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PubMedDate not supplied

Reorganization of alpha-fodrin induced by stimulation in secretory cells.

Spectrin is an ubiquitous protein composed of heterodimers with alpha and beta subunits. It was first described in erythrocyte cell membranes (see ref. 1 for review) and subsequently in brain, intestinal brush borders, kidney, liver and adrenals. Brain spectrin (fodrin) alpha-subunit, responsible for actin binding, has a relative molecular mass (Mr) of 240,000, whereas the beta-subunit, involved in membrane attachment, has an Mr of 235,000 (refs 1, 3, 9-13). The membrane of secretory granules from adrenal chromaffin cells membrane of secretory granules from adrenal chromaffin cells increases the viscosity of F-actin solution, and spectrin-like protein is associated with storage granule and plasma membranes. Here, we report the localization of fodrin in secretory cells using monospecific antibodies against the alpha-subunit of fodrin using indirect immunofluorescence. We find that the alpha-subunit forms an intensely stained continuous ring in the subplasmalemmal region of resting chromaffin cells. On stimulation of the cell with nicotine, high potassium or ionophores in the presence of calcium, fodrin forms patches. This aggregation is inhibited by trifluoperazine, hence the entrance of calcium into cells following cell depolarization seems to be the calmodulin-dependent stimulus initiating patch formation.

Adrenal Medulla↗
PubMed1997

Effect of glycine on prelethal and postlethal increases in calpain activity in rat renal proximal tubules.

The effect of glycine on hypoxia- and ionomycin-induced increases in calpain activity in rat proximal tubules was determined. Calpain activity was determined both in vitro and in the intact cell using the fluorescent substrate N-succinyl-Leu-Leu-Val-Tyr-7-amido-4-methyl coumarin (N-succinyl-Leu-Leu-Val-Tyr-AMC) and Western blotting for calpain-specific spectrin breakdown products (BDP), respectively. At 7.5 minutes of hypoxia (prelethal injury model) there was a significant (10-fold) increase in in vitro calpain activity that was not inhibited by glycine. At 15 minutes of hypoxia (postlethal injury model) there was a further increase in calpain activity that was inhibited by glycine. Normoxic tubules incubated with the calcium ionophore ionomycin (5 microM) for two minutes and 10 minutes had a significant increase in calpain activity that was not inhibited by glycine. After 15 minutes of hypoxia in the presence of glycine, there was an increase in calpain-specific spectrin breakdown products (BDP) in both Triton X-100 soluble and cytosolic extracts from proximal tubules. Glycine in concentrations up to 10 mM had no direct effect on the in vitro calpain activity of purified calpains. The present study demonstrates that: (1) prelethal increases in calpain activity stimulated by hypoxia and ionomycin treatment are not affected by glycine; (2) calpain-mediated spectrin breakdown during hypoxia occurs in the presence of glycine; (3) glycine does prevent the additional postlethal increase in calpain activity probably by maintaining membrane integrity to calcium influx.

Animals↗
PubMed1978

Cross-linking of erythrocyte membrane proteins by periodate and intramembrane particle distribution.

Treatment of isolated human erythrocyte membranes at pH 7.4 with 0.1-0.5 mM-sodium periodate specifically cross-linked some of the spectrin polypeptides. Treatment with 2 mM-periodate resulted in complete cross-linking of spectrin and partial cross-linking of other polypeptides. The latter treatment also caused aggregation of the intramembrane particles made visible by freeze-fracturing. When membranes that had been treated with 2 mM-periodate were depleted of spectrin by treatment with 0.1 mM-EDTA, extensive aggregation of the intramembrane particles occurred.

Amino Acids↗
PubMed1982

Hereditary spherocytosis of man. Altered binding of cytoskeletal components to the erythrocyte membrane.

