Characterization of protein 4.1R in erythrocytes of zebrafish (Danio rerio): unique binding properties with transmembrane proteins and calmodulin

The NF-κB p65/miR-23a-27a-24 cluster is a target for leukemia treatment

p65 is a transcription factor that is involved in many physiological and pathologic processes. Here we report that p65 strongly binds to the miR-23a-27a-24 cluster promoter to up-regulate its expression. As bone marrow-derived cells differentiate into red blood cells in vitro, p65/miR-23a-27a-24 cluster expression increases sharply and then declines before the appearance of red blood cells, suggesting that this cluster is negatively related to erythroid terminal differentiation. Bioinformatic and molecular biology experiments confirmed that the miR-23a-27a-24 cluster inhibited the expression of the erythroid proteome and contributed to erythroleukemia progression. In addition, high level of the p65/miR-23a-27a-24 cluster was found in APL and AML cell lines and in nucleated peripheral blood cells from leukemia patients. Furthermore, anti-leukemia drugs significantly inhibited the expression of the p65/miR-23a-27a-24 cluster in leukemia cells. Administration of the p65 inhibitor parthenolide significantly improved hematology and myelogram indices while prolonging the life span of erythroleukemia mice. Meanwhile, stable overexpression of these three miRNAs in mouse erythroleukemia cells enhanced cell malignancy. Our findings thus connect a novel regulation pathway of the p65/miR-23a-27a-24 cluster with the erythroid proteome and provide an applicable approach for treating leukemia.

Phosphatidylinositol-4,5 bisphosphate (PIP(2)) inhibits apo-calmodulin binding to protein 4.1

Membrane skeletal protein 4.1R(80) plays a key role in regulation of erythrocyte plasticity. Protein 4.1R(80) interactions with transmembrane proteins, such as glycophorin C (GPC), are regulated by Ca(2+)-saturated calmodulin (Ca(2+)/CaM) through simultaneous binding to a short peptide (pep11; A(264)KKLWKVCVEHHTFFRL) and a serine residue (Ser(185)), both located in the N-terminal 30 kDa FERM domain of 4.1R(80) (H·R30). We have previously demonstrated that CaM binding to H·R30 is Ca(2+)-independent and that CaM binding to H·R30 is responsible for the maintenance of H·R30 β-sheet structure. However, the mechanisms responsible for the regulation of CaM binding to H·R30 are still unknown. To investigate this, we took advantage of similarities and differences in the structure of Coracle, the Drosophila sp. homologue of human 4.1R(80), i.e. conservation of the pep11 sequence but substitution of the Ser(185) residue with an alanine residue. We show that the H·R30 homologue domain of Coracle, Cor30, also binds to CaM in a Ca(2+)-independent manner and that the Ca(2+)/CaM complex does not affect Cor30 binding to the transmembrane protein GPC. We also document that both H·R30 and Cor30 bind to phosphatidylinositol-4,5 bisphosphate (PIP2) and other phospholipid species and that that PIP2 inhibits Ca(2+)-free CaM but not Ca(2+)-saturated CaM binding to Cor30. We conclude that PIP2 may play an important role as a modulator of apo-CaM binding to 4.1R(80) throughout evolution.

Characterization of cytoskeletal protein 4.1R interaction with NHE1 (Na(+)/H(+) exchanger isoform 1)

NHE1 (Na(+)/H(+) exchanger isoform 1) has been reported to be hyperactive in 4.1R-null erythrocytes [Rivera, De Franceschi, Peters, Gascard, Mohandas and Brugnara (2006) Am. J. Physiol. Cell Physiol. 291, C880-C886], supporting a functional interaction between NHE1 and 4.1R. In the present paper we demonstrate that 4.1R binds directly to the NHE1cd (cytoplasmic domain of NHE1) through the interaction of an EED motif in the 4.1R FERM (4.1/ezrin/radixin/moesin) domain with two clusters of basic amino acids in the NHE1cd, K(519)R and R(556)FNKKYVKK, previously shown to mediate PIP(2) (phosphatidylinositol 4,5-bisphosphate) binding [Aharonovitz, Zaun, Balla, York, Orlowski and Grinstein (2000) J. Cell. Biol. 150, 213-224]. The affinity of this interaction (K(d) = 100-200 nM) is reduced in hypertonic and acidic conditions, demonstrating that this interaction is of an electrostatic nature. The binding affinity is also reduced upon binding of Ca(2+)/CaM (Ca(2+)-saturated calmodulin) to the 4.1R FERM domain. We propose that 4.1R regulates NHE1 activity through a direct protein-protein interaction that can be modulated by intracellular pH and Na(+) and Ca(2+) concentrations.

