The cytoskeletal system of nucleated erythrocytes. II. presence of a high molecular weight calmodulin-binding protein

A calmodulin and alpha-subunit binding domain in human erythrocyte spectrin

Human erythrocyte spectrin binds calmodulin weakly under native conditions. This binding is enhanced in the presence of urea. The site responsible for this enhanced binding in urea has now been shown to reside in a specific region of the spectrin beta-subunit. Cleavage of spectrin with trypsin, cyanogen bromide or 2-nitro-5-thiocyanobenzoic acid generates fragments of the molecule which retain the ability to bind calmodulin under denaturing conditions. The origin of these fragments, identified by two-dimensional peptide mapping, is the terminal region of the spectrin beta-IV domain. The smallest peptide active in calmodulin binding is a 10 000 Mr fragment generated by cyanogen bromide cleavage. Only the intact 74 000 Mr fragment generated by trypsin (the complete beta-IV domain) retains the capacity to reassociate with the isolated alpha-subunit of spectrin. The position of a putative calmodulin binding site near a site for subunit-subunit association and protein 4.1 and actin binding suggests a possible role in vivo for calmodulin regulation of the spectrin-actin membrane skeleton or for regulation of subunit-subunit associations. This beta-subunit binding site in erythrocyte spectrin is found in a region near the NH2-terminus at a position analogous to the alpha-subunit calmodulin binding site previously identified in a non-erythroid spectrin by ultrastructural studies.

Mechanism of cytoskeletal regulation (I): functional differences correlate with antigenic dissimilarity in human brain and erythrocyte spectrin

Human erythrocyte and brain spectrin (fodrin, calspectin) have been compared quantitatively with respect to the extent and sites of antigenic and functional similarity. Brain spectrin cross-reacts strongly with approx. 1% of the epitopes in erythrocyte spectrin, but weakly with at least 50%. The distribution of shared determinants is not uniform. Brain spectrin is most deficient in epitopes characteristic of the 80 kDa and 52 kDa domains of the alpha-subunit (alpha-I and alpha-III) and of terminal portions of the 28 kDa and 74 kDa domains of the beta-subunit (beta-I and beta-IV). The functions associated with these domains also differ between the two proteins. Brain spectrin does not undergo extensive polymerization and binds calmodulin at a different site. The unique ability of erythrocyte spectrin to oligomerize beyond the tetramer reflects its role in the membrane skeleton. Non-erythroid spectrins probably function as specific linkers between membrane receptors and the filamentous cytoskeleton. In this sense, they may act as regulated transducers of information flow between the membrane and the cytoplasmic matrix.

Analysis of the calcium-dependent interaction of calmodulin with bovine serum albumin

An air-driven ultracentrifuge has been used to investigate the calcium-dependent association between calmodulin and bovine serum albumin. Procedures were described which allowed the interaction to be analyzed to yield the equilibrium constant. At low ionic strength (25 mM Tris-HCl, pH 7.5, pCa 6.68, 9 degrees C) the equilibrium constant for the interaction was estimated to be 2.1 X 10(4) M-1, while at high ionic strength (25 mM Tris-HCl, pH 7.5, 150 mM KCl, pCa 6.68, 9 degrees C) the value was 4.5 X 10(3) M-1. Under similar conditions, calmodulin was also found to interact with beta-lactoglobulin A and gelatin, but no detectable association was observed with ovalbumin.

Ankyrin and synapsin: spectrin-binding proteins associated with brain membranes

Brain membranes contain an actin-binding protein closely related in structure and function to erythrocyte spectrin. The proteins that attach brain spectrin to membranes are not established, but, by analogy with the erythrocyte membrane, may include ankyrin and protein 4.1. In support of this idea, proteins closely related to ankyrin and 4.1 have been purified from brain and have been demonstrated to associate with brain spectrin. Brain ankyrin binds with high affinity to the spectrin beta subunit at the midregion of spectrin tetramers. Brain ankyrin also has binding sites for the cytoplasmic domain of the erythrocyte anion channel (band 3), as well as for tubulin. Ankyrins from brain and erythrocytes have a similar domain structure with protease-resistant domains of Mr = 72,000 that contain spectrin-binding activity, and domains of Mr = 95,000 (brain ankyrin) or 90,000 (erythrocyte ankyrin) that contain binding sites for both tubulin and the anion channel. Brain ankyrin is present at about 100 pmol/mg membrane protein, or about twice the number of copies of spectrum beta chains. Brain ankyrin thus is present in sufficient amounts to attach spectrin to membranes, and it has the potential to attach microtubules to membranes as well as to interconnect microtubules with spectrin-associated actin filaments. Another spectrin-binding protein has been purified from brain membranes, and this protein cross-reacts with erythrocyte 4.1. Brain 4.1 is identical to the membrane protein synapsin, which is one of the brain's major substrates for cAMP-dependent and Ca/calmodulin-dependent protein kinases with equivalent physical properties, immunological cross-reaction, and peptide maps. Synapsin (4.1) is present at about 60 pmol/mg membrane protein, and thus is a logical candidate to regulate certain protein linkages involving spectrin.

