Increased methyl esterification of membrane proteins in aged red-blood cells. Preferential esterification of ankyrin and band-4.1 cytoskeletal proteins

The enzymatic carboxyl methyl esterification of erythrocyte membrane proteins has been investigated in three different age-related fractions of human erythrocytes. When erythrocytes of different mean age, separated by density gradient centrifugation, were incubated under physiological conditions (pH 7.4, 37 degrees C) in the presence of L-[methyl-3H]methionine, the precursor in vivo of the methyl donor S-adenosylmethionine, a fourfold increase in membrane-protein carboxyl methylation was observed in the oldest cells compared with the youngest ones. The identification of methylated species, based on comigration of radioactivity with proteins stained with Coomassie blue, analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, shows, in all cell fractions, a pattern similar to that reported for unfractionated erythrocytes. However in the membrane of the oldest erythrocytes the increase in methylation of the cytoskeletal proteins, bands 2.1 and 4.1, appears to be significantly more marked compared with that observed in the other methylated polypeptides. Furthermore the turnover rate of incorporated [3H]methyl groups in the membrane proteins of the oldest cells markedly increases during cell ageing. Particularly in band 4.1 the age-related increase in methyl esterification is accompanied by a significant reduction of the half-life of methyl esters. The activity of cytoplasmic protein methylase II does not change during cell ageing, while the isolated ghosts from erythrocytes of different age show an age-related increased ability to act as methyl-accepting substrates, when incubated in presence of purified protein methylase II and methyl-labelled S-adenosylmethionine, therefore the relevance of membrane structure in determining membrane protein methylation levels can be postulated. Finally the possible correlation of this posttranslational protein modification with erythrocyte ageing is discussed.     

Membrane alterations in irreversibly sickled cells: Hemoglobin–membrane interaction

Irreversibly sickled cells (ISCs) are sickle erythrocytes which retain bipolar enlongated shapes despite reoxygenation and owe their biophysical abnormalities to acquired membrane alterations. Freeze-etched membranes both of ISCs produced in vitro and ISCs isolated in vivo reveal microbodies fixed to the internal (PS) surface which obscure spectrin filaments. Intramembranous particles (IMPs) on the intramembrane (PF) surface aggregate over regions of subsurface microbodies. Electron microscopy of diaminobenzidine-treated ISC ghosts show the microbodies to contain hemoglobin and/or hemoglobin derivatives. Scanning electron microscopy and freeze-etching demonstrate that membrane–hemoglobin S interaction in ISCs enhances the membrane loss by microspherulation. Membrane-bound hemoglobin is five times greater in in vivo ISCs than non-ISCs, and increases during ISC production, paralleling depletion of adenosine triphosphate. Polyacrylamide gel electrophoresis of ISC membranes shows the presence of high-molecular-weight heteropolymers in the pre–band 1 region, a decrease in band 4.1 and an increase in bands 7, 8, and globin. The role of cross-linked membrane protein polymers in the generation of ISCs is discussed and is synthesized in terms of a unified concept for the determinants of the genesis of ISCs.     

Ubiquitination of spectrin regulates the erythrocyte spectrin-protein-4.1-actin ternary complex dissociation: implications for the sickle cell membrane skeleton.

It has been demonstrated by our laboratory that the irreversibly sickled cell (ISC) spectrin-4.1-actin complex dissociates slowly as compared to ternary complexes formed out of control (AA) and reversibly sickle cell (RSCs) core skeletons. These studies indicated that the molecular basis for the inability of irreversibly sickled cells (ISCs) to change shape is a skeleton that disassembles, and therefore reassembles, very slowly. The present study is based on the following observations: a) alpha-spectrin repeats 20 and 21 contain ubiquitination sites, and b) The spectrin repeats beta-1 and beta-2 are in direct contact with spectrin repeats alpha-20 and alpha-21 during spectrin heterodimer formation, and contain the protein 4.1 binding domain. We demonstrate here that alpha-spectrin ubiquitination at repeats 20 and 21 increases the dissociation of the spectrin-protein-4.1-actin ternary complex thereby regulating protein 4.1's ability to stimulate the spectrin-actin interaction. Performing in vitro ternary complex dissociation assays with AA control and sickle cell SS spectrin (isolated from high-density sickle cells), we further demonstrate that reduced ubiquitination of alpha-spectrin is, in part, responsible for the locked membrane skeleton in sickle cell disease.     