Human erythrocytes possess a lattice work of extrinsic proteins on the inner face of the membrane (;cytoskeleton') that maintains the shape and deformability of the cell. The major proteins of the cytoskeleton are spectrin and actin, which are attached to the membrane by protein bands 2.1 (;ankyrin') and 4.1. The interactions of spectrin/actin with erythrocyte membranes from normal subjects and from patients with hereditary spherocytosis (HS) have been studied by using an air-driven ultracentrifuge, which can rapidly separate membranes from soluble proteins (150000g for 30s). The total amount of spectrin/actin in HS and normal ghosts is similar. However, the rate of dissociation of spectrin and actin from HS erythrocyte membranes at low ionic strength is significantly lower than that observed for normal membranes. Spectrin and actin isolated from either HS or normal membranes re-associated in a similar manner to spectrin/actin-depleted vesicles prepared from normal cells. Scatchard analysis showed an average binding capacity of 278mug/mg of membrane protein. However, spectrin/actin-depleted vesicles prepared from HS cells bound significantly less spectrin/actin prepared from either the normal or abnormal cells (average binding capacity 158mug/mg of membrane protein). The defect was defined further by studying the cytoskeleton obtained by Triton X-100 extraction of membranes. Under conditions of low ionic strength cytoskeletons prepared from HS membranes dissociated more slowly than those prepared from normal membranes, and only 80% of the protein from HS cytoskeletons could be solubilized after 180min compared with 100% for normal cytoskeletons. The difference between HS and normal membranes, which persists in isolated cytoskeletons, suggests that alterations in either the primary structure or the degree of phosphorylation of protein bands 2.1 or 4.1 may be central to the molecular basis of hereditary spherocytosis.

Actins↗
PubMed1983

Hereditary spherocytosis of man. Defective cytoskeletal interactions in the erythrocyte membrane.

Hereditary spherocytosis (HS) is an inherited abnormality of red cell shape and results from defective interactions amongst the components of the cytoskeleton. It is known that spectrin/actin dissociates in low ionic strength media from ghosts and cytoskeletons at a rate which is slower for HS than normal preparations. Hybridization experiments have established that this behaviour is not due to a defective spectrin or actin but resides in a spectrin-binding component of the membrane [Hill, Sawyer, Howlett & Wiley (1981) Biochem. J. 201, 259-266]. In the present study erythrocyte shells have been examined in low ionic strength media and a similar difference in the rate of solubilization has been revealed. Since band 4.1 (but not band 2.1) is a common component of cytoskeletons and shells it is possible that 4.1 may be abnormal in the HS condition. The interaction of band 4.1 with spectrin/actin was examined by low shear falling ball viscometry. The addition of a mixture of band 2.1 and 4.1 to a solution of actin and spectrin tetramer increased the viscosity due to cross-linking of the cytoskeletal elements by band 4.1. When band 2.1/4.1 mixtures were derived from five HS families the viscosity was increased to a greater extent than in the normal controls. This difference was not a result of alterations in the calcium dependence of the spectrin/actin-band 4.1 interaction. The results imply that band 4.1 may be defective in the HS condition.

Actins↗
PubMed2003

Fodrin CaM-binding domain cleavage by Pet from enteroaggregative Escherichia coli leads to actin cytoskeletal disruption.

We have previously shown that the plasmid-encoded toxin (Pet) of enteroaggregative Escherichia coli produces cytotoxic and enterotoxic effects. Pet-intoxicated epithelial cells reveal contraction of the cytoskeleton and loss of actin stress fibres. Pet effects require its internalization into epithelial cells. We have also shown that Pet degrades erythroid spectrin. Pet delivery within the intestine suggests that Pet may degrade epithelial fodrin (non-erythroid spectrin). Here we demonstrate that Pet has affinity for alpha-fodrin (formally named alphaII spectrin) in vitro and in vivo and cleaves epithelial fodrin, causing its redistribution within the cells. When Pet has produced its cytoskeletal effects, fodrin is found in intracellular aggregates as membrane blebs. Pet cleaves recombinant GST-fodrin, generating two breakdown products of 37 and 72 kDa. Sequencing of the 37 kDa fragment demonstrated that the cleavage site occurred within fodrin's 11th repetitive unit between M1198 and V1199, in the calmodulin binding domain. Site-directed mutagenesis of these amino acids prevented fodrin degradation by Pet. Pet also cleaves epithelial fodrin from cultured Pet-treated cells. A mutant in the Pet serine protease motif was unable to cause fodrin redistribution or to cleave GST-fodrin. This is the first report showing cleavage of alpha-fodrin by a bacterial protease. Cleavage occurs in the middle of the calmodulin binding domain, which leads to cytoskeleton disruption.

Actins↗
PubMed2000

Temporal profile and cell subtype distribution of activated caspase-3 following experimental traumatic brain injury.

