Bundling of microtubules in vitro by fodrin.

Fodrin is a spectrin-like protein present in the cortical cytoplasm of neurons and binds to F-actin to induce gelation of actin. We found that fodrin purified from porcine brains co-sedimented with microtubules which were assembled from phosphocellulose-purified tubulin prepared from porcine brains. This indicates that fodrin bound to microtubules. An unusual enhancement of turbidity at 350 nm was observed when microtubules were assembled in the presence of fodrin. Microscopic observations showed that fodrin bundled the microtubules. The interaction between fodrin and microtubules was decreased by microtubule-associated proteins (MAPs), indicating that fodrin and MAPs interacted with microtubules competitively. These data raise the possibility that microtubules are involved in the submembranous cytoskeleton of neurons with fodrin.