Calcium-induced phosphorylations and [125I]calmodulin binding in renal membrane preparations

Calcium-induced phosphorylated intermediates and calmodulin-binding proteins in membrane preparations from the renal cortex were analyzed by SDS-polyacrylamide gel electrophoresis at low pH, protein electroblotting and [125I]calmodulin overlay. Two calcium-induced phosphoproteins were found, with a molecular mass of 135 and 115 kDa, respectively. By comparing different preparations characterized by marker enzymes, it was shown that the 135 kDa phosphoprotein is localized in the basal-lateral fragment of the plasma membrane, whereas the 115 kDa phosphoprotein is more pronounced in preparations containing a high proportion of endoplasmic reticulum. A prominent calmodulin-binding protein comigrated with the 135 kDa phosphoprotein; there was no calmodulin binding to polypeptides in the molecular mass range of the 115 kDa phosphoprotein. Partial proteolysis by trypsin and the effect of 20 microM La2+ on the formation of phosphoproteins before and after trypsinization support the conclusion that the 135 kDa protein can be identified with the plasma membrane calcium pump, whereas the 115 kDa phosphoprotein is the phosphorylated intermediate of a different type of calcium pump probably originating from the endoplasmic reticulum. Calmodulin binding in renal membrane preparations analyzed on Laemmli-type slab gels revealed that there are many calmodulin-binding proteins in our preparations. We have identified one band with the renal calcium pump localized in the basal-lateral membrane. Another calmodulin-binding protein migrating at 108 kDa, is not localized in the basal-lateral membrane and could be one of the calmodulin-binding proteins originating from the cytoskeleton.

Heterogeneity of spectrin distribution among avian muscle fiber types

Muscle spectrin has been examined in avian fast, slow, and mixed muscles using the techniques of immunofluorescence microscopy and immunoautoradiography. By immunofluorescence, fibers of the fast-twitch pectoralis major (PM) are seen to contain alpha-spectrin antigen primarily at the sarcolemma, while in the slow-tonic anterior latissimus dorsi (ALD), alpha-spectrin antigen is found in high concentrations throughout the sarcoplasm as well as being present in association with the sarcolemma. In mixed (fast- and slow-twitch) muscles of the leg, two populations of fibers can be distinguished: those which resemble the fibers of the PM and another group which displays interior staining similar to the fibers of the ALD. Histochemical staining for actomyosin ATPase reveals that the fibers of mixed muscles which contain the most spectrin antigen correspond to the slow-twitch fibers. Supportive data were obtained using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoautoradiography. In slow muscle, an approximate threefold increase in alpha-spectrin concentration relative to other proteins is evident. These results suggest that the distribution of alpha-spectrin may be modified by the physiological state of the myofiber.

The cytoskeletal system of nucleated erythrocytes. III. Marginal band function in mature cells

Marginal bands (MBs) of microtubules are believed to function during morphogenesis of nonmammalian vertebrate erythrocytes, but there has been little evidence favoring a continuing role in mature cells. To test MB function, we prepared dogfish erythrocytes with and without MBs at the same temperature by (a) stabilization of the normally cold-labile MB at 0 degree C by taxol, and (b) inhibition of MB reassembly at room temperature by nocodazole or colchicine. We then compared the responses of these cells to mechanical stress by fluxing them through capillary tubes. Before fluxing , cells with or without MBs had normal flattened elliptical shape. After fluxing , deformation was consistently observed in a much greater percentage of cells lacking MBs. The difference in percent deformation between the two cell types was highly significant. That the MB is an effector of cell shape was further documented in studies of the formation of singly or doubly pointed dogfish erythrocytes that appear during long-term incubation of normal cells at room temperature. On-slide perfusion experiments revealed that the pointed cells contain MBs of corresponding pointed morphology. Incubation of cells with and without MBs showed that they become pointed only when they contain MBs, indicating that the MB acts as a flexible frame which can deform and support the cell surface from within. To test this idea further, cells with and without MBs were exposed to hyperosmotic conditions. Many of the cells without MBs collapsed and shriveled , whereas those with MBs did not. The results support the view that the MB has a continuing function in mature erythrocytes, resisting deformation and/or rapidly returning deformed cells to an efficient equilibrium shape in the circulation.

