Translocation of ferritin and biomineralization of goethite in the radula of the limpet Cellana toreuma reeve.

The radula of the limpet, Cellana toreuma, consists of a continuous series of teeth in various stages of iron biomineralization. The major iron-binding protein of the limpet's iron-containing granule (siderosome) has been purified and identified as ferritin. Limpet ferritin has a M(r) of 575 kDa and can be resolved into two bands by SDS-PAGE analysis, with respective M(r)s of 26 and 21 kDa. The partial N-terminal amino acid sequences of these two subunits were confirmed, and antisera against them were respectively generated. The specifity of these two antisera shows no difference between them. By using transmission electron microscopy and immunogold staining techniques the following two events were revealed: (1) in the superior epithelial cell of the radula, ferritin was disassembled through autophagy or heterophagy before exocytosis; (2) of the limpet ferritin, at least the 26-kDa subunit was found to pass through the microvilli, resulting in the accumulation of iron in the extracellular tooth chamber and the formation of goethite needles. Intracellular ferritin being translocated to the extracellular environment is discussed in the text.