Calpactins: calcium-regulated membrane-skeletal proteins

The calpactins are a novel group of proteins associated with the membrane skeleton. The two main forms, calpactin I and II, have been shown to bind to the cytoskeletal proteins actin and spectrin, as well as to anionic phospholipids, which may imply some sort of bridging role. By raising monoclonal antibodies to the heavy and light chains of calpactin I, and to calpactin II, the protein subunits were shown to be coordinately expressed, and the existence of separate calpactin pools hypothesized. Calcium-binding studies suggest that the calpactins may translate Ca²⁺ signals into cellular responses at the membrane. Structural studies have revealed two distinct domains and are beginning to throw light on heavy--light chain interaction and cytoskeletal attachment.     

Regulation of calpactin I phospholipid binding by calpactin I light-chain binding and phosphorylation by p60v-src.

Calpactins I and II are proteins that bind Ca2+, phospholipids, actin and spectrin; they are also major substrates of oncogene and growth-factor-receptor tyrosine kinases. Since calpactins have been proposed to provide a link between membrane lipids and the cytoskeleton, we examined in detail the interactions between purified calpactin I and phospholipid liposomes. We focused on the Ca2+-dependence, the effects of phosphorylation of calpactin I by p60v-src (the protein kinase coded for by the Rous-sarcoma-virus oncogene), and the effects of the binding of calpactin I light chain to calpactin I heavy chain. Binding of the light chain to the heavy chain increased the affinity of calpactin I for phosphatidylserine (PS) liposomes. The opposite effect was observed for phosphorylation by p60v-src; phosphorylation decreased the affinity of calpactin I for PS liposomes. These two opposite effects appeared to be independent, since phosphorylation did not prevent light-chain binding to the heavy chain. Calpactin I was found, by the use of three different techniques, to bind to phospholipid liposomes at less than 10(-8) M free Ca2+. This result is in contrast with those of previous studies, which indicated that greater than 10(-6) M free Ca2+ was required. Our findings suggest that calpactin I may be bound to phospholipids in vivo at Ca2+ concentrations of about 1.5 x 10(-7) M, typical of resting unstimulated cells, and that this interaction may be modulated by light-chain binding and phosphorylation by p60v-src.     

Association of the S-100-related calpactin I light chain with the NH2-terminal tail of the 36-kDa heavy chain

Calpactin I, a Ca2+- and phospholipid-binding cytoskeletal protein, which serves as a major substrate of protein-tyrosine kinases, was isolated from bovine intestine and lung as a species containing two 36-kDa heavy chains and two 10-kDa light chains. The heavy chain is comprised of two distinct domains which can be identified by limited proteolysis: a COOH-terminal 33-kDa core, which contains the Ca2+- and phospholipid-binding sites, and an NH2-terminal tail, which contains the major site of phosphorylation by pp60v-src. To determine the site of association of the light chain on the heavy chain, we analyzed the association states of the light chain, core, and tail by sucrose gradient centrifugation after limited chymotryptic digestion. The core was not detected in higher Mr complexes with the light chain, and the tail cosedimented with a light chain dimer. The tail, isolated from chymotryptic digests and radiolabeled with 125I, was found to form a specific complex with the light chain, but not the core. The authentic tail and a synthetic peptide corresponding to residues 1-29 of the calpactin I heavy chain were both able to specifically inhibit the reassociation between heavy and light chain, whereas a synthetic peptide corresponding to residues 15-33 was inactive. These results suggest that the tail may serve as a site of regulation by light chain or phosphorylation.     

The protein-tyrosine kinase substrate, calpactin I heavy chain (p36), is part of the primer recognition protein complex that interacts with DNA polymerase alpha

Primer recognition proteins (PRP) stimulate the activity of DNA polymerase alpha on DNA substrates with long single-stranded template containing few primers. Purified PRP from HeLa cells and human placenta are composed of two subunits of 36,000 (PRP 1) and 41,000 (PRP 2) daltons. By amino acid sequence homology, we have identified PRP 2 as the glycolytic enzyme 3-phosphoglycerate kinase. Here we present data that establishes PRP 1 to be the protein-tyrosine kinase substrate, calpactin I heavy chain. Amino acid sequence analysis of six tryptic peptides of PRP 1 followed by homology search in a protein sequence data base revealed 100% identity of all six peptides with the deduced amino acid sequence of human calpactin I heavy chain. The activities of PRP and calpactin I coelute on gel filtration columns, and a high correlation of PRP and calpactin I activities was seen at different stages of purification. A rabbit polyclonal anti-chicken calpactin I antibody was shown to cross-react with PRP 1 polypeptide at various stages of PRP purification, and the homogeneous preparation of PRP exhibits 3-phosphoglycerate kinase (PRP 2) and calpactin I (PRP 1) activities. PRP activity is neutralized by a mouse monoclonal anti-calpactin II antibody although having no effect on the polymerase alpha activity itself. Calpactin II has a 50% amino acid sequence homology with calpactin I. However, PRP 1 is not calpactin II as shown by lack of cross-reaction to a monoclonal anti-calpactin II antibody on Western blots. Calpactin I and 3-phosphoglycerate kinase, purified independently, cannot be efficiently reconstituted into the PRP complex, indicating that their association in the PRP complex involves specific protein-protein interactions that remain to be elucidated. The biochemical and immunological data presented here revealing the identity of PRP 1 as calpactin I provide evidence for one physiological role of calpactin I in the cell.     

