Natural enrichment of ferritin mRNA in mRNP particles

Distribution and activity of alternatively spliced Alzheimer amyloid peptide precursor and scrapie PrP mRNAs on rat brain polysomes

Mammalian brains contain low levels of the Alzheimer amyloid precursor variants (AAPPs) and the normal form of the scrapie agent protease-resistant protein (PrPc); however, their mRNAs are readily detectable. To understand these discrepancies we have investigated some aspects of the translational regulation of these mRNAs. An accurate blot-hybridization procedure was developed to measure absolute amounts of mRNA. Rat brain contains the following mRNA levels (ng/g tissue) AAPP(695), 170; AAPP(751/770), 63; PrPc, 144; actin, 615; glyceraldehyde-3-phosphate dehydrogenase (G3PDH), 359; ferritin, 148. The method was also used to determine the distribution of mRNAs between translationally active polysomes and translationally inactive ribonucleoprotein protein particles (mRNPs). More than 90% of G3PDH and actin mRNAs were associated with polysomal RNA; whereas, ferritin light chain mRNA was predominantly (90%) in mRNP RNA. The degree of cross-contamination of mRNPs with polysomes was less than 10%. Probes specific for the scrapie PrP protein and the AAPP(695) splice junction revealed that 70% of these mRNAs were associated with polysomes. One-half of AAPP(751/770) mRNAs (which comprise 20-30% of all AAPP mRNA in brain) were found in polysomes. We conclude therefore that both scrapie and AAPP mRNAs are subject to translational regulation in rat brain. Evidence from in vitro translational experiments confirm the message distribution determined by blot hybridization and corroborate the hypothesis that AAPP is subject to partial post-transcriptional regulation. Nevertheless, the low tissue levels of AAPP and PrPc must result primarily from their relatively rapid turnover.

Complementarity between ferritin H mRNA and 28 S ribosomal RNA

We have found an interesting complementarity in sequences of human ferritin H mRNA and 28 S ribosomal RNA. Immediately upstream of the initiating AUG in the ferritin mRNA is a stretch of 67 nucleotides which contains sequences complementary to several regions in 28 S RNA. One such region can form 55 base pairings with the 5' noncoding region of the ferritin H mRNA. Most of the complementarity is due to repeats of CCG in the ferritin mRNA and GGC in the ribosomal RNA. The regions of complementarity in the 28 S RNA appear to be expansion sequences that have arisen in the evolution of eukaryotic ribosomal RNA. We suggest that interaction of ferritin mRNA and 28 S RNA may function to regulate the stability and/or translatability of ferritin mRNA.

Molecular size heterogeneity of ferritin in mouse liver

As much as 4% of the total protein in pure liver ferritin from mice with short-term parenteral iron overload produces a minor band migrating anodally to the major (alpha) band of holoferritin with non-denaturing polyacrylamide gel electrophoresis. The components in this minor band and the alpha band have been isolated to purity by preparative electrophoretic fractionation. The protein in the minor band is ferritin, since it contains ferric iron and fulfills defining criteria at the level of biochemistry, immunology and ultrastructure. Native polyacrylamide electrophoresis with pore-size-gradient gels shows that the ferritin molecules in the minor band have a slightly smaller diameter than the holoferritin in the alpha band. Isoelectric focusing reveals that the smaller ferritin has an identical number and range of charge isomers (pI 4.9-5.3) as the larger ferritin, but the relative amount of each size class within some isoferritin bands differs. The smaller ferritin molecules are structurally intact and are made from polypeptide subunits with Mr 18 000; the larger ferritin molecules have subunits with Mr 22 000. The minor species of hepatic ferritin thus has a smaller molecular size because it is made mainly from smaller subunits. No minor electrophoretic band can be detected in liver ferritin obtained from mice with normal iron levels. These results demonstrate that siderosis induces the formation of molecular size polymorphism (macroheterogeneity) in mouse liver ferritin. The new smaller hepatic ferritin could serve to redistribute excess iron into the main storage organs during the early response to iron overload, since it appears to be identical to one of the two types of serum ferritin molecules present in these siderotic mice.

Molecular mechanisms regulating the synthesis of transferrin receptors and ferritin in human erythroleukemic cell lines

The effects of either iron salts or iron chelators on the biosynthesis of transferrin receptors in human erythroleukemic lines was investigated. Addition of these compounds induced a rapid and marked decrease (iron salts) or increase (iron chelators) in the level of transferrin receptors synthesis. Both phenomena were inhibited by actinomycin D. In contrast, the increase in the synthesis of ferritin induced in these cells by addition of iron salts was not inhibited by actinomycin D. These results suggest that iron salts modulate the synthesis of transferrin receptors and ferritin via different molecular mechanisms, of transcriptional and translation type, respectively.

Isolation of ferritin from human hepatocellular carcinoma

Ferritin was extracted from human hepatocellular carcinoma tissue and purified using column chromatography, gradient gel electrophoresis and cadmium sulphate crystallization. DEAE cellulose chromatography showed a difference between hepatoma and normal liver ferritin, indicative of a more acidic isoferritin profile in the tumour. Column-purified and crystalline ferritin and that remaining in the mother-liquor after crystallization was subjected to isoelectric focusing. Hepatoma ferritin showed higher concentrations of acidic isoferritins than liver ferritin. This was most obvious with mother-liquor ferritin, as crystallization tended to select out more basic isoferritins. Subunit analysis of hepatoma and liver ferritin showed a higher proportion of heavy subunits in the tumour ferritin, in keeping with the presence of acidic isoferritins. An antibody against hepatoma mother-liquor ferritin was raised in rabbits. However, hepatoma ferritin proved to be antigenically identical with normal liver ferritin, and we were thus unable to develop a specific radioimmunoassay for hepatoma ferritin.

Variable amounts of translatable ferritin mRNA in bean leaves with various iron contents

RNA was isolated from bean leaves with various iron and ferritin contents. The amount of translatable ferritin mRNA in these RNAs was determined by wheat-germ cell-free translation and immunoprecipitation with anti-bean seed ferritin serum. The levels of ferritin mRNA present in total RNA of the leaves are reflected by the in vivo levels of ferritin in the corresponding leaf tissues. This implies that the iron-dependent regulation of phytoferritin synthesis, in contrast with the regulation of animal ferritin synthesis, occurs at the level of transcription.

Studies on heterogeneity in ferritin subunits

Subunits prepared by dissociating rat and human ferritins by acid/urea or SDS can be resolved by isoelectric focusing in urea/Triton gels into many discrete forms. Most of these are not true isosubunits but aggregation artefacts formed during electrofocusing. The distribution of H and L subunit classes in these aggregates indicates that HeLa and heart ferritins contain similar classes of H and L subunits but that one or both of these classes is different in liver and kidney ferritins. To avoid aggregation artefacts, we examined subunits synthesised in vitro from exogenous mRNA. Our results indicate that HeLa and rat liver cells synthesise only one class of L subunit but two classes of H subunit.