Differences in the regulation of messenger RNA for housekeeping and specialized-cell ferritin. A comparison of three distinct ferritin complementary DNAs, the corresponding subunits, and identification of the first processed in amphibia.

The ferritin family is a widespread group of proteins that maintain iron in a soluble form and also protect against the toxic effects of excess iron. The structure and sequence of the proteins are highly conserved. However, the cell-specific features of structure which occur within the same organism indicate cell specificity of gene expression and may be related to variations in types of iron storage, i.e. specialized-cell ferritin (stored iron is for other cell types) versus housekeeping ferritin (stored iron is for intracellular purposes related to normal or stress metabolism); the protein structure may also affect rates of iron turnover. Iron induces ferritin synthesis and accumulation by recruiting stored ferritin mRNA that is efficiently translated in cells specialized for iron storage. For the first time we show the occurrence of three different cDNAs from bullfrog tadpoles, corresponding to three subunits of the protein: H, M, and L. Thus, ferritin can be encoded by at least three different mRNAs and probably three different genes, in contrast to the older idea of two, H and L; the subunits maintain the conserved sequences of known ferritins and have similar predicted masses, 20.5, 20.6, and 19.9 kDa, but have distinct mobilities in denaturing gels. Ferritin subunit expression is cell specific; more of the H and L chain mRNAs are expressed in red cells than in liver. Ferritin expression is regulated by transcription (or mRNA stability) in adult red cells; cellular levels of ferritin mRNA were 20% that of embryonic red cells, and L subunit mRNA increased 2.5 times with excess iron. Ferritin expression is also regulated during translation in adult red cells; iron recruits stored ferritin mRNA, but only during certain stages of red cell maturation, in contrast to embryonic red cells. The developmental differences in ferritin expression are discussed in relation to the shift from specialized-cell ferritin to housekeeping ferritin in red cells of the embryonic versus adult lines.