Calpactins: two distinct Ca++-regulated phospholipid- and actin-binding proteins isolated from lung and placenta

Three forms of calpactin, the 36,000 Mr Ca++-binding cytoskeletal protein, were isolated in large amounts from bovine lung and human placenta using cycles of calcium-dependent precipitation followed by solubilization with EGTA-containing buffers. Calpactin-I as a tetramer of heavy (36 kD) and light (11 kD) chains was the predominant form of calpactin isolated, however milligram amounts of the calpactin-I heavy chain monomer and calpactin-II, a related but distinct molecule, were also isolated by this method. Calpactin-II was characterized in some detail and found to bind two Ca++ ions with Kd's of 10 microM in the presence of phosphatidylserine. Both calpactin-I and -II were found to aggregate liposomes at micromolar Ca++ concentrations, suggesting that at least two phospholipid-binding sites are present on these molecules. Both calpactin monomers bind to and bundle actin filament at high (1 mM) but not low (less than 1 microM) Ca++ concentrations. Amino-terminal sequence analysis of a lower molecular mass variant of calpactin-II revealed that this protein was the previously identified human "lipocortin" molecule. Antibodies were elicited to calpactin-I and -II and the cell and subcellular distribution of each was compared. Calpactin-II was only present at high levels in tissues (lung, placenta) which contained high levels of calpactin-I. Other tissues (intestine) contained high calpactin-I and undetectable levels of calpactin-II. Double-label immunofluorescence microscopy on human fibroblasts revealed that, like calpactin-I, calpactin-II is present in a submembraneous reticular network, although the distribution of the two calpactins is not identical.     

Epidermal growth factor induces the accumulation of calpactin II on the cell surface during membrane ruffling

Confluent and proliferatively quiescent T51B rat liver epithelial cells provide a cellular model for the study of epidermal growth factor (EGF) effects in non-neoplastic cells. Immunoreactive calpactin II, a well-known substrate for EGF-receptor kinase, was found predominantly in the cytosol, although a second immunoreactive pool was found in a Triton X-100-extractable membrane fraction. Stimulation with EGF resulted in a rapid and transient (2-5 min) formation of ruffles at the cell surface and at the cell-cell contacts. Both calpactin II and filamentous actin were found co-localized at the membrane ruffles. Immunoprecipitations of membrane-bound calpactin II from 32P-labeled cells indicate a transient EGF-dependent phosphorylation of calpactin II correlating with membrane ruffling. These results suggest a temporal (2-5 min) function for calpactin II at the plasma membrane during the EGF-induced mitogenesis of T51B cells.     

Identification of a 34-kD polypeptide as a light chain of microtubule- associated protein-1 (MAP-1) and its association with a MAP-1 peptide that binds to microtubules

We examined the association of a 34-kD light chain component to the heavy chains of MAP-1 using a monoclonal antibody that specifically binds the 34-kD component and labels neuronal microtubules in a specific and saturable manner. Immunoprecipitation of MAP-1 heavy chains together with the 34-kD component by the antibody indicates that the 34-kD polypeptide forms a complex with MAP-1 heavy chains. Both major isoforms of MAP-1 heavy chains (MAP-1A and MAP-1B) were found in the immunoprecipitate. Digestion of MAP-1 with alpha-chymotrypsin and analysis of the chymotryptic peptides reveals a 120-kD fragment of the MAP-1 heavy chain that binds to microtubules and is precipitable with the 34-kD light chain antibody, suggesting that the 34-kD light chain also binds to this domain of the molecule. Since microtubules that contain the 120-kD fragment lack the long lateral projections characteristic of microtubules with intact MAP-1, the 34-kD light chains may be localized at or near the microtubule surface.     

cDNA structure and expression of calpactin, a peptide involved in Ca2(+)-dependent cell aggregation in sponges.

Aggregation of cells of the marine sponge Geodia cydonium is mediated by an aggregation factor (AF) particle of Mr 1.3 X 10(8). It is now reported that the AF particle is associated with calpactin, which was ascribed a role in the cell-adhesion process. In order to identify the sequence similarity to other members of the lipocortin family, the cDNA of sponge calpactin was cloned and found to display an 80% sequence similarity to vertebrate calpactin II but only a 47% similarity to calpactin I. The calpactin gene, which contains the consensus sequence coding for the amino acids G-T-D-E, was expressed in Escherichia coli and subsequently purified to a 37000-Mr polypeptide. Both the p32 and the p37 are provided with approximately two Ca2+ ions/molecule and the property to bind to phospholipids. The dissociation constant (calpactin-Ca2+) was in the absence of phospholipids in the range 500-700 microM-Ca2+ but in