literature.