The role of Ca2+-dependent biochemical changes in the ageing process in normal red cells and in the development of irreversibly sickled cells

During the ageing process of normal red cells and in the formation of irreversibly sickled cells (ISCs) there is a progressive increase in the intracellular concentration of Ca2+. This is parallelled by the development of a variety of morphological and biochemical changes in older fractions of normal cells and in ISCs which are similar to those seen in normal cells exposed to Ca2+ ionophore. These changes include cell shrinkage, loss of membrane lipid and degradation of cytoskeletal proteins and polyphosphoinositides. In this paper we consider the ways in which the Ca2+-dependent biochemical changes may be related to the morphological alterations which are characteristic of ageing and irreversible sickling.     

Methylation of erythrocyte membrane proteins at extracellular and intracellular D-aspartyl sites in vitro. Saturation of intracellular sites in vivo

A cytosolic protein carboxyl methyltransferase (S-adenosyl-L-methionine:protein O-methyltransferase, E.C. 2.1.1.24) purified from human erythrocytes catalyzes the methylation of erythrocyte membrane proteins in vitro using S-adenosyl-L-[methyl-3H]methionine as the methyl group donor. The principal methyl-accepting proteins have been identified by sodium dodecyl sulfate-gel electrophoresis at pH 2.4 and fluorography as the anion transport protein (band 3), ankyrin (band 2.1), and integral membrane proteins with molecular weights of 45,000, 28,000, and 21,000. Many of the methylation sites associated with intrinsic membrane proteins may reside in their extracellular portions, since these same proteins are methylated when intact cells are used as the substrate. The maximal number of methyl groups transferred in these experiments is approximately 30 pmol/mg of membrane protein, a value which represents less than one methyl group/50 polypeptide chains of any methyl-accepting species. The number of methylation sites associated with the membranes is increased, but not to stoichiometric levels, by prior demethylation of the membranes. The additional sites are associated primarily with bands 2.1 and 4.1, the principal methyl acceptors in vivo, suggesting that most methylation sites are fully modified in vivo. Extracellular methylation sites are not increased by demethylation of membranes. The aspartic acid beta-methyl ester which can be isolated from carboxypeptidase Y digests of [3H]methylated membranes is in the unusual D-stereoconfiguration. Similar results have been obtained with [3H]methylated membranes isolated from intact cells (McFadden, P.N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2460-2464). It is proposed that the methyltransferase recognizes D-aspartyl residues in proteins and is involved with the metabolism of damaged proteins in vivo.     

Identification of glucose and nucleoside transport proteins in neonatal pig erythrocytes using monoclonal antibodies against band 4.5 polypeptides of adult human and pig erythrocytes

Cytochalasin B and nitrobenzylthioinosine (NBMPR), which inhibit membrane transport of glucose and nucleosides, respectively, have served as photoaffinity ligands that become covalently linked at inhibitor binding sites on transporter-associated proteins. Thus, when membranes from erythrocytes of neonatal pigs with site-bound [3H]cytochalasin B or [3H]NBMPR were irradiated with uv light, two labeled membrane polypeptides (peak Mr values: 55,000 and 64,000, respectively) were identified. Treatment of the photolabeled membranes with endoglycosidase F increased the mobility of [3H]cytochalasin B- and [3H]NBMPR-labeled material (peak Mr values: 44,000 and 57,000, respectively) and limited digestion with trypsin yielded different polypeptide fragments (Mr values: 18,000-23,000 and 43,000, respectively). Identification of the photolabeled polypeptides as transporter components was established using monoclonal antibodies (MAbs) raised against partially purified preparations of band 4.5 from erythrocytes of adult pigs and humans. MAbs 65D4 and 64C7 (anti-human band 4.5), raised in this study, reacted with [3H]cytochalasin B-labeled material from membranes of human erythrocytes and bound to permeabilized erythrocytes but not to intact cells. MAb 65D4 also bound to erythrocytes of mice and neonatal pigs and to a variety of cultured cells (mouse, human, rat), including AE1 mouse lymphoma cells, which lack an NBMPR-sensitive nucleoside transporter. Also employed was MAb 11C4 (anti-pig band 4.5), which recognizes the NBMPR-binding protein of erythrocyte membranes from adult pigs. When membrane proteins from neonatal and adult pigs were subjected to electrophoretic analysis and blots were probed with different MAbs, MAb 65D4 (anti-human band 4.5) bound to material that comigrated with [3H]cytochalasin B-labeled polypeptides (band 4.5) from neonatal, but not adult, pig erythrocytes, whereas MAb 11C4 (anti-pig band 4.5) bound to material that comigrated with [3H]NBMPR-labeled band 4.5 polypeptides of erythrocytes from both neonatal and adult pigs. These results, which indicate structural differences in the cytochalasin B- and NBMPR-binding proteins of pig erythrocytes, establish the presence of both proteins in erythrocytes of neonatal pigs and suggest that only the NBMPR-binding protein is present in erythrocytes of adult pigs.     

