Calmodulin modulates protein 4.1 binding to human erythrocyte membranes

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.

Fluorescence correlation spectroscopy analysis of the hydrophobic interactions of protein 4.1 with phosphatidyl serine liposomes

Fluorescence correlation spectroscopy (FCS) was applied to examine the interactions between a protein and a membrane lipid. The protein 4.1-phosphatidyl serine (PS) interactions served as the model system to demonstrate the membrane lipid-protein interactions. This protein was labeled with rhodamine and its interactions with PS-liposomes were measured by FCS. The present results clearly demonstrated that a small protein molecule, protein 4.1, interacts specifically with a large particle, a PS-liposome. This interaction appears to be hydrophobic and not electrostatic, since the bound protein 4.1 did not dissociate in solution and was specifically released from PS-liposomes by treatment with phospholipase A(2) (PLA(2)). In the present study, using FCS we could demonstrate that the serine residue of PS is required for protein 4.1 to bind to PS-liposomes and that the bound protein 4.1 is closely associated with the fatty acid of the PS molecule in the liposomes.

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

Cytoskeletal protein 4.1R is instrumental in regulating erythrocyte plasticity. 4.1R is comprised of four domains identified after chymotryptic digestion: an N-terminal 30 kDa domain responsible for interaction with membrane proteins, a unique domain, a spectrin-actin binding (SAB) domain, and a C-terminal domain (CTD). 4.1R 30 kDa domain interactions with transmembrane proteins are regulated by the Ca(2+)/calmodulin (CaM) complex. Unlike mature mammalian erythrocytes, fish erythrocytes remain nucleated. Comparing their cytoskeleton architecture and functional properties is therefore of great interest. Here we characterized the recently cloned zebrafish (Danio rerio, ZF) 4.1R and compared its properties with human 4.1R. We identified three ZF4.1R mRNA transcripts in erythrocytes, all characterized by exclusion of the central domains. The major transcript, referred to as BL31, included a full length 30 kDa domain (ZFR30) and parts of the unique region Ua and of CTD. Two minor transcripts, referred to as BL42 and BL56, expressed parts of ZFR30 and of the unique region Ub and full length SAB and CTD domains. Antibodies to ZFR30, ZF4.1R CTD and ZF glycophorin C (GPC) labeled the ZF erythrocyte plasma membrane. ZFR30 bound to CaM in presence or absence of Ca(2+). Resonant mirror detection binding assays revealed that ZFR30 bound to human Band3 with low K((D)) ( approximately 10nM), and to GPC with higher K((D)) ( approximately 1nM). The Ca(2+)/CaM complex did not affect ZFR30 binding to Band3 and GPC. Finally, we confirmed ZFR30 binding to erythrocyte plasma membrane proteins by pulling down ZFR30 with human erythrocyte inside-out vesicles (IOV). This study defines unique structural and functional properties for ZF4.1R.

Characterization of zebrafish merlot/chablis as non-mammalian vertebrate models for severe congenital anemia due to protein 4.1 deficiency

The red blood cell membrane skeleton is an elaborate and organized network of structural proteins that interacts with the lipid bilayer and transmembrane proteins to maintain red blood cell morphology, membrane deformability and mechanical stability. A crucial component of red blood cell membrane skeleton is the erythroid specific protein 4.1R, which anchors the spectrin-actin based cytoskeleton to the plasma membrane. Qualitative and quantitative defects in protein 4.1R result in congenital red cell membrane disorders characterized by reduced cellular deformability and abnormal cell morphology. The zebrafish mutants merlot (mot) and chablis (cha) exhibit severe hemolytic anemia characterized by abnormal cell morphology and increased osmotic fragility. The phenotypic analysis of merlot indicates severe hemolysis of mutant red blood cells, consistent with the observed cardiomegaly, splenomegaly, elevated bilirubin levels and erythroid hyperplasia in the kidneys. The result of electron microscopic analysis demonstrates that mot red blood cells have membrane abnormalities and exhibit a severe loss of cortical membrane organization. Using positional cloning techniques and a candidate gene approach, we demonstrate that merlot and chablis are allelic and encode the zebrafish erythroid specific protein 4.1R. We show that mutant cDNAs from both alleles harbor nonsense point mutations, resulting in premature stop codons. This work presents merlot/chablis as the first characterized non-mammalian vertebrate models of hereditary anemia due to a defect in protein 4.1R integrity.

