The affinity of DNA-microtubule protein complexes and their disruption by tubulin binding drugs.

Using the gel shift assay system, we have measured the apparent affinity constant for the interaction of two different DNAs with MAP proteins found in both total calf brain microtubules and heat stable brain preparations. Both DNAs studied contained centromere/kinetochore sequences- one was enriched in the calf satellite DNA; the other was a large restriction fragment containing the yeast CEN11 DNA sequence. Complexes formed using both DNAs had similar Kapp values in the range of 2.1 x 10(7) M-1 to 2.0 x 10(8) M-1. CEN11 DNA-MTP complexes had by far the highest Kapp value of 2.0 x 10(8) M-1. The CEN11 DNA sequence is where the yeast kinetochore of chromosome 11 is formed and where the single yeast microtubule is bound in vivo. The CEN11 conserved region II known binding sites-(dA/dT)n runs- for mammalian MAP2 protein, are in good agreement with this higher Kapp value. The effects of the classical tubulin binding drugs colchicine, podophyllotoxin and vinblastine on the DNA-MAP protein complex stability were investigated by determining the drug concentrations where the complexes were destabilized. Only the complexes formed from total microtubule protein (tubulin containing) were destabilized over a wide drug concentration range. Heat stable brain protein complexes (no tubulin) were largely unaffected. Furthermore, it took 10-100 fold higher drug concentrations to disrupt the CEN11 DNA complexes compared to the calf thymus satellite DNA enriched complexes. These data support our previous results suggesting that there is a DNA sequence dependent interaction with MAP proteins that appears to be conserved in evolution (Marx et. al., Biochim. Biophys. Acta. 783, 383-392, 1984; Marx and Denial, Molecular Basis of Cancer 172B, 65-75 1985). In addition, these results imply that the classical tubulin binding drugs may exert their biological effects in cells at least in part by disrupting DNA-Protein complexes of the type we have studied here.