Structural stabilization of protein 4.1R FERM domain upon binding to apo-calmodulin: novel insights into the biological significance of the calcium-independent binding of calmodulin to protein 4.1R

In erythrocytes, 4.1R80 (80 kDa isoform of protein 4.1R) binds to the cytoplasmic tail of the transmembrane proteins band 3 and GPC (glycophorin C), and to the membrane-associated protein p55 through the N- (N-terminal), α- (α-helix-rich) and C- (C-terminal) lobes of R30 [N-terminal 30 kDa FERM (4.1/ezrin/radixin/moesin) domain of protein 4.1R] respectively. We have shown previously that R30 binds to CaM (calmodulin) in a Ca2+-independent manner, the equilibrium dissociation constant (Kd) for R30-CaM binding being very similar (in the submicromolar range) in the presence or absence of Ca2+. In the present study, we investigated the consequences of CaM binding on R30's structural stability using resonant mirror detection and FTIR (Fourier-transform IR) spectroscopy. After a 30 min incubation above 40° C, R30 could no longer bind to band 3 or to GPC. In contrast, R30 binding to p55, which could be detected at a temperature as low as 34° C, was maintained up to 44° C in the presence of apo-CaM. Dynamic light scattering measurements indicated that R30, either alone or complexed with apo-CaM, did not aggregate up to 40° C. FTIR spectroscopy revealed that the dramatic variations in the structure of the β-sheet structure of R30 observed at various temperatures were minimized in the presence of apo-CaM. On the basis of Kd values calculated at various temperatures, ΔCp and ΔG° for R30 binding to apo-CaM were determined as -10 kJ · K(-1) · mol-1 and ~ -38 kJ · mol(-1) at 37° C (310.15 K) respectively. These data support the notion that apo-CaM stabilizes R30 through interaction with its β-strand-rich C-lobe and provide a novel function for CaM, i.e. structural stabilization of 4.1R80.

Insights into the Function of the Unstructured N-Terminal Domain of Proteins 4.1R and 4.1G in Erythropoiesis

Membrane skeletal protein 4.1R is the prototypical member of a family of four highly paralogous proteins that include 4.1G, 4.1N, and 4.1B. Two isoforms of 4.1R (4.1R(135) and 4.1R(80)), as well as 4.1G, are expressed in erythroblasts during terminal differentiation, but only 4.1R(80) is present in mature erythrocytes. One goal in the field is to better understand the complex regulation of cell type and isoform-specific expression of 4.1 proteins. To start answering these questions, we are studying in depth the important functions of 4.1 proteins in the organization and function of the membrane skeleton in erythrocytes. We have previously reported that the binding profiles of 4.1R(80) and 4.1R(135) to membrane proteins and calmodulin are very different despite the similar structure of the membrane-binding domain of 4.1G and 4.1R(135). We have accumulated evidence for those differences being caused by the N-terminal 209 amino acids headpiece region (HP). Interestingly, the HP region is an unstructured domain. Here we present an overview of the differences and similarities between 4.1 isoforms and paralogs. We also discuss the biological significance of unstructured domains.