Photoaffinity labeling of procaine-binding sites in normal and sickle cell membranes

A photoaffinity probe, procaine azide, was employed to determine the sites of interaction of procaine in normal and sickle cell erythrocytes. Studies show that the number of binding sites and affinity of procaine to membranes derived from normal and sickled cell erythrocytes were similar, although procaine retards the in vitro formation of irreversibly sickled cells from cells. The results show that procaine azide, a photoaffinity analogue of procaine, is covalently incorporated into both protein (60–70%) and lipid (40–30%) components of the membrane. Sodium dodecyl sulfate-gel electrophoresis of the labeled ghosts show that procaine binds specifically to band 3 and periodic acid-Schiff staining bands in membranes derived from labeled erythrocytes. Binding of procaine or covalent incorporation of procaine azide into membrane proteins does not affect the phosphate transport. Moreover, pre-treatment of intact erythrocytes with 4,4′-diisothiocyano-2,2′-stilbene disulfonate, an anion transport inhibitor, did not affect either the binding or covalent incorporation of procaine azide into erythrocytes. These results indicate that the binding of procaine azide to Band 3 protein occurs at a locus different than that involved in anion translocation process.     

Membrane protein carboxyl methylation increases with human erythrocyte age. Evidence for an increase in the number of methylatable sites

The level of carboxyl methylation of membrane proteins has been measured in intact human erythrocyte populations of different ages separated by density gradient centrifugation. Age separation was confirmed by measurement of cytosolic pyruvate kinase specific activity in each fraction. When cells of different ages were incubated with L-[methyl-3H]methionine, the steady state level of 3H radioactivity covalently bound to membrane proteins is observed to be at least 3-fold higher in older erythrocytes. Because the specific radioactivity of the methyl group donor S-adenosyl-L-[methyl-3H]methionine was identical in all age fractions, this represents an increase in the extent of modification of membrane proteins by carboxyl methylation. Of the three major methylated erythrocyte membrane proteins, this increase in carboxyl methylation with age is 4 to 7-fold for bands 2.1 and 3, while the increase in band 4.1 is 3 to 4-fold. This increase in the steady state level of methylation with age cannot be explained by changes in either the intrinsic rate of methyl transfer or by changes in the rate constant of methyl turnover. We, therefore, propose that the age-dependent change in carboxyl methylation is due to an increase in the number of available acceptor sites as the erythrocyte ages in vivo. Since methylation of acidic residues on erythrocyte membrane proteins has been detected exclusively on D-aspartic acid residues (McFadden, P. N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2460-2464), these results are consistent with an accumulation of D-aspartic acid in membrane protein due to spontaneous racemization a the cell ages. The relationship of these observations to possible functions of erythrocyte membrane protein carboxyl methylation is discussed.     