This study investigated the temporal expression and cell subtype distribution of activated caspase-3 following cortical impact-induced traumatic brain injury in rats. The animals were killed and examined for protein expression of the proteolytically active subunit of caspase-3, p18, at intervals from 6 h to 14 days after injury. In addition, we also investigated the effect of caspase-3 activation on proteolysis of the cytoskeletal protein alpha-spectrin. Increased protein levels of p18 and the caspase-3-specific 120-kDa breakdown product to alpha-spectrin were seen in the cortex ipsilateral to the injury site from 6 to 72 h after the trauma. Immunohistological examinations revealed increased expression of p18 in neurons, astrocytes, and oligodendrocytes from 6 to 72 h following impact injury. In contrast, no evidence of caspase-3 activation was seen in microglia at all time points investigated. Quantitative analysis of caspase-3-positive cells revealed that the number of caspase-3-positive neurons exceeded the number of caspase-3-positive glia cells from 6 to 72 h after injury. Moreover, concurrent assessment of nuclear histopathology using hematoxylin identified p18-immunopositive cells exhibiting apoptotic-like morphological profiles in the cortex ipsilateral to the injury site. In contrast, no evidence of increased p18 expression or alpha-spectrin proteolysis was seen in the ipsilateral hippocampus, contralateral cortex, or hippocampus up to 14 days after the impact. Our results are the first to demonstrate the concurrent expression of activated caspase-3 in different CNS cells after traumatic brain injury in the rat. Our findings also suggest a contributory role of activated caspase-3 in neuronal and glial apoptotic degeneration after experimental TBI in vivo.

Animals↗
PubMed2000

Marking synaptic activity in dendritic spines with a calpain substrate exhibiting fluorescence resonance energy transfer.

Excitatory synaptic activity can evoke transient and substantial elevations of postsynaptic calcium. Downstream effects of elevated calcium include the activation of the calcium-dependent protease calpain. We have developed a reagent that identifies dendritic spines in which calpain has been activated. A fusion protein was expressed that contained enhanced yellow and enhanced cyan fluorescent protein (EYFP and ECFP, respectively) linked by a peptide that included the micro-calpain cleavage site from alpha-spectrin. A PDZ-binding site fused to ECFP anchored this protein to postsynaptic densities. The fusion protein exhibited fluorescence resonance energy transfer (FRET), and diminution of FRET by proteolysis was used to localize calpain activity in situ by fluorescence microscopy. Incubation of the fusion protein with calpain in the presence of calcium resulted in the separation of EYFP and ECFP into monomeric fluorophores. In transiently transfected cell lines and dissociated hippocampal neurons, FRET was diminished by raising intracellular calcium levels with an ionophore or with glutamatergic agonists. Calpain inhibitors blocked these changes. Under control conditions, FRET levels in different dendritic spines of cultured neurons and in hippocampal slices were heterogeneous but showed robust decreases upon treatment with glutamatergic agonists. Immunostaining of cultured neurons with antibodies to a spectrin epitope produced by calpain-mediated digestion revealed an inverse correlation between the amount of FRET present at postsynaptic elements and the concentration of spectrin breakdown products. These results suggest that the FRET methodology identifies sites of synaptically induced calpain activity and that it may be useful in analyzing synapses undergoing changes in efficacy.

Amino Acid Sequence↗
PubMed2002

Headpiece domain of dematin is required for the stability of the erythrocyte membrane.

Dematin is an actin-binding and bundling protein of the erythrocyte membrane skeleton. Dematin is localized to the spectrin-actin junctions, and its actin-bundling activity is regulated by phosphorylation of cAMP-dependent protein kinase. The carboxyl terminus of dematin is homologous to the "headpiece" domain of villin, an actin-bundling protein of the microvillus cytoskeleton. The headpiece domain contains an actin-binding site, a cAMP-kinase phosphorylation site, plays an essential role in dematin self-assembly, and bundles F-actin in vitro. By using homologous recombination in mouse embryonic stem cells, the headpiece domain of dematin was deleted to evaluate its function in vivo. Dematin headpiece null mice were viable and born at the expected Mendelian ratio. Hematological evaluation revealed evidence of compensated anemia and spherocytosis in the dematin headpiece null mice. The headpiece null erythrocytes were osmotically fragile, and ektacytometry/micropore filtration measurements demonstrated reduced deformability and filterability. In vitro membrane stability measurements indicated significantly greater membrane fragmentation of the dematin headpiece null erythrocytes. Finally, biochemical characterization, including the vesicle/cytoskeleton dissociation, spectrin self-association, and chemical crosslinking measurements, revealed a weakened membrane skeleton evidenced by reduced association of spectrin and actin to the plasma membrane. Together, these results provide evidence for the physiological significance of dematin and demonstrate a role for the headpiece domain in the maintenance of structural integrity and mechanical properties of erythrocytes in vivo.

