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Career Choice↗
Biomedical subjects
Samuel E Lux
Publications and source records attributed to Samuel E Lux.
Identification and functional characterization of protein 4.1R and actin-binding sites in erythrocyte beta spectrin: regulation of the interactions by phosphatidylinositol-4,5-bisphosphate.
The ternary complex of spectrin, F-actin, and protein 4.1R defines the erythrocyte membrane skeletal network, which governs the stability and elasticity of the membrane. It has been shown that both 4.1R and actin bind to the N-terminal region (residues 1-301) of the spectrin beta chain, which contains two calponin homology domains, designated CH1 and CH2. Here, we show that 4.1R also binds to the separate CH1 and CH2 domains. Unexpectedly, truncation of the CH2 domain by its 20 amino acids, corresponding to its N-terminal alpha helix, was found to greatly enhance its binding to 4.1R. The intact N terminus and the CH1 but not the CH2 domain bind to F-actin, but again, deletion of the first 20 amino acids of the latter exposes an actin-binding activity. As expected, the polypeptide 1-301 inhibits the binding of spectrin dimer to actin and formation of the spectrin-actin-4.1R ternary complex in vitro. Furthermore, the binding of 4.1R to 1-301 is greatly enhanced by PIP(2), implying the existence of a regulatory switch in the cell.
Actins↗
Hereditary spherocytosis--defects in proteins that connect the membrane skeleton to the lipid bilayer.
The molecular causes of hereditary spherocytosis (HS) have been unraveled in the past decade. No frequent defect is found, and nearly every family has a unique mutation. In dominant HS, nonsense and frameshift mutations of ankyrin, band 3, and beta-spectrin predominate. Recessive HS is most often due to compound heterozygosity of defects in ankyrin, alpha-spectrin, or protein 4.2. Common combinations include a defect in the promoter or 5'-untranslated region of ankyrin paired with a missense mutation, a low expression allele of alpha-spectrin plus a missense mutation, and various mutations in the gene for protein 4.2. In most patients' red cells, no abnormal protein is present. Only rare missense mutations, like ankyrin Walsrode (V463I) or beta-spectrin Kissimmee (W202R), have given any insight into the functional domains of the respective proteins. Although the eminent role of the spleen in the premature hemolysis of red cells in HS is unquestioned, the molecular events that cause splenic conditioning of spherocytes are unclear. Electron micrographs show that small membrane vesicles are shed during the formation of spherocytes. Animal models give further insight into the pathogenetic consequences of membrane protein defects as well as the causes of the variability of disease severity.
5' Untranslated Regions↗
Identification of quantitative trait loci that modify the severity of hereditary spherocytosis in wan, a new mouse model of band-3 deficiency.
Defects in red blood cell (RBC) membrane skeleton components cause hereditary spherocytosis (HS). Clinically, HS varies significantly even among individuals with identical gene defects, illustrating the profound effects of genetic background on disease severity. We exploited a new spontaneous mouse model, wan, which arose on the inbred C3H/HeJ strain, to identify quantitative trait loci (QTL) that modify the HS phenotype. Homozygous wan mice have severe HS due to a complete deficiency of erythroid band 3. A QTL analysis of RBC count, hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin content (MCHC) was performed in wan/wan mice from an F2 intercross between C3H/HeJ(+/wan) and CAST/Ei(+/+) F1 hybrids. Hematologic and survival data from C3H, CAST/Ei F2 wan homozygotes support the hypothesis that genetic modifiers significantly influence the band-3 null HS phenotype. Significant QTL were identified for the MCV trait only, suggesting that RBC membrane characteristics are a target for modifier gene action. The most significant quantitative trait locus, Hsm1 (hereditary spherocytosis modifier 1), localizes to mouse Chromosome 12 and is dominant. The peak LOD score was obtained with a marker for Spnb1 encoding erythroid beta-spectrin, an obvious candidate gene.
Animals↗
Cell-specific mitotic defect and dyserythropoiesis associated with erythroid band 3 deficiency.
Most eukaryotic cell types use a common program to regulate the process of cell division. During mitosis, successful partitioning of the genetic material depends on spatially coordinated chromosome movement and cell cleavage. Here we characterize a zebrafish mutant, retsina (ret), that exhibits an erythroid-specific defect in cell division with marked dyserythropoiesis similar to human congenital dyserythropoietic anemia. Erythroblasts from ret fish show binuclearity and undergo apoptosis due to a failure in the completion of chromosome segregation and cytokinesis. Through positional cloning, we show that the ret mutation is in a gene (slc4a1) encoding the anion exchanger 1 (also called band 3 and AE1), an erythroid-specific cytoskeletal protein. We further show an association between deficiency in Slc4a1 and mitotic defects in the mouse. Rescue experiments in ret zebrafish embryos expressing transgenic slc4a1 with a variety of mutations show that the requirement for band 3 in normal erythroid mitosis is mediated through its protein 4.1R-binding domains. Our report establishes an evolutionarily conserved role for band 3 in erythroid-specific cell division and illustrates the concept of cell-specific adaptation for mitosis.
Amino Acid Sequence↗
Simultaneous (AC)n microsatellite polymorphism analysis and single-stranded conformation polymorphism screening is an efficient strategy for detecting ankyrin-1 mutations in dominant hereditary spherocytosis.
Nonsense/stop mutations in the ankyrin-1 gene (ANK1) are a major cause of dominant HS (dHS) (frequency of 23% in German dHS patients). To date, no common mutation has been found and therefore a simple mutation screening is not feasible. The reduced expression of one cDNA allele in the (AC)n microsatellite polymorphism of the ankyrin-1 gene, as seen in about 20% of Czech patients with dHS, may identify candidates with a possible frameshift/nonsense mutation. In order to verify the efficiency of this screening we screened the ankyrin-1 gene of 22 Czech dHS patients for both the reduced cDNA allele expression in the frequent (AC)n and the common exonic 26/39 polymorphisms, as well as for polymerase chain reaction (PCR) single-stranded conformation polymorphisms in any one of the 42 exons of ANK1. Anomalous PCR products were sequenced. We found seven new ANK1 frameshift/nonsense mutations in nine patients with, but in none of six patients without, a reduced cDNA allele expression (efficiency of 78%). We conclude that screening of dHS patients for such a reduced allele expression in common ANK1 polymorphisms is an efficient procedure for the identification of candidates for frameshift/nonsense mutations in the ankyrin-1 gene.
Ankyrins↗