Erythroid anion transporter assembly is mediated by a developmentally regulated recruitment onto a preassembled membrane cytoskeleton

Analysis of the expression and assembly of the anion transporter by metabolic pulse-chase and steady-state protein and RNA measurements reveals that the extent of association of band 3 with the membrane cytoskeleton varies during chicken embryonic development. Pulse-chase studies have indicated that band 3 polypeptides do not associate with the membrane cytoskeleton until they have been transported to the plasma membrane. At this time, band 3 polypeptides are slowly recruited, over a period of hours, onto a preassembled membrane cytoskeletal network and the extent of this cytoskeletal assembly is developmentally regulated. Only 3% of the band 3 polypeptides are cytoskeletal-associated in 4-d erythroid cells vs. 93% in 10-d erythroid cells and 36% in 15-d erythroid cells. This observed variation appears to be regulated primarily at the level of recruitment onto the membrane cytoskeleton rather than by different transport kinetics to the membrane or differential turnover of the soluble and insoluble polypeptides and is not dependent upon the lineage or stage of differentiation of the erythroid cells. Steady-state protein and RNA analyses indicate that the low levels of cytoskeletal band 3 very early in development most likely result from limiting amounts of ankyrin and protein 4.1, the membrane cytoskeletal binding sites for band 3. As embryonic development proceeds, ankyrin and protein 4.1 levels increase with a concurrent rise in the level of cytoskeletal band 3 until, on day 10 of development, virtually all of the band 3 polypeptides are cytoskeletal bound. After day 10, the levels of total and cytoskeletal band 3 decline, whereas ankyrin and protein 4.1 continue to accumulate until day 18, indicating that the cytoskeletal association of band 3 is not regulated solely by the availability of membrane cytoskeletal binding sites at later stages of development. Thus, multiple mechanisms appear to regulate the recruitment of band 3 onto the erythroid membrane cytoskeleton during chicken embryonic development.     

Changes in membrane polypeptides, polyphosphoinositides and phosphatidate in dense fractions of sickle cells

When sickle erythrocytes were fractionated on discontinuous isotonic stractan gradients the denser fractions, which were rich in irreversibly sickled cells contained less polyphosphoinositides and more phosphatidate than either lighter sickle cell fractions or normal cells. These changes could be due to activation of a polyphosphoinositide phosphodiesterase in the denser cells. Membrane polypeptide analysis of the denser fractions also showed a marked depletion of band 4.1 and a protein of molecular mass about 110 kDa but an increased amount of a 180 kDa polypeptide which might be a breakdown product of ankyrin. These biochemical alterations could be consequences of Ca2+ accumulation in the denser sickle cells and may contribute to the structural alterations which give rise to irreversibly sickled cells.     

The cytoskeletal system of nucleated erythrocytes. I. Composition and function of major elements

We have studied the dogfish erythrocyte cytoskeletal system, which consists of a marginal band of microtubules (MB) and trans-marginal band material (TBM). The TBM appeared in whole mounts as a rough irregular network and in thin sections as a surface-delimiting layer completely enclosing nucleus and MB. In cells incubated at 0 degrees C for 30 min or more, the MB disappeared but the TBM remained. MB reassembly occurred with rewarming, and was inhibited by colchicine. Flattened elliptical erythrocyte morphology was retained even when MBs were absent. Total solubilization of MB and TBM at low pH, or dissolution of whole anucleate cytoskeletons, yielded components comigrating with actin, spectrin, and tubulin standards during gel electrophoresis. Mass-isolated MBs, exhibiting ribbonlike construction apparently maintained by cross-bridges, contained four polypeptides in the tubulin region of the gel. Only these four bands were noticeably increased in the soluble phase obtained from cells with 0 degrees C- disassembled MBs. The best isolated MB preparations contained tubulin but no components comigrating with high molecular weight microtubule- associated proteins, spectrin, or actin. Actin and spectrin therefore appear to be major TBM constituents, with tubulin localized in the MB. The results are interpreted in terms of an actin- and spectrin- containing subsurface cytoskeletal layer (TBM), related to that of mammalian erythrocytes, which maintains cell shape in the absence of MBs. Observations on abnormal pointed erythrocytes containing similarly pointed MBs indicate further that the MB can deform the TBM from within so as to alter cell shape. MBs may function in this manner during normal cellular morphogenesis and during blood flow in vivo.     

