Abstract
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.
Highlights
From the Department of Biochemistry, North Carolina State University, Raleigh, North Carolina 27695 and the llDepartments of Medicine and Biochemistry, Duke University Medical Center, Durham, NorthCarolina 27710
*., subunit-specific hybridization probes and inductionby excess iron.Previous studies showed that iron-induced accumulations of ferritin of up to 40-fold occurred in cells specialized for iron storage, with no change in either the concentration or the subunit ratio of ferritin mRNA in hepatocytes or red cells of the embryonic cell line [4,5,6], but resulted from the recruitment of stored ferritin mRNA and competitive translation [6,14,23].We report that fohrousekeeping ferritin induction by iron is accompanied by changes in the ratiosof ferritin subunit mRNA and protein; such differences in the mode of induction may be related to the different role of storage iron in the ferritin of the cell types studied
Mass tadpole red cells a minor third subunit can be detected after electrophoresis of the native protein or the protein synthecDNA clone in2:s mobility prMedaiscsted Naumminboer Homology from cDNA acids sized under the direction of poly(A+) RNA from red cells in wheat germ extracts (Fig. 1).Previously we described aferritin kDa kDa subunit encodedincDNA clonepJD5F12from a tadpole pJD5F12 H 22.8
Summary
The third apoferritin subunit observed in tadpole red cell ferritin (Fig. 1)is encoded in cDNA clone pJDlD8, which was isolated from the same reticulocyte cDNA library [5]used for the other cDNA clones described above.
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