Similarities between the Mr 245,000 calmodulin-binding protein of the dogfish erythrocyte cytoskeleton and alpha-fodrin

The Mr 245,000 calmodulin-binding protein of the dogfish erythrocyte cytoskeleton (D245) has been compared with human erythrocyte spectrin and mammalian brain fodrin [J. Levine and M. Willard (1981) J. Cell Biol. 90, 631-643]. Mammalian erythrocyte alpha-spectrin, brain alpha-fodrin, and D245 are all localized in the cell surface-associated cytoskeleton, and have similar molecular weights. Like mammalian erythrocyte spectrin, D245 was extracted from erythrocyte ghosts under low-ionic-strength conditions. However, D245 failed to bind an antibody which reacted strongly with both subunits of human erythrocyte spectrin. Unlike mammalian erythrocyte alpha- and beta-spectrin, D245 bound calmodulin in the absence of urea both in a "gel-binding" assay and in situ using azidocalmodulin [D.C. Bartelt, R.K. Carlin, G.A. Scheele, and W.D. Cohen (1982) J. Cell Biol. 95, 278-284]. Striking similarities were noted between D245 and alpha-fodrin in that both exhibited (a) comparable calcium-dependent calmodulin binding properties, (b) strong reactivity with two different anti-fodrin antibody preparations, (c) similar reactivity with antibody to brain CBP-I, now believed to be fodrin, (d) proteolytic degradation yielding an Mr 150,000 calmodulin-binding fragment, and (e) lack of reactivity with an anti-spectrin antibody. A protein with calmodulin-binding and anti-fodrin-binding properties similar to D245 was detected in cytoskeletal preparations of chicken erythrocytes. Moderate and consistent cross-reactivity of anti-fodrin with human erythrocyte alpha-spectrin was also observed. The data indicate that D245 is functionally and immunologically more closely related to alpha-fodrin than to alpha-spectrin of the mammalian erythrocyte.

Identification of fodrin as a major calmodulin-binding protein in postsynaptic density preparations

A major protein of postsynaptic densities (PSDs), a doublet of 230,000 and 235,000 Mr that becomes enriched in PSDs after treatment of synaptic membranes with 0.5% Triton X-100, has been found to be identical to fodrin (Levine, J., and M. Willard, 1981, J. Cell Biol. 90:631) by the following criteria. The upper bands of the PSD doublet and purified fodrin (alpha-fodrin) were found to be identical since both bands (a) co-migrated on SDS gels, (b) reacted with antifodrin, (c) bound calmodulin, and (d) had identical peptide maps after Staphylococcus aureus protease digestion. The lower bands of the PSD doublet and of purified fodrin (beta-fodrin) were found to be identical since both bands co-migrated on SDS gels and both had identical peptide maps after S. aureus protease digestion. The binding of calmodulin to alpha-fodrin was confirmed by cross-linking azido-125I-calmodulin to fodrin before running the protein on SDS gels. No binding of calmodulin to beta-fodrin was observed with either the gel overlay or azido-calmodulin techniques. A second calmodulin binding protein in the PSD has been found to be the proteolytic product of alpha-fodrin. This band (140,000 Mr), which can be created by treating fodrin with chymotrypsin, both binds calmodulin and reacts with antifodrin.

Spectrin and ankyrin in brain

Further similarity between mammalian erythrocyte spectrin and pig brain spectrin has been demonstrated by (a) formation of hybrid molecules with brain alpha-chains and erythrocyte beta-chains and by (b) identification of an ankyrin protein in brain membranes. Hybrid spectrin molecules prepared from brain alpha-chains and erythrocyte beta-chains were visualized by low-angle rotary shadowing as double-stranded rods (dimers) 100 nM in length. 125I-labeled brain alpha-chain that was hybridized with erythrocyte beta-subunit acquired ability to bind to ankyrin sites on erythrocyte membranes. 125I-labeled brain alpha-chain bound only to beta-subunits of erythrocyte and brain spectrin following transfer of these polypeptides to nitrocellulose paper from sodium dodecyl sulfate (SDS) gels. Thus brain spectrin and mammalian erythrocyte spectrin have shared functional sites involved in association of their subunits. Additional evidence for similarity of brain and erythrocyte membranes is the finding of a 210,000 Mr membrane protein in brain that cross-reacts with erythrocyte ankyrin and has a water-soluble domain of 72,000 Mr that is produced by protease digestion. The 72,000 Mr domain of brain ankyrin has been isolated by affinity chromatography on erythrocyte spectrin-Sepharose, and was demonstrated to bind directly to erythrocyte and brain spectrin. The brain 72,000 Mr fragment has distinct peptide maps from the erythrocyte 72,000 Mr ankyrin fragment and thus is not a result of erythrocyte contamination.