Membrane alterations in G6PD- and PK-deficient erythrocytes exposed to oxidizing agents

After in vitro treatment of normal, glucose-6-phosphate dehydrogenase-deficient or pyruvate kinase-deficient human erythrocytes with three different oxidizing agents, the extent of lipid peroxidative degradation and the alterations of membrane proteins were evaluated. Exposure to tert-butylhydroperoxide induced, most markedly in G6PD- and PK-deficient erythrocytes, a reduction of protein bands 1, 2, 2.1, 3, 4.1, 4.2, and 5, with the appearance of high-molecular-weight aggregates and of "new" polypeptide components in the 29- to 23-kDa region and with a marked increase of membrane-bound globin. Malonyldialdehyde production was highest in G6PD-deficient cells and relatively low in PK-deficient ones. Methylene blue, which had similar but less relevant effects on lipid peroxidation, in G6PD-deficient erythrocytes caused a conspicuous appearance of high-molecular-weight aggregates and a simultaneous relevant decrease of bands 1 and 2 and of membrane-bound globin; it brought about an almost opposite effect in PK-deficient red cells. Acetylphenylhydrazine, which under our conditions appeared the mildest agent, failed, in normal and PK-deficient erythrocytes, to increase malonyldialdehyde production or to alter membrane proteins, whereas it caused, in G6PD-deficient cells, a slight decrease of bands 1 and 2, a more pronounced decrease of band 3, and a marked increase of bands 4.5 and 4.9.     

Isolation of bovine aortic endothelial cell plasma membranes: identification of membrane-associated cytoskeletal proteins

The plasma membrane of bovine aortic endothelium was isolated, characterized, and found to contain at least four membrane-associated cytoskeletal proteins. Exposure of the plasma membranes to salt media (up to 1M KCl) resulted in the release of 30% of the total plasma membrane-associated proteins and extraction with 1% Triton X-100, 60%. At least four heavily glycosylated bands (185-, 165-, 150-, and 130,000 mol-wt) were evident. The Triton-insoluble pellet fraction contained several major polypeptides (30-, 43-, 58-, and 240,000 mol-wt), two of which were identified by immunoblotting as cytoplasmic actin (43,000 mol-st) and vimentin (58,000 mol-wt). Strikingly, vimentin and a 240,000 mol-wt polypeptide were routinely present in approximately a mole ratio of 4:1 in more than 60% of the plasma membrane preparations. We also report the presence of a 2.1-like and a 4.1-like protein associated with plasma membranes. The 2.1-like protein demonstrated similar solubilities and apparent molecular weight (210,000) as erythroid protein 2.1. Likewise, the endothelial 4.1-like protein exhibited similar solubilities and apparent molecular weight as erythroid protein 4.1. Immunofluorescence staining of fixed and permeabilized cultures with anti-2.1 antibodies showed a fibrillar pattern. In contrast, cells stained with anti-protein 4.1 were brightly fluorescent, bearing both a diffuse and punctate pattern. This paper presents several novel observations pertaining to the composition of bovine aortic endothelial cell plasma membranes, namely: the presence of two erythroid-like cytoskeletal polypeptides; the presence of vimentin and a 240,000 mol-wt polypeptide in a 4:1 mole ratio in more than 60% of the plasma membrane preparations and the co-elution in a 4:1 mol ratio with a protein perturbant; and the inability to release actin from the plasma membrane preparations, suggesting the association of actin with other molecules in the plasma membrane preparation.     

Clathrin is axonally transported as part of slow component b: the microfilament complex

During axonal transport, membranes travel down axons at a rapid rate, whereas the cytoskeletal elements travel in either of two slow components, SCa (with tubulin and neurofilament protein) and SCb (with actin). Clathrin, the highly ordered, structural coat protein of coated vesicles, has recently been shown to be able to interact in vitro with cytoskeletal proteins in addition to membranes. The present study examines whether clathrin travels preferentially with the membrane elements or the cytoskeletal elements when it is axonally transported. Guinea pig visual system was labeled with tritiated amino acids. Radioactive SDS-polyacrylamide gel electrophoresis profiles from the major components of transport were coelectrophoresed with clathrin. Only SCb had a band comigrating with clathrin. In addition, radioactive clathrin was purified from guinea pig brain containing only radioactive SCb polypeptides. Kinetic analysis of the putative clathrin band in SCb revealed that it travels entirely within the SCb wave. Thus we conclude that clathrin travels preferentially with the cytoskeletal proteins making up SCb, rather than with the membranes and membrane-associated proteins in the fast component.     

