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Mitochondrial iron is essential for the biosynthesis of heme and iron-sulfur ([Fe-S]) clusters in mammalian cells. In developing erythrocytes, iron is imported into the mitochondria by MFRN1 (mitoferrin-1, SLC25A37). Although loss of MFRN1 in zebrafish and mice leads to profound anemia, mutant animals showed no overt signs of porphyria, suggesting that mitochondrial iron deficiency does not result in an accumulation of protoporphyrins. Here, we developed a gene trap model to provide in vitro and in vivo evidence that iron regulatory protein-1 (IRP1) inhibits protoporphyrin accumulation. Mfrn1(+/gt);Irp1(-/-) erythroid cells exhibit a significant increase in protoporphyrin levels. IRP1 attenuates protoporphyrin biosynthesis by binding to the 5'-iron response element (IRE) of alas2 mRNA, inhibiting its translation. Ectopic expression of alas2 harboring a mutant IRE, preventing IRP1 binding, in Mfrn1(gt/gt) cells mimics Irp1 deficiency. Together, our data support a model whereby impaired mitochondrial [Fe-S] cluster biogenesis in Mfrn1(gt/gt) cells results in elevated IRP1 RNA-binding that attenuates ALAS2 mRNA translation and protoporphyrin accumulation.

Spectrin and protein 4.1 cross-link F-actin protofilaments into a network called the membrane skeleton. Actin and 4.1 bind to one end of beta-spectrin. The adjacent end of alpha-spectrin, called the EF-domain, is calmodulin-like, with calcium-dependent and calcium-independent EF-hands. It has no known function. However, the sph(1J)/sph(1J) mouse has very fragile red cells and lacks the last 13 amino acids in the EF-domain, suggesting the domain is critical for skeletal integrity. Using pulldown binding assays, we find the alpha-spectrin EF-domain either alone or incorporated into a mini-spectrin binds native and recombinant protein 4.2 at a previously identified region of 4.2 (G(3) peptide). Native 4.2 binds with an affinity comparable with other membrane skeletal interactions (K(d) = 0.30 microM). EF-domains bearing the sph(1J) mutation are inactive. Binding of protein 4.2 to band 3 (K(d) = 0.45 microM) does not interfere with the spectrin-4.2 interaction. Spectrin-4.2 binding is amplified by micromolar concentrations of Ca(2+) (but not Mg(2+)) by three to five times. Calmodulin also binds to the EF-domain (K(d) = 17 microM), and Ca(2+)-calmodulin blocks Ca(2+)-dependent binding of protein 4.2 but not Ca(2+)-independent binding. The data suggest that protein 4.2 is located near protein 4.1 at the spectrin-actin junctions. Because proteins 4.1 and 4.2 also bind to band 3, the erythrocyte anion channel, we suggest that one or both of these proteins cause a portion of band 3 to localize near the spectrin-actin junctions and provide another point of attachment between the membrane skeleton and the lipid bilayer.

Dematin and adducin are actin-binding proteins of the erythrocyte "junctional complex." Individually, they exert modest effects on erythrocyte shape and membrane stability, and their homologues are expressed widely in non-erythroid cells. Here we report generation and characterization of double knock-out mice lacking beta-adducin and the headpiece domain of dematin. The combined mutations result in altered erythrocyte morphology, increased membrane instability, and severe hemolysis. Peripheral blood analysis shows evidence of severe hemolytic anemia with reduced number of erythrocytes/hematocrit/hemoglobin and an approximately 12-fold increase in the number of circulating reticulocytes. The presence of a variety of misshapen and fragmented erythrocytes correlates with increased osmotic fragility and reduced in vivo life span. Despite the apparently normal protein composition of the mutant erythrocyte membrane, the retention of the spectrin-actin complex in the membrane under low ionic strength conditions is significantly reduced by the double mutation. Atomic force microscopy reveals an increase in grain size and a decrease in filament number of the mutant membrane cytoskeleton, although the volume parameter is similar to wild type erythrocytes. Aggregated, disassembled, and irregular features are visualized in the mutant membrane, consistent with the presence of large protein aggregates. Importantly, purified dematin binds to the stripped inside-out vesicles in a saturable manner, and dematin-membrane binding is abolished upon pretreatment of membrane vesicles with trypsin. Together, these results reveal an essential role of dematin and adducin in the maintenance of erythrocyte shape and membrane stability, and they suggest that the dematin-membrane interaction could link the junctional complex to the plasma membrane in erythroid cells.