Actins↗
PubMed1996

A new function for adducin. Calcium/calmodulin-regulated capping of the barbed ends of actin filaments.

Adducin is a membrane skeleton protein originally described in human erythrocytes that promotes the binding of spectrin to actin and also binds directly to actin and bundles actin filaments. Adducin is associated with regions of cell-cell contact in nonerythroid cells, where it is believed to play a role in regulating the assembly of the spectrin-actin membrane skeleton. In this study we demonstrate a novel function for adducin; it completely blocks elongation and depolymerization at the barbed (fast growing) ends of actin filaments, thus functioning as a barbed end capping protein (Kcap approximately 100 nM). This barbed end capping activity requires the intact adducin molecule and is not provided by the NH2-terminal globular head domains alone nor by the COOH-terminal extended tail domains, which were previously shown to contain the spectrin-actin binding, calmodulin binding, and phosphorylation sites. A novel difference between adducin and other previously described capping proteins is that it is down-regulated by calmodulin in the presence of calcium. The association of stoichiometric amounts of adducin with the short erythrocyte actin filaments in the membrane skeleton indicates that adducin could be the functional barbed end capper in erythrocytes and play a role in restricting actin filament length. Our experiments also suggest novel possibilities for calcium regulation of actin filament assembly by adducin in erythrocytes and at cell-cell contact sites in nonerythroid cells.

Actin Cytoskeleton↗
PubMed1998

Direct association of the gap junction protein connexin-43 with ZO-1 in cardiac myocytes.

The gap junction protein connexin-43 is normally located at the intercalated discs of cardiac myocytes, and it plays a critical role in the synchronization of their contraction. The mechanism by which connexin-43 is localized within cardiac myocytes is unknown. However, localization of connexin-43 likely involves an interaction with the cytoskeleton; immunofluorescence microscopy showed that in cardiac myocytes, connexin-43 specifically colocalizes with the cytoskeletal proteins ZO-1 and alpha-spectrin. In transfected HEK293 cells, immunoprecipitation experiments using coexpressed epitope-tagged connexin-43 and ZO-1 indicated that ZO-1 links connexin-43 with alpha-spectrin. The domains responsible for the protein-protein interaction between connexin-43 and ZO-1 were identified using affinity binding assays with deleted ZO-1 and connexin-43 fusion proteins. Immunoblot analysis of associated proteins showed that the C-terminal domain of connexin-43 binds to the N-terminal domain of ZO-1. The role of this linkage in gap junction formation was examined by a dominant-negative assay using the N-terminal domain of ZO-1. Overexpression of the N-terminal domain of ZO-1 in connexin-43-expressing cells resulted in redistribution of connexin-43 from cell-cell interfaces to cytoplasmic structures; this intracellular redistribution of connexin-43 coincided with a loss of electrical coupling. We therefore conclude that the linkage between connexin-43 and alpha-spectrin, via ZO-1, may serve to localize connexin-43 at the intercalated discs, thereby generating functional gap junctions in cardiac myocytes.

Amino Acid Sequence↗
PubMed1991

Effect of X-irradiation on erythrocyte membrane proteins. Primary radicals.

The effect of X-irradiation on the major proteins of human erythrocyte membranes have been examined. Samples of human erythrocyte ghosts and stripped ghosts were irradiated (up to 1.5 kGy) under air, N2 or N2O. The effects on the main erythrocyte membrane proteins as well as on aggregate formation were investigated using sodium dodecyl sulphate-polyacrylamide gel electrophoresis and high-performance gel permeation chromatography. Experiments were carried out with or without dithiothreitol as a reducing agent. The main peripheral protein of the erythrocyte membrane, spectrin, is more radio-sensitive than the other membrane proteins. Degradation was mainly due to aggregation and was increased by excluding oxygen. Since radiolysis under N2O instead of N2 enhanced the loss of spectrin, OH radicals seemed to be especially effective. Under anaerobic conditions the degradation of band 3 material could also be observed. In stripped erythrocyte ghosts the radiosensitivity of this integral protein was similar to that of spectrin.