Comparative distribution of glyceraldehyde 3-phosphate dehydrogenase activity in human, guinea-pig, rabbit and mouse erythrocytes

1. Guinea-pig erythrocytes contain half the activity of the enzyme glyceraldehyde-3-phosphate dehydrogenase (G3PD) present in human cells. About 60% of their total activity is membrane-bound. 2. Rabbit erythrocytes also contain half the amount of the enzyme of human red cells. The distribution of G3PD in rabbit cells, however, is similar to that of human cells with 70% of the enzyme being membrane-bound. 3. Mouse erythrocytes contain about two-thirds of G3PD activity present in human cells. All their enzyme activity is present in membrane-free hemolysate. 4. Non-human erythrocyte membrane proteins, in addition, have relatively greater amount of band 2.1, lack band 2.2, have a more heterogenous band 3 than its human counterpart, and have overlapping bands 4.1 and 4.2.     

Synthesis and assembly of membrane skeletal proteins in mammalian red cell precursors

The synthesis of membrane skeletal proteins in avian nucleated red cells has been the subject of extensive investigation, whereas little is known about skeletal protein synthesis in bone marrow erythroblasts and peripheral blood reticulocytes in mammals. To address this question, we have isolated nucleated red cell precursors and reticulocytes from spleens and from the peripheral blood, respectively, of rats with phenylhydrazine-induced hemolytic anemia and pulse-labeled them with [35S]methionine. Pulse-labeling of nucleated red cell precursors shows that the newly synthesized alpha- and beta-spectrins are present in the cytosol, with a severalfold excess of alpha-spectrin over beta-spectrin. However, in the membrane-skeletal fraction, newly synthesized alpha- and beta-spectrins are assembled in stoichiometric amounts, suggesting that the association of alpha-spectrin with the membrane skeleton may be rate-limited by the amount of beta-spectrin synthesized, as has been shown recently in avian erythroid cells (Blikstad, I., W. J. Nelson, R. T. Moon, and E. Lazarides, 1983. Cell, 32:1081-1091). Pulse-chase experiments in the rat nucleated red cell precursors show that the newly synthesized alpha- and beta-spectrin of the cytosol turn over coordinately and extremely rapidly. In contrast, in the membrane-skeletal fraction, the newly synthesized polypeptides of spectrin are stable. In contrast to nucleated erythroid cells, in reticulocytes the synthesis of alpha- and beta-spectrins is markedly diminished compared with the synthesis and assembly of proteins comigrating with bands 2.1 and 4.1 on SDS gels. Thus, in nucleated red cell precursors, the newly synthesized spectrin may be attached to the plasma membrane before proteins 2.1 and 4.1 are completely synthesized and incorporated in the membrane.     

The interaction of calmodulin with erythrocyte membrane proteins

The method of sedimentation equilibrium in an air-driven ultracentrifuge (Airfuge) has been employed to investigate the interaction of 125I-calmodulin with the cytoskeletal components of the human red cell membrane. The results indicate significant calcium-dependent calmodulin binding activity in the low and high ionic strength extracts of the human erythrocyte membrane. The interaction of 125I-calmodulin with the low ionic strength extract proteins is analysed quantitatively. Further purification of the high ionic strength extract comprising mainly band 2.1 and band 4.1 results in the elution of calmodulin binding activity in a purified fraction of band 4.1.     

Identification of cytoskeletal components involved in attachment of L929 cells and macrophages to polystyrene

We have previously shown that lactoperoxidase (LPO) covalently coupled to polystyrene tissue culture flasks can be used to radioiodinate monolayer cell proteins that come into intimate contact with the LPO- polystyrene surface. These studies have now been extended to include a detailed examination of the class of iodinated polypeptides migrating with apparent molecular weights of 50,000 and 55,000 in SDS polyacrylamide gels. Whereas in cultured L929 cells the 55,000 band is predominantly iodinated, in thioglycollate-activated murine peritoneal macrophages the 55,000 and 50,000 bands are of equal intensity. It is possible that the marked degree of exposure of the 50,000 mol wt polypeptide to immobilized LPO is related to the unique strength of macrophages attachment. After labeling of both L929 cells and macrophages with immobilized LPO, all polypeptides in this molecular weight region were subjected to peptide mapping by simultaneous limited proteolysis and electrophoresis in a second SDS polyacrylamide slab gel. The results clearly show that the two major polypeptides in this region are identical within the limits of resolution of this technique. The 55,000 mol wt polypeptide can also be identified in Triton X-100 cytoskeletons from L929 cells after labeling with soluble LPO either before or after detergent lysis. We conclude that this cell surface polypeptide is in continuity with the cytoskeleton and is preferentially exposed to the substratum during attachment to polystyrene.     