In mammals, the functional unit for definitive erythropoiesis is the erythroblastic island, a multicellular structure composed of a central macrophage surrounded by developing erythroblasts. Erythroblast-macrophage interactions play a central role in the terminal maturation of erythroblasts, including enucleation. One possible mediator of this cell-cell interaction is the protein Emp (erythroblast macrophage protein). We used targeted gene inactivation to define the function of Emp during hematopoiesis. Emp null embryos die perinatally and show profound alterations in the hematopoietic system. A dramatic increase in the number of nucleated, immature erythrocytes is seen in the peripheral blood of Emp null fetuses. In the fetal liver virtually no erythroblastic islands are observed, and the number of F4/80-positive macrophages is substantially reduced. Those present lack cytoplasmic projections and are unable to interact with erythroblasts. Interestingly, wild type macrophages can bind Emp-deficient erythroblasts, but these erythroblasts do not extrude their nuclei, suggesting that Emp impacts enucleation in a cell autonomous fashion. Previous studies have implicated the actin cytoskeleton and its reorganization in both erythroblast enucleation as well as in macrophage development. We demonstrate that Emp associates with F-actin and that this interaction is important in the normal distribution of F-actin in both erythroblasts and macrophages. Thus, Emp appears to be required for erythroblast enucleation and in the development of the mature macrophages. The availability of an Emp null model provides a unique experimental system to study the enucleation process and to evaluate the function of macrophages in definitive erythropoiesis.

N-Myristoyltransferase (NMT) transfers myristate to an amino-terminal glycine of many eukaryotic proteins. In yeast, worms, and flies, this enzyme is essential for viability of the organism. Humans and mice possess two distinct but structurally similar enzymes, NMT1 and NMT2. These two enzymes have similar peptide specificities, but no one has examined the functional importance of the enzymes in vivo. To address this issue, we performed both genetic and biochemical studies. Northern blots with RNA from adult mice and in situ hybridization studies of day 13.5 embryos revealed widespread expression of both Nmt1 and Nmt2. To determine whether the two enzymes are functionally redundant, we generated Nmt1-deficient mice carrying a beta-galactosidase marker gene. beta-Galactosidase staining of tissues from heterozygous Nmt1-deficient (Nmt1+/-) mice and embryos confirmed widespread expression of Nmt1. Intercrosses of Nmt1+/- mice yielded no viable homozygotes (Nmt1-/-), and heterozygotes were born at a less than predicted frequency. Nmt1-/- embryos died between embryonic days 3.5 and 7.5. Northern blots revealed lower levels of Nmt2 expression in early development than at later time points, a potential explanation for the demise of Nmt1-/- embryos. To explore this concept, we generated Nmt1-/- embryonic stem (ES) cells. The Nmt2 mRNA could be detected in Nmt1-/- ES cells, but the total NMT activity levels were reduced by approximately 95%, suggesting that Nmt2 contributes little to total enzyme activity levels in these early embryo cells. The Nmt1-/- ES cells were functionally abnormal; they yielded small embryoid bodies in in vitro differentiation experiments and did not contribute normally to organogenesis in chimeric mice. We conclude that Nmt1 is not essential for the viability of mammalian cells but is required for development, likely because it is the principal N-myristoyltransferase in early embryogenesis.

We isolated cDNAs that encode a 77-kDa peptide similar to repeats 10-16 of beta-spectrins. Its gene localizes to human chromosome 19q13.13-q13.2 and mouse chromosome 7, at 7.5 centimorgans. A 289-kDa isoform, similar to full-length beta-spectrins, was partially assembled from sequences in the human genomic DNA data base and completely cloned and sequenced. RNA transcripts are seen predominantly in the brain, and Western analysis shows a major peptide that migrates as a 72-kDa band. This new gene, spectrin betaIV, thus encodes a full-length minor isoform (SpbetaIVSigma1) and a truncated major isoform (SpbetaIVSigma5). Immunostaining of cells shows a micropunctate pattern in the cytoplasm and nucleus. In mesenchymal stem cells, the staining concentrates at nuclear dots that stain positively for the promyelocytic leukemia protein (PML). Expression of SpbetaIVSigma5 fused to green fluorescence protein in cells produces nuclear dots that include all PML bodies, which double in number in transfected cells. Deletion analysis shows that partial repeats 10 and 16 of SpbetaIVSigma5 are necessary for nuclear dot formation. Immunostaining of whole-mount nuclear matrices reveals diffuse positivity with accentuation at PML bodies. Spectrin betaIV is the first beta-spectrin associated with a subnuclear structure and may be part of a nuclear scaffold to which gene regulatory machinery binds.

The structural protein ankyrin functions in red blood cells to link the spectrin-based membrane skeleton to the plasma membrane. Ankyrin proteins are now known to occur in most cell types, and two distinct ankyrin genes have been identified (erythroid (Ank-1) and brain (Ank-2)). We have characterized transcripts of the mouse erythroid ankyrin gene by cDNA cloning and DNA sequencing. Ank-1 transcripts of 7.5 and 9.0 kilobases are found in erythroid tissues, and a 9.0-kilobase transcript is found in cerebellum. RNA hybridization blot analysis of 13 additional mouse tissues has detected four novel Ank-1 transcripts (5.0, 3.5, 2.0, and 1.6 kilobases in size). Sequencing of Ank-1 cDNA clones isolated from mouse reticulocyte, spleen, and cerebellar libraries has identified (i) multiple 5' ends that indicate possible multiple promoters; (ii) alternative polyadenylation sites that probably account for the 7.5- and 9.0-kilobase size difference; (iii) a variety of small insertions and deletions that could produce transcripts (and ultimately proteins) of nearly identical size, but different functions; and (iv) clones with large deletions of coding sequence that account for the smaller transcripts seen in spleen, skeletal muscle, and heart.