Air↗
PubMed1985

Components of the cytoskeleton in the retinal pigmented epithelium of the chick.

The retinal pigmented epithelium (RPE) is a simple cuboidal epithelium with apical processes which, unlike many epithelia, do not extend freely into a lumen but rather interdigitate closely with the outer segments of the neural retina. To determine whether this close association was reflected in the cytoskeletal organization of the RPE, we studied the components of the cytoskeleton of the RPE and their localization in the body of the cell and in the apical processes. By relative mobility on SDS gels and by immunoblotting, we identified actin, vimentin, myosin, spectrin (240/235), and alpha-actinin as major components, and vinculin as a minor component. In addition, the RPE cytoskeleton contains polypeptides of Mr 280,000 and 250,000; the latter co-electrophoreses with actin-binding protein. By immunofluorescence, the terminal web region appeared similar to the comparable region of the intestinal epithelium that consists of broad belts of microfilaments containing myosin, actin, spectrin, and alpha-actinin. However, the components of the apical processes were very different from those of intestinal microvilli. We observed staining along the process for myosin, actin, spectrin, alpha-actinin, and vinculin. The presence in the apical processes of contractile proteins and also of proteins typically found at sites of cell attachments suggests that the RPE may actively adhere to, and exert tension on, the neural retina.

Animals↗
PubMed1986

A lymphoma plasma membrane-associated protein with ankyrin-like properties.

In this study we have used several complementary techniques to isolate and characterize a 72-kD polypeptide that is tightly associated with a major mouse T-lymphoma membrane glycoprotein, gp 85 (a wheat germ agglutinin-binding protein), in a 16 S complex. These two proteins do not separate in the presence of high salt but can be dissociated by treatment with 2 M urea. Further analysis indicates that the 72-kD protein has ankyrin-like properties based on the following criteria: (a) it cross-reacts with specific antibodies raised against erythrocyte and brain ankyrin; (b) it displays a peptide mapping pattern and a pI (between 6.5 and 6.8) similar to that of the 72-kD proteolytic fragment of erythrocyte ankyrin; (c) it competes with erythrocyte ghost membranes (spectrin-depleted preparations) for spectrin binding; and (d) it binds to purified spectrin and fodrin molecules. Most importantly, in intact lymphoma cells this ankyrin-like protein is localized directly underneath the plasma membrane and is found to be preferentially accumulated beneath receptor cap structures as well as associated with a membrane-cytoskeleton complex preparation. It is proposed that the ankyrin-like 72-kD protein may play an important role in linking certain surface glycoprotein(s) to fodrin which, in turn, binds to actin filaments required for lymphocyte cap formation.

Animals↗
PubMed1986

Dynamics of membrane-skeleton (fodrin) organization during development of polarity in Madin-Darby canine kidney epithelial cells.

Madin-Darby canine kidney (MDCK) epithelial cells exhibit a polarized distribution of membrane proteins between the apical and basolateral domains of the plasma membrane. We have initiated studies to investigate whether the spectrin-based membrane skeleton plays a role in the establishment and maintenance of these membrane domains. MDCK cells express an isoform of spectrin composed of two subunits, Mr 240,000 (alpha-subunit) and Mr 235,000 (gamma-subunit). This isoform is immunologically and structurally related to fodrin in lens and brain cells, which is a functional and structural analog of alpha beta-spectrin, the major component of the erythrocyte membrane skeleton. Analysis of fodrin in MDCK cells by immunoblotting, immunofluorescence, and metabolic labeling revealed significant changes in the biophysical properties, subcellular distribution, steady-state levels, and turnover of the protein during development of a continuous monolayer of cells. The changes in the cellular organization of fodrin did not appear to coincide with the distributions of microfilaments, microtubules, or intermediate filaments. These changes result in the formation of a highly insoluble, relatively dense and stable layer of fodrin which appears to be localized to the cell periphery and predominantly in the region of the basolateral plasma membrane of MDCK cells in continuous monolayers. The formation of this structure coincides temporally and spatially with extensive cell-cell contact, and with the development of the polarized distribution of the Na+, K+-ATPase, a marker protein of the basolateral plasma membrane.

