Modulation of the G protein regulator phosducin by Ca2+/calmodulin-dependent protein kinase II phosphorylation and 14-3-3 protein binding

Phototransduction is a canonical G protein-mediated cascade of retinal photoreceptor cells that transforms photons into neural responses. Phosducin (Pd) is a Gbetagamma-binding protein that is highly expressed in photoreceptors. Pd is phosphorylated in dark-adapted retina and is dephosphorylated in response to light. Dephosphorylated Pd binds Gbetagamma with high affinity and inhibits the interaction of Gbetagamma with Galpha or other effectors, whereas phosphorylated Pd does not. These results have led to the hypothesis that Pd down-regulates the light response. Consequently, it is important to understand the mechanisms of regulation of Pd phosphorylation. We have previously shown that phosphorylation of Pd by cAMP-dependent protein kinase moderately inhibits its association with Gbetagamma. In this study, we report that Pd was rapidly phosphorylated by Ca(2+)/calmodulin-dependent kinase II, resulting in 100-fold greater inhibition of Gbetagamma binding than cAMP-dependent protein kinase phosphorylation. Furthermore, Pd phosphorylation by Ca(2+)/calmodulin-dependent kinase II at Ser-54 and Ser-73 led to binding of the phosphoserine-binding protein 14-3-3. Importantly, in vivo decreases in Ca(2+) concentration blocked the interaction of Pd with 14-3-3, indicating that Ca(2+) controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca(2+)-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.

Human angiotensin II type 1 receptor isoforms encoded by messenger RNA splice variants are functionally distinct

Human tissues that express the angiotensin II (Ang II) type 1 receptor (hAT(1)R) can synthesize four distinct alternatively spliced hAT(1)R mRNA transcripts. In this study, we show that the relative abundance of these mRNA transcripts varies widely in human tissues, suggesting that each splice variant is functionally distinct. Here we demonstrate, for the first time, that the hAT(1)R-B mRNA splice variant encodes a novel long hAT(1)R isoform in vivo that has significantly diminished affinity for Ang II (i.e. >3-fold) when compared with the short hAT(1)R isoform (encoded by hAT(1)R-A mRNA splice variant). This reduced agonist affinity caused a significant shift to the right in the dose-response curve for Ang II-induced inositol trisphosphate production and Ca(2+) mobilization of the long hAT(1)R when compared with that of the short hAT(1)R. The functional differences between these isoforms allows Ang II responsiveness to be fine-tuned by regulating the relative abundance of the long and short hAT(1)R isoform expressed in a given human tissue.

Functional roles of the two domains of phosducin and phosducin-like protein

Phosducin and phosducin-like protein regulate G protein signaling pathways by binding the betagamma subunit complex (Gbetagamma) and blocking Gbetagamma association with Galpha subunits, effector enzymes, or membranes. Both proteins are composed of two structurally independent domains, each constituting approximately half of the molecule. We investigated the functional roles of the two domains of phosducin and phosducin-like protein in binding retinal G(t)betagamma. Kinetic measurements using surface plasmon resonance showed that: 1) phosducin bound G(t)betagamma with a 2. 5-fold greater affinity than phosducin-like protein; 2) phosphorylation of phosducin decreased its affinity by 3-fold, principally as a result of a decrease in k(1); and 3) most of the free energy of binding comes from the N-terminal domain with a lesser contribution from the C-terminal domain. In assays measuring the association of G(t)betagamma with G(t)alpha and light-activated rhodopsin, both N-terminal domains inhibited binding while neither of the C-terminal domains had any effect. In assays measuring membrane binding of G(t)betagamma, both the N- and C-terminal domains inhibited membrane association, but much less effectively than the full-length proteins. This inhibition could only be described by models that included a change in G(t)betagamma to a conformation that did not bind the membrane. These models yielded a free energy change of +1.5 +/- 0.25 kcal/mol for the transition from the G(t)alpha-binding to the Pd-binding conformation of G(t)betagamma.