Phosphorylation of serine residues affects the conformation of the calmodulin binding domain of human protein 4.1

We have previously characterized the calcium-dependent calmodulin (CaM)-binding domain (Ser76-Ser92) of the 135-kDa human protein 4.1 isoform using fluorescence spectroscopy and chemically synthesized nonphosphorylated or serine phosphorylated peptides [Leclerc, E. & Vetter, S. (1998) Eur. J. Biochem. 258, 567-671]. Here we demonstrate that phosphorylation of two serine residues within the 17-residue peptide alters their ability to adopt alpha helical conformation in a position-dependent manner. The helical content of the peptides was determined by CD-spectroscopy and found to increase from 36 to 45% for the Ser80 phosphorylated peptide and reduce to 28% for the Ser84 phosphorylated peptide; the di-phosphorylated peptide showed 32% helical content. Based on secondary structure prediction methods we propose that initial helix formation involves the central residues Leu82-Phe86. The ability of the peptides to adopt alpha helical conformations did not correlate with the observed binding affinities to CaM. We suggest that the reduced CaM-binding affinities observed for the phosphorylated peptides are more likely to be the result of unfavorable sterical and electrostatic interactions introduced into the CaM peptide-binding interface by the phosphate groups, rather than being due to the effect of phosphorylation on the secondary structure of the peptides.

Regulation of the glycophorin C-protein 4.1 membrane-to-skeleton bridge and evaluation of its contribution to erythrocyte membrane stability

The band 3-ankyrin-spectrin bridge and the glycophorin C-protein 4.1-spectrin/actin bridge constitute the two major tethers between the erythrocyte membrane and its spectrin skeleton. Although a structural requirement for the band 3-ankyrin bridge is well established, the contribution of the glycophorin C-protein 4.1 bridge to red cell function remains to be defined. In order to explore this latter bridge further, we have identified and/or characterized five stimuli that sever the linkage in intact erythrocytes and have examined the impact of this rupture on membrane mechanical properties. We report here that elevation of cytosolic 2,3-bisphosphoglycerate, an increase in intracellular Ca(2+), removal of cell O(2), a decrease in intracellular pH, and activation of erythrocyte protein kinase C all promote dissociation of protein 4.1 from glycophorin C, leading to reduced retention of glycophorin C in detergent-extracted spectrin/actin skeletons. Significantly, where mechanical studies could be performed, we also observe that rupture of the membrane-to-skeleton bridge has little or no impact on the mechanical properties of the cell, as assayed by ektacytometry and nickel mesh filtration. We, therefore, suggest that, although regulation of the glycophorin C-protein 4.1-spectrin/actin bridge likely occurs physiologically, the role of the tether and the associated regulatory changes remain to be established.

Regulation of protein 4.1R, p55, and glycophorin C ternary complex in human erythrocyte membrane

Three binary protein-protein interactions, glycophorin C (GPC)-4.1R, GPC-p55, and p55-4.1R, constitute the GPC-4.1R-p55 ternary complex in the erythrocyte membrane. Little is known regarding the molecular basis for the interaction of 4.1R with either GPC or p55 and regarding the role of 4.1R in regulating the various protein-protein interactions that constitute the GPC-4.1R-p55 ternary complex. In the present study, we present evidence that sequences in the 30-kDa domain encoded by exon 8 and exon 10 of 4.1R constitute the binding interfaces for GPC and p55, respectively. We further show that 4.1R increases the affinity of p55 binding to GPC by an order of magnitude, implying that 4.1R modulates the interaction between p55 and GPC. Finally, we document that binding of calmodulin to 4.1R decreases the affinity of 4.1R interactions with both p55 and GPC in a Ca(2+)-dependent manner, implying that the GPC-4.1R-p55 ternary protein complex can undergo dynamic regulation in the erythrocyte membrane. Taken together, these findings have enabled us to identify an important role for 4.1R in regulating the GPC-4.1R-p55 ternary complex in the erythrocyte membrane.

Ca(2+)-dependent and Ca(2+)-independent calmodulin binding sites in erythrocyte protein 4.1. Implications for regulation of protein 4.1 interactions with transmembrane proteins

In vitro protein binding assays identified two distinct calmodulin (CaM) binding sites within the NH(2)-terminal 30-kDa domain of erythrocyte protein 4.1 (4.1R): a Ca(2+)-independent binding site (A(264)KKLWKVCVEHHTFFRL) and a Ca(2+)-dependent binding site (A(181)KKLSMYGVDLHKAKDL). Synthetic peptides corresponding to these sequences bound CaM in vitro; conversely, deletion of these peptides from a 30-kDa construct reduced binding to CaM. Thus, 4.1R is a unique CaM-binding protein in that it has distinct Ca(2+)-dependent and Ca(2+)-independent high affinity CaM binding sites. CaM bound to 4.1R at a stoichiometry of 1:1 both in the presence and absence of Ca(2+), implying that one CaM molecule binds to two distinct sites in the same molecule of 4.1R. Interactions of 4.1R with membrane proteins such as band 3 is regulated by Ca(2+) and CaM. While the intrinsic affinity of the 30-kDa domain for the cytoplasmic tail of erythrocyte membrane band 3 was not altered by elimination of one or both CaM binding sites, the ability of Ca(2+)/CaM to down-regulate 4. 1R-band 3 interaction was abrogated by such deletions. Thus, regulation of protein 4.1 binding to membrane proteins by Ca(2+) and CaM requires binding of CaM to both Ca(2+)-independent and Ca(2+)-dependent sites in protein 4.1.