Animals↗
PubMed1998

Regulation mechanism of the lateral diffusion of band 3 in erythrocyte membranes by the membrane skeleton.

Mechanisms that regulate the movement of a membrane spanning protein band 3 in erythrocyte ghosts were investigated at the level of a single or small groups of molecules using single particle tracking with an enhanced time resolution (0.22 ms). Two-thirds of band 3 undergo macroscopic diffusion: a band 3 molecule is temporarily corralled in a mesh of 110 nm in diameter, and hops to an adjacent mesh an average of every 350 ms. The rest (one-third) of band 3 exhibited oscillatory motion similar to that of spectrin, suggesting that these band 3 molecules are bound to spectrin. When the membrane skeletal network was dragged and deformed/translated using optical tweezers, band 3 molecules that were undergoing hop diffusion were displaced toward the same direction as the skeleton. Mild trypsin treatment of ghosts, which cleaves off the cytoplasmic portion of band 3 without affecting spectrin, actin, and protein 4.1, increased the intercompartmental hop rate of band 3 by a factor of 6, whereas it did not change the corral size and the microscopic diffusion rate within a corral. These results indicate that the cytoplasmic portion of band 3 collides with the membrane skeleton, which causes temporal confinement of band 3 inside a mesh of the membrane skeleton.

Anion Exchange Protein 1, Erythrocyte↗
PubMed1977

On the mechanism of ATP-induced shape changes in human erythrocyte membranes. II. The role of ATP.

In the preceding paper (Sheetz, M. and S.J. Singer. 1977. J Cell Biol. 73:638-646) it was shown that erythrocyte ghosts undergo pronounced shape changes in the presence of mg-ATP. The biochemical effects of the action of ATP are herein examined. The biochemical effects of the action of ATP are herein examined. Phosphorylation by ATP of spectrin component 2 of the erythrocyte membrane is known to occur. We have shown that it is only membrane protein that is significantly phosphorylated under the conditions where the shape changes are produced. The extent of this phosphorylation rises with increasing ATP concentration, reaching nearly 1 mol phosphoryle group per mole of component 2 at 8mM ATP. Most of this phosphorylation appears to occur at a single site on the protein molecule, according to cyanogen bromide peptide cleavage experiments. The degree of phosphorylation of component 2 is apparently also regulated by a membrane-bound protein phosphatase. This activity can be demonstrated in erythrocyte ghosts prepared from intact cells prelabeled with [(32)P]phosphate. In addition to the phosphorylation of component 2, some phosphorylation of lipids, mainly of phosphatidylinositol, is also known to occur. The ghost shape changes are, however, shown to be correlated with the degree of phosphorylation of component 2. In such experiment, the incorporation of exogenous phosphatases into ghosts reversed the shape changes produced by ATP, or by the membrane-intercalating drug chlorpromazine. The results obtained in this and the preceding paper are consistent with the proposal that the erythrocyte membrane possesses kinase and phosphates activities which produce phosphorylation and dephosphorylation of a specific site on spectrin component 2 molecules; the steady-state level of this phosphorylation regulates the structural state of the spectrin complex on the cytoplasmic surface of the membrane, which in turn exerts an important control on the shape of the cell.

Adenosine Triphosphate↗
PubMed1984

Exclusion of erythrocyte-specific membrane proteins from clathrin-coated pits during differentiation of human erythroleukemic cells.

When human erythroleukemic cells are induced to differentiate, they produce globin and redistribute glycophorin and spectrin to one pole of the cell. This process was accompanied by an alteration in the clathrin-coated pits at the cell surface. In nondifferentiating cells, receptors for Concanavalin A have been shown, using electron microscopy, to be concentrated into coated pits and rapidly internalized. Glycophorin was also internalized via coated pits, but was not greatly concentrated into these portions of the surface membrane. Ligands attached to glycophorin were, therefore, cleared from the cell surface more slowly than Concanavalin A. In nondifferentiating cells, immunoelectron microscopy showed that spectrin is largely excluded from coated pits. After erythroid differentiation proceeded for several days, glycophorin was totally excluded from the coated pits along with spectrin. This did not reflect a general cessation of endocytosis, however, because Concanavalin A receptors continued to be internalized. It is possible that the specific exclusion of glycophorin from coated pits is part of the remodeling process that occurs when the precursor cell membrane differentiates into that of the mature erythrocyte.

Cell Differentiation↗