Molecular cloning and characterization of the human phosducin-like protein (hPhLP) promoter

Phosducin-like protein (PhLP) is an inducible Gbetagamma binding protein which is hypothesized to be a ubiquitous G protein regulator. To elucidate the mechanisms regulating the expression of the human PhLP (hPhLP) gene, we have cloned and characterized its 5'-flanking region. Primer extension analysis identified a major transcription initiation site 172 bp upstream of the ATG start codon. Analysis of the 5'-flanking region revealed that, although it lacked a TATA box element, the hPhLP promoter did contain several consensus binding motifs including AP4, CCAAT, CREB, NF-kappaB, SP1 and E2F. Transient transfection analyses using a series of 5'-flanking deletion/luciferase reporter gene constructs identified a 25 bp sequence (-80 to -55 bp) that is necessary for basal level transcription of the hPhLP gene in all the cell lines investigated. Interestingly, dependent upon the cell line, distinct transcription factors bind to this region suggesting that basal level hPhLP gene transcription may be regulated in a tissue-specific manner.

The immunolocalization and divergent roles of phosducin and phosducin-like protein in the retina

PURPOSE

These investigations were undertaken to compare and contrast the roles of phosducin and phosducin-like protein in the retina.

METHODS

Phosducin and phosducin-like protein were compared in an in vitro assay measuring their inhibition of transducin binding to light-activated rhodopsin. The two proteins were localized within the retina by immunoblot analyses and immunocytochemistry using affinity-purified antibodies with high specificity for each of the two homologs. The sensitivity of phosducin-like protein to phosphorylation was probed using in vitro protein kinase reactions.

RESULTS

Phosducin and phosducin-like protein were found to have similar, though not identical, transducin inhibiting activity in vitro. These two proteins were found to be localized dissimilarly within the retina, with spatial overlap limited to the inner segments of the photoreceptors. Phosducin is found exclusively in photoreceptor cells, including the synaptic and nuclear layers, while phosducin-like protein is found throughout the inner retinal layers, most abundantly in the bipolar cells of the inner nuclear layer. Phosducin-like protein is not efficiently phosphorylated by the protein kinases tested, indicating that its regulation differs from that of phosducin.

CONCLUSIONS

It appears that phosducin and phosducin-like protein play distinct roles in the retina. While phosducin is likely to be important in feedback regulation of the visual signal, such as in light adaptation, phosducin-like protein probably has little if any function in the phototransduction cascade. Phosducin-like protein may have a role in regulating the processing of visual signals by the neural cells of the inner retina.

Human phosducin-like protein (hPhLP) messenger RNA stability is regulated by cis-acting instability elements present in the 3'-untranslated region

Phosducin (Pd) and phosducin-like protein (PhLP) have been shown to regulate G-protein signaling by binding G beta gamma subunits. To better define the function and regulation of PhLP, and to begin to investigate its potential role in human pathophysiological states, we have cloned the human PhLP (hPhLP) cDNA. The hPhLP shows 92% identity with the rat PhLP (rPhLP). However, unlike the rPhLP, no evidence of hPhLP isoforms were detected in the human tissues investigated. Additionally, unlike the rPhLP, alternative polyadenylation sites were detected in hPhLP cDNA clones which corresponded with two distinct mRNA transcripts, 1.2 kb and 3.1 kb, respectively. Interestingly, the predominantly expressed long transcript contains multiple AU-rich elements (AREs) in its 3'-untranslated region (3'-UTR) which have been shown to correlate with rapid mRNA turnover and translational control. This study shows that the hPhLP AREs are functional both in vitro and in vivo, with the long transcript exhibiting a much shorter mRNA half-life. We also demonstrate that subcloning of either the full-length 3'-UTR or the ARE-rich region of the long transcript immediately following the stop codon of luciferase reporter gene confers instability to the luciferase mRNA and results in a ninefold reduction of luciferase activity in the cell types investigated. Taken together, these findings suggest that the AREs present in the long hPhLP mRNA may play a critical role in the regulation of hPhLP gene expression.

A molecular mechanism for the phosphorylation-dependent regulation of heterotrimeric G proteins by phosducin

Visual signal transduction is a nearly noise-free process that is exquisitely well regulated over a wide dynamic range of light intensity. A key component in dark/light adaptation is phosducin, a phosphorylatable protein that modulates the amount of transducin heterotrimer (Gt alpha beta gamma) available through sequestration of the beta gamma subunits (Gt beta gamma). The structure of the phosphophosducin/Gt beta gamma complex combined with mutational and biophysical analysis provides a stereochemical mechanism for the regulation of the phosducin-Gt beta gamma interaction. Phosphorylation of serine 73 causes an order-to-disorder transition of a 20-residue stretch, including the phosphorylation site, by disrupting a helix-capping motif. This transition disrupts phosducin's interface with Gt beta gamma, leading to the release of unencumbered Gt beta gamma, which reassociates with the membrane and Gt alpha to form a signaling-competent Gt alpha beta gamma heterotrimer.