Calculation of a Gap restoration in the membrane skeleton of the red blood cell: possible role for myosin II in local repair

Human red blood cells contain all of the elements involved in the formation of nonmuscle actomyosin II complexes (V. M. Fowler. 1986. J. Cell. Biochem. 31:1-9; 1996. Curr. Opin. Cell Biol. 8:86-96). No clear function has yet been attributed to these complexes. Using a mathematical model for the structure of the red blood cell spectrin skeleton (M. J. Saxton. 1992. J. Theor. Biol. 155:517-536), we have explored a possible role for myosin II bipolar minifilaments in the restoration of the membrane skeleton, which may be locally damaged by major mechanical or chemical stress. We propose that the establishment of stable links between distant antiparallel actin protofilaments after a local myosin II activation may initiate the repair of the disrupted area. We show that it is possible to define conditions in which the calculated number of myosin II minifilaments bound to actin protofilaments is consistent with the estimated number of myosin II minifilaments present in the red blood cells. A clear restoration effect can be observed when more than 50% of the spectrin polymers of a defined area are disrupted. It corresponds to a significant increase in the spectrin density in the protein free region of the membrane. This may be involved in a more complex repair process of the red blood cell membrane, which includes the vesiculation of the bilayer and the compaction of the disassembled spectrin network.

Regulation of CD44-protein 4.1 interaction by Ca2+ and calmodulin. Implications for modulation of CD44-ankyrin interaction

Erythrocyte membrane skeletal protein 4.1 isoforms have been identified in a variety of non-erythroid cells. However, interactions between protein 4.1 and its binding partners in non-erythroid cell membranes are poorly understood. In the erythrocyte membrane, protein 4.1 binds to the cytoplasmic domain of band 3 and, through this interaction, modulates ankyrin binding to band 3. The sequences LRRRY or IRRRY in band 3 mediate the interaction between band 3 and protein 4.1. The cytoplasmic domain of CD44, a transmembrane glycoprotein found in erythroid as well as non-erythroid cells, has internal sequences SRRRC and QKKKL. We wanted to determine if protein 4.1 binds to CD44 in a fashion analogous to its binding to band 3 and through this interaction modulates ankyrin binding to CD44. We report here that protein 4.1 binds to the cytoplasmic domain of CD44 with a dissociation constant on the order of 10(-7) M and that Ca2+ and calmodulin reduce the affinity of this interaction. Furthermore, although independent binding of both protein 4.1 and ankyrin to CD44 could be documented, binding of protein 4.1 prevented subsequent ankyrin binding. These studies have enabled us to identify a potentially important functional role for protein 4.1 in modulating ankyrin binding to CD44.

Cytoplasmic calcium buffers in intact human red cells

1. Precise knowledge of the cytoplasmic Ca2+ buffering behaviour in intact human red cells is essential for the characterization of their [Ca2+]i-dependent functions. This was investigated by using a refined method and experimental protocols which allowed continuity in the estimates of [Ca2+]i, from nanomolar to millimolar concentrations, in the presence and absence of external Ca2+ chelators. 2. The study was carried out in human red cells whose plasma membrane Ca2+ pump was inhibited either by depleting the cells of ATP or by adding vanadate to the cell suspension. Cytoplasmic Ca2+ buffering was analysed from plots of total cell calcium content vs. ionized cytoplasmic Ca2+ concentration ([CaT]i vs. [Ca2+]i) obtained from measurements of the equilibrium distribution of 45Ca2+ at different external Ca2+ concentrations ([Ca2+]o), in conditions known to clamp cell volume and pH. The equilibrium distribution of 45Ca2+ was induced by the divalent cation ionophore A23187. 3. The results showed the following. (i) The known red cell Ca2+ buffer represented by alpha, with a large capacity and low Ca2+ affinity, was the main cytoplasmic Ca2+ binding agent. (ii) The value of alpha was remarkably constant; the means for each of four donors ranged from 0.33 to 0.35, with a combined value of all independent measurements of 0.34 +/- 0.01 (mean +/- S.E.M., n = 16). This contrasts with the variability previously reported. (iii) There was an additional Ca2+ buffering complex with a low capacity (approximately 80 micromol (340 g Hb)(-1)) and intermediate Ca2+ affinity (apparent dissociation constant, K(D,app) approximately 4-50 microM) whose possible identity is discussed. (iv) The cell content of putative Ca2+ buffers with submicromolar Ca2+ dissociation constants was below the detection limit of the methods used here (less than 2 micromol (340 g Hb)(-1)). 4. Vanadate (1 mM) inhibited the Vmax of the Ca2+ pump in inosine-fed cells by 99.7%. The cytoplasmic Ca2+ buffering behaviour in these cells was similar to that found in ATP-depleted cells.