Two different unique cardiac isoforms of protein 4.1R in zebrafish, Danio rerio, and insights into their cardiac functions as related to their unique structures

Protein 4.1R (4.1R) has been identified as the major component of the human erythrocyte membrane skeleton. The members of the protein 4.1 gene family are expressed in a tissue-specific alternative splicing manner that increases their functions in each tissue; however, the exact roles of cardiac 4.1R in the developing myocardium are poorly understood. In zebrafish (ZF), we identified two heart-specific 4.1R isoforms, ZF4.1RH2 and ZF4.1RH3, encoding N-terminal 30 kDa (FERM) domain and spectrin-actin binding domain (SABD) and C-terminal domain (CTD), separately. Applying immunohistochemistry using specific antibodies for 30 kDa domain and CTD separately, the gene product of ZF4.1RH2 and ZF4.1RH3 appeared only in the ventricle and in the atrium, respectively, in mature hearts. During embryogenesis, both gene expressions are expressed starting 24 h post-fertilization (hpf). Following whole-mount in situ hybridization, ZF4.1RH3 gene expression was detected in the atrium of 37 hpf embryos. These results indicate that the gene product of ZF4.1RH3 is essential for normal morphological shape of the developing heart and to support the repetitive cycles of its muscle contraction and relaxation.

Similarities and differences in the structure and function of 4.1G and 4.1R135, two protein 4.1 paralogues expressed in erythroid cells

Membrane skeletal protein 4.1R is the prototypical member of a family of four highly paralogous proteins that include 4.1G, 4.1N and 4.1B. Two isoforms of 4.1R (4.1R135 and 4.1R80), as well as 4.1G, are expressed in erythroblasts during terminal differentiation, but only 4.1R80 is present in mature erythrocytes. Although the function of 4.1R isoforms in erythroid cells has been well characterized, there is little or no information on the function of 4.1G in these cells. In the present study, we performed detailed characterization of the interaction of 4.1G with various erythroid membrane proteins and the regulation of these interactions by calcium-saturated calmodulin. Like both isoforms of 4.1R, 4.1G bound to band 3, glycophorin C, CD44, p55 and calmodulin. While both 4.1G and 4.1R135 interact with similar affinity with CD44 and p55, there are significant differences in the affinity of their interaction with band 3 and glycophorin C. This difference in affinity is related to the non-conserved N-terminal headpiece region of the two proteins that is upstream of the 30 kDa membrane-binding domain that harbours the binding sites for the various membrane proteins. The headpiece region of 4.1G also contains a high-affinity calcium-dependent calmodulin-binding site that plays a key role in modulating its interaction with various membrane proteins. We suggest that expression of the two paralogues of protein 4.1 with different affinities for band 3 and glycophorin C is likely to play a role in assembly of these two membrane proteins during terminal erythroid differentiation.

Marked difference in membrane-protein-binding properties of the two isoforms of protein 4.1R expressed at early and late stages of erythroid differentiation

Two major isoforms of protein 4.1R, a 135 kDa isoform (4.1R(135)) and an 80 kDa isoform (4.1R(80)), are expressed at distinct stages of terminal erythroid differentiation. The 4.1R(135) isoform is exclusively expressed in early erythroblasts and is not present in mature erythrocytes, whereas the 4.1R(80) isoform is expressed at late stages of erythroid differentiation and is the principal component of mature erythrocytes. These two isoforms differ in that the 4.1R(135) isoform includes an additional 209 amino acids designated as the HP (head-piece) at the N-terminus of 4.1R(80). In the present study, we performed detailed characterization of the interactions of the two 4.1R isoforms with various membrane-binding partners and identified several isoform-specific differences. Although both 4.1R(135) and 4.1R(80) bound to cytoplasmic domains of GPC (glycophorin C) and band 3, there is an order of magnitude difference in the binding affinities. Furthermore, although both isoforms bound CaM (calmodulin), the binding of 4.1R(80) was Ca2+-independent, whereas the binding of 4.1R(135) was strongly Ca2+-dependent. The HP of 4.1R(135) mediates this Ca2+-dependent binding. Ca2+-saturated CaM completely inhibited the binding of 4.1R(135) to GPC, whereas it strongly reduced the affinity of its binding to band 3. Interestingly, in spite of the absence of spectrin-binding activity, the 4.1R(135) isoform was able to assemble on to the membrane of early erythroblasts suggesting that its ability to bind to membrane proteins is sufficient for its membrane localization. These findings enable us to offer potential new insights into the differential contribution of 4.1R isoforms to membrane assembly during terminal erythroid differentiation.