Identification of a 250 kDa putative microtubule-associated protein as bovine ferritin. Evidence for a ferritin-microtubule interaction

We reported previously on the purification and partial characterization of a putative microtubule-associated protein (MAP) from bovine adrenal cortex with an approximate molecular mass of 250 kDa. The protein was expressed ubiquitously in mammalian tissues, and bound to microtubules in vitro and in vivo, but failed to promote tubulin polymerization into microtubules. In the present study, partial amino acid sequencing revealed that the protein shares an identical primary structure with the widely distributed iron storage protein, ferritin. We also found that the putative MAP and ferritin are indistinguishable from each other by electrophoretic mobility, immunological properties and morphological appearance. Moreover, the putative MAP conserves the iron storage and incorporation properties of ferritin, confirming that the two are structurally and functionally the same protein. This fact led us to investigate the interaction of ferritin with microtubules by direct electron microscopic observations. Ferritin was bound to microtubules either singly or in the form of large intermolecular aggregates. We suggest that the formation of intermolecular aggregates contributes to the intracellular stability of ferritin. The interactions between ferritin and microtubules observed in this study, in conjunction with the previous report that the administration of microtubule depolymerizing drugs increases the serum release of ferritin in rats [Ramm GA, Powell LW & Halliday JW (1996) J Gastroenterol Hepatol11, 1072-1078], support the probable role of microtubules in regulating the intracellular concentration and release of ferritin under different physiological circumstances.