Topography of protein kinase C substrates analyzed by membrane splitting

We have used the methods of planar cell and membrane monolayer formation and monolayer splitting to study structural details of the transmembrane signaling process mediated by protein kinase C. We analyzed human red cell membrane proteins phosphorylated by phorbol ester activation of protein kinase C. Planar single membrane preparations, extraction procedures, and gel electrophoresis coupled with silver staining and autoradiography confirmed that two bands in the 100 kDa region, and bands 4.1, and 4.9, were peripheral and phosphorylated by treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). TPA also stimulated minor incorporation of [32 P]Pi into most integral membrane proteins, including band 3, glycophorin A, the band 4.5 region (glucose transporter) and band 7. Planar cell and membrane-splitting methods revealed that neither integral nor peripheral phosphorylated polypeptides were cleaved by freeze fracture, that all phosphorylated peripheral proteins partitioned intact with the cytoplasmic side of the membrane, and that the percentages of [32P]Pi-labeled peripheral proteins were the same in split membrane cytoplasmic leaflets as in intact membranes. As a unique approach to examining protein topographies membrane splitting provides strong evidence that the major phosphorylated products of the polyphosphatidylinositide pathway are topographically associated with the cytoplasmic leaflet of the human erythrocyte plasma membrane. We further conclude that TPA-induced phosphorylation of red cell peripheral proteins does not significantly alter their transbilayer partitioning patterns after membrane splitting.     

Phosphorylation of rabbit and human erythrocyte membranes by soluble adenosine 3':5'-monophosphate-dependent and -independent protein kinases.

Previous reports from this laboratory and others have established that both the rabbit and human erythrocyte membranes contain multiple protein kinase and phosphate acceptor activities. We now report that these membranes also contain phosphoryl acceptor sites for the soluble cyclic AMP-dependent and -independent protein kinases from rabbit erythrocytes. The rabbit erythrocyte membrane, which does not contain a cyclic AMP-dependent protein kinase, has at least four polypeptides (Bands 2.1, 2.3, 4.5, and 4.8) which are phosphorylated in the presence of the soluble cyclic AMP-dependent protein kinases I, IIa, and IIb isolated from rabbit erythrocyte lysates. The resulting phosphoprotein profile is very similar to that obtained for the cyclic AMP-mediated autophosphorylation of human erythrocyte membranes. The activities of the soluble cyclic AMP-dependent protein kinases toward the membranes have been studied at several pH values. Although the substrate specificity of the three kinases is similar, polypeptide 2.3 appears to be phosphorylated to a greater extent by kinase IIa than by I or IIb. This occurs at all pH values studied. Also apparent is that the pH profile for membrane phosphorylation is different from that of histone phosphorylation. The phosphorylation of membrane proteins can also be catalyzed by the soluble erythrocyte casein kinases. These enzymes are not regulated by cyclic nucleotides and can use either ATP or GTP as their phosphoryl donor. Polypeptides 2.1, 2.9, 4.1, 4.5, 4.8, and 5 of both human and rabbit erythrocyte membranes are phosphorylated in the presence of GTP and the casein kinases. This reaction is optimal at pH 7.5. Experiments were performed to determine whether the phosphorylation of the membranes by the soluble and membrane-bound kinases is additive or exclusive. Our results indicate that after maximal autophosphorylation of the erythrocyte membranes, phosphoryl acceptor sites are available to the soluble cyclic AMP-dependent and -independent protein kinases. Furthermore, after maximal phosphorylation of the membranes with one type of soluble kinase, further 32P incorporation can occur as a result of exposure to the other type of soluble kinase.