Development and testing of a bacterial biosensor for toluene-based environmental contaminants

A bacterial biosensor for benzene, toluene, and similar compounds has been constructed, characterized, and field tested on contaminated water and soil. The biosensor is based on a plasmid incorporating the transcriptional activator xylR from the TOL plasmid of Pseudomonas putida mt-2. The XylR protein binds a subset of toluene-like compounds and activates transcription at its promoter, Pu. A reporter plasmid was constructed by placing the luc gene for firefly luciferase under the control of XylR and Pu. When Escherichia coli cells were transformed with this plasmid vector, luminescence from the cells was induced in the presence of benzene, toluene, xylenes, and similar molecules. Accurate concentration dependencies of luminescence were obtained and exhibited K1/2 values ranging from 39.0 +/- 3.8 microM for 3-xylene to 2,690 +/- 160 microM for 3-methylbenzylalcohol (means +/- standard deviations). The luminescence response was specific for only toluene-like molecules that bind to and activate XylR. The biosensor cells were field tested on deep aquifer water, for which contaminant levels were known, and were able to accurately detect toluene derivative contamination in this water. The biosensor cells were also shown to detect BETX (benzene, toluene, and xylene) contamination in soil samples. These results demonstrate the capability of such a bacterial biosensor to accurately measure environmental contaminants and suggest a potential for its inexpensive application in field-ready assays.

Regulation of the kinetics of phosducin phosphorylation in retinal rods

Phosducin (Pd) is a widely expressed phosphoprotein that regulates G-protein (G) signaling. Unphosphorylated Pd binds to Gbetagamma subunits and blocks their interaction with Galpha. This binding sequesters Gbetagamma and inhibits both receptor-mediated activation of Galpha and direct interactions between Gbetagamma and effector enzymes. When phosphorylated by cAMP-dependent protein kinase, Pd does not affect these functions of Gbetagamma. To further understand the role of Pd in regulating G-protein signaling in retinal rod photoreceptor cells, we have measured the abundance of Pd in rods and examined factors that control the rate of Pd phosphorylation. Pd is expressed at a copy number comparable to that for the rod G-protein, transducin (Gt). The ratio of rhodopsin (Rho) to Pd is 15. 5 +/- 3.5 to 1. The rate of Pd phosphorylation in rod outer segment preparations was dependent on [cAMP]. K1/2 for cAMP was 0.56 +/- 0. 09 microM, and the maximal rate of phosphorylation was approximately 500 pmol PO4 incorporated/min/nmol Rho. In the presence of Gtbetagamma this rate was decreased approximately 50-fold. From these data, one can estimate a t1/2 of approximately 3 min for the rephosphorylation of Pd in rods during the recovery period after a light response. This relatively slow rephosphorylation of the Pd.Gtbetagamma complex may provide a period of molecular memory in which sensitivity to further light stimuli is reduced as a result of sequestration of Gtbetagamma by Pd.

Regulation of phosducin phosphorylation in retinal rods by Ca2+/calmodulin-dependent adenylyl cyclase

The phosphoprotein phosducin (Pd) regulates many guanine nucleotide binding protein (G protein)-linked signaling pathways. In visual signal transduction, unphosphorylated Pd blocks the interaction of light-activated rhodopsin with its G protein (Gt) by binding to the beta gamma subunits of Gt and preventing their association with the Gt alpha subunit. When Pd is phosphorylated by cAMP-dependent protein kinase, it no longer inhibits Gt subunit interactions. Thus, factors that determine the phosphorylation state of Pd in rod outer segments are important in controlling the number of Gts available for activation by rhodopsin. The cyclic nucleotide dependencies of the rate of Pd phosphorylation by endogenous cAMP-dependent protein kinase suggest that cAMP, and not cGMP, controls Pd phosphorylation. The synthesis of cAMP by adenylyl cyclase in rod outer segment preparations was found to be dependent on Ca2+ and calmodulin. The Ca2+ dependence was within the physiological range of Ca2+ concentrations in rods (K1/2 = 230 +/- 9 nM) and was highly cooperative (n app = 3.6 +/- 0.5). Through its effect on adenylyl cyclase and cAMP-dependent protein kinase, physiologically high Ca2+ (1100 nM) was found to increase the rate of Pd phosphorylation 3-fold compared to the rate of phosphorylation at physiologically low Ca2+ (8 nM). No evidence for Pd phosphorylation by other (Ca2+)-dependent kinases was found. These results suggest that Ca2+ can regulate the light response at the level of Gt activation through its effect on the phosphorylation state of Pd.

The phosphorylation state of phosducin determines its ability to block transducin subunit interactions and inhibit transducin binding to activated rhodopsin

Heterotrimeric GTP-binding proteins (G-proteins) serve many different signal transduction pathways. Phosducin, a 28-kDa phosphoprotein, is expressed in a variety of mammalian cell types and blocks activation of several classes of G-proteins. Phosphorylation of phosducin by cyclic AMP-dependent protein kinase prevents phosducin-mediated inhibition of G-protein GTPase activity (Bauer, P. H., Müller, S., Puzicha, M., Pippig, S., Obermaier, B., Helmreich, E. J. M., and Lohse, M. J. (1992) Nature 358, 73-76). In retinal rods, phosducin inhibits transducin (Gt) activation by binding its beta gamma subunits. While rod phosducin is phosphorylated in the dark and dephosphorylated after illumination (Lee, R.-H., Brown, B. M., and Lolley, R. N. (1984) Biochemistry 23, 1972-1977), the significance of these reactions is still unclear. The data presented here permit a more precise characterization of phosducin function and the consequences of its phosphorylation. Dephosphophosducin blocked binding of the Gt alpha 1 subunit to activated rhodopsin in the presence of stoichiometric amounts of Gt beta gamma, whereas phosphophosducin did not. Surprisingly, the binding affinity of phosphophosducin for Gt beta gamma was not significantly reduced compared with the binding affinity of dephosphophosducin. However, the association of phosducin with Gt beta gamma in a size exclusion column matrix was dependent on the phosphorylation state of phosducin. Moreover, the ability of phosducin to compete with Gt alpha for binding to Gt beta gamma was also dependent on the phosphorylation state of phosducin. No interaction was found between phosducin and Gt alpha. These data indicate that phosducin decreases rod responsiveness by binding to the beta gamma subunits of Gt and preventing their interaction with Gt alpha, thereby inhibiting Gt alpha activation by the activated receptor. Moreover, phosphorylation of phosducin blocks its ability to compete with Gt alpha for binding to Gt beta gamma.

Cooperative binding of the retinal rod G-protein, transducin, to light-activated rhodopsin

Direct measurements of the binding between light-activated rhodopsin (Rho*) and transducin, the retinal rod G-protein, revealed a strongly cooperative interaction. Cooperativity was assessed by measuring the association of 125I-labeled transducin (Gt) to Rho* in urea-stripped rod outer segment membranes at equilibrium. Analysis of 125I-Gt binding curves gave a Hill coefficient of 1.8. These data were consistent with a two-site model in which binding of the first 125I-Gt to Rho* increased the binding of the second 125I-Gt approximately 40-fold (Kd values were 80 +/- 30 and 1.9 +/- 0.7 nM, respectively). The effects of GDP on the binding were also investigated. GDP decreased the affinity between Rho* and Gt approximately 100-fold but did not decrease the degree of cooperativity. Binding curves of 125I-Gt in the presence of 1 mM GDP showed a Hill coefficient of 1.9. The data were also consistent with a two-binding site model in which binding of the first 125I-Gt increased the binding of the second 125I-Gt approximately 70-fold (Kd values were 13.7 +/- 5.4 and 0.20 +/- 0.08 microM, respectively). The Gt alpha subunit in the absence of Gt beta gamma also bound Rho* in a cooperative manner. These data implicate a role for the cooperative association of Rho* and Gt in the light activation cascade of retinal rods.

Localization of the protein 4.1-binding site on the cytoplasmic domain of erythrocyte membrane band 3

Of the several proteins that bind along the cytoplasmic domain of erythrocyte membrane band 3, only the sites of interaction of proteins 4.1 and 4.2 remain to be at least partially localized. Using five independent techniques, we have undertaken to map and characterize the binding site of band 4.1 on band 3. First, transfer of a radioactive cross-linker (125I-2-(p-azido-salicylamido)ethyl-1-3-dithiopropionate) from purified band 4.1 to its binding sites on stripped inside-out erythrocyte membrane vesicles (stripped IOVs) revealed major labeling of band 3, glycophorin C, and glycophorin A. Proteolytic mapping of the stripped IOVs then demonstrated that the label on band 3 was confined largely to a fragment comprising residues 1-201. Second, competitive binding experiments with Fab fragments of monoclonal and peptide-specific polyclonal antibodies to numerous epitopes along the cytoplasmic domain of band 3 displayed stoichiometric competition only with Fabs to epitopes between residues 1 and 91 of band 3. Weak competition was also observed with Fabs to a sequence of the cytoplasmic domain directly adjacent to the membrane-spanning domain, but only at 50-100-fold excess of Fab. Third, band 4.1 protected band 3 from chymotryptic hydrolysis at tyrosine 46 and to a much lesser extent at a site within the junctional peptide connecting the membrane-spanning and cytoplasmic domains of band 3. Fourth, ankyrin, which has been previously shown to interact with band 3 both near a putative central hinge and at the N terminus competed with band 4.1 for band 3 in stripped IOVs. Since band 4.1 does not associate with band 3 near the flexible central hinge, the competition with ankyrin can be assumed to derive from a mutual association with the N terminus. Finally, a synthetic peptide corresponding to residues 1-15 of band 3 was found to mildly inhibit band 4.1 binding to stripped IOVs. Taken together, these data suggest that band 4.1 binds band 3 predominantly near the N terminus, with a possible secondary site near the junction of the cytoplasmic domain and the membrane.

Contribution of the band 3-ankyrin interaction to erythrocyte membrane mechanical stability

In an effort to evaluate the role of the band 3-ankyrin linkage in maintenance of red blood cell membrane integrity, solution conditions were sought that would selectively dissociate the band 3-ankyrin linkage, leaving other membrane skeletal interactions intact. For this purpose erythrocytes were equilibrated overnight in nutrient-containing buffers at a range of elevated pHs and then examined for changes in mechanical stability and membrane skeletal composition. Band 3 was found to be released from interaction with the membrane skeleton over a pH range (8.4 to 9.5) that was observed to dissociate the band 3-ankyrin interaction in vitro. In contrast, all other membrane skeletal associations appeared to remain intact up to pH 9.3, after which they were also seen to dissociate. Whereas hemolysis of mechanically unstressed cells did not begin until approximately pH 9.3, where the membrane skeletons began to disintegrate, enhanced fragmentation of shear stressed membranes was seen to begin near pH 8, where band 3 dissociation was first observed. Furthermore, the shear-induced fragmentation rate was found to reach a maximum at pH 9.4, ie, where band 3 dissociation was essentially complete. Based on these correlations, we hypothesize that the band 3-ankyrin linkage of the membrane skeleton to the lipid bilayer is essential for red blood cell stability in the face of mechanical distortion but not for cellular integrity in the absence of mechanical stress.

Localization of the ankyrin-binding site on erythrocyte membrane protein, band 3

The predominant attachment site of the spectrin-based cytoskeleton to the erythrocyte membrane occurs via the interaction of ankyrin with the cytoplasmic domain of band 3 (cdb3). In order to further characterize this interaction, we have conducted experiments to localize the ankyrin-binding site on cdb3. Four monoclonal and three antipeptide polyclonal antibodies were raised against cdb3 and used in competition studies to identify regions of close association of cdb3 with ankyrin. Antibodies to regions of cdb3 near the cytoplasmic domain-membrane spanning domain junction had no effect on 125I-ankyrin binding. Likewise, an antibody to a highly conserved region between residues 142 and 154 did not inhibit ankyrin binding. However, antibodies at or near the cysteine 201-317 cluster and the proposed proline-rich hinge in the center of cdb3 were potent inhibitors of ankyrin association, as were antibodies to the acidic NH2 terminus. Additional evidence for interaction of ankyrin with the NH2-terminal region of cdb3 was obtained by demonstrating the ability of ankyrin to inhibit tyrosine phosphorylation of cdb3 at its NH2 terminus by a purified calf thymus tyrosine kinase. These studies reveal two regions of cdb3, distant in primary sequence, which interact with ankyrin. A specific conformation of cdb3 may be required to permit these regions to simultaneously associate with ankyrin and allow binding to occur.

The redox state of cysteines 201 and 317 of the erythrocyte anion exchanger is critical for ankyrin binding

Previous studies have demonstrated that modification of erythrocyte membrane cysteine residues via disulfide cross-briding or direct derivatization with thiol reagents promotes massive morphological, rheological, and structural changes in the cell. To determine whether disruption of the band 3-ankyrin interaction, the major membrane-cytoskeletal linkage, might contribute to the above lesions, we quantitatively measured the band 3-ankyrin interaction following modification of Cys-201 and/or Cys-317 of the cytoplasmic domain of band 3. It was observed that irreversible alkylating agents (e.g. N-ethylmaleimide or iodoacetamide and its derivatives), reversible derivatizing compounds (.e.g. p-chloromercuribenzenesulfonate or glutathione), and native disulfide bond formation all blocked the ankyrin interaction. Comparison of the extent of sulfhydryl modification with the degree of inhibition of ankyrin binding further confirmed that cysteine modification was directly responsible for the inhibition. However, analysis of the site of sulfhydryl derivatization revealed that inhibition of ankyrin binding could be initiated in some cases with derivatization of Cys-201, while in other cases obstruction of Cys-317 appeared to be essential. This apparent discrepancy was resolved by demonstrating that Cys-201 of one strand of the cytoplasmic domain of band 3 dimer could disulfide bond with Cys-317 of the opposite strand, thus demonstrating that all four cysteines of the band 3 dimer are clustered at the interface between subunits. We argue that derivatization or disulfide cross-linking of these cysteines can block ankyrin binding by both conformational and steric mechanisms.

Tyrosine phosphorylation of band 3 inhibits peripheral protein binding

The cytoplasmic domain of band 3 (cdb3) of the human erythrocyte membrane is a good substrate of endogenous and exogenous protein-tyrosine kinases. Because one site of tyrosine phosphorylation is within the glycolytic enzyme/hemoglobin-binding region at the N terminus of the polypeptide, we have investigated whether tyrosine phosphorylation of cdb3 might influence its interaction with the above peripheral proteins. Using p40, a protein-tyrosine kinase isolated from bovine thymus, we demonstrate that aldolase binding to cdb3 linked to Affi-Gel 15 is significantly inhibited by phosphorylation of the immobilized band 3. Importantly, upon dephosphorylation of the gel with acid phosphatase, aldolase binding returns to prephosphorylated values. Similarly, cdb3 phosphorylation was found to inhibit glyceraldehyde-3-phosphate dehydrogenase, phosphofructokinase, and hemoglobin binding to immobilized cdb3. In the converse experiment, untreated soluble cdb3 was shown to bind to immobilized aldolase, whereas phosphorylated cdb3 (approximately equal to 1.8 mol of Pi/mol of cdb3) did not. Furthermore, phosphorylated cdb3 was unable to inhibit aldolase catalysis, whereas untreated cdb3, as shown previously by others, was a potent inhibitor. Taken together, these results demonstrate that phosphorylation of cdb3 on tyrosine residues inhibits peripheral protein binding at the polypeptide's N terminus. In view of the known effect of glycolytic enzyme binding to band 3 on catalytic activity, tyrosine phosphorylation of band 3 may modulate glycolysis in vivo.

Heinz bodies induce clustering of band 3, glycophorin, and ankyrin in sickle cell erythrocytes

In earlier model studies we demonstrated that artificially denatured hemoglobin binds to and clusters the protein, band 3, in the plane of the erythrocyte membrane. To determine whether denatured hemoglobin also clusters band 3 in vivo, we have compared the locations of denatured hemoglobin aggregates (Heinz bodies) with band 3 in sickle cells using phase contrast and immunofluorescence microscopy. We report that where Heinz bodies are found associated with the cytoplasmic surface of the membrane, clusters of band 3 are usually colocalized within the membrane. In contrast, normal erythrocyte membranes and regions of sickle cell membranes devoid of Heinz bodies display an uninterrupted staining of band 3. Similarly, ankyrin and glycophorin are periodically seen to aggregate at Heinz body sites, but the degree of colocalization is lower than for band 3. These data demonstrate that the binding of denatured hemoglobin to the membrane forces a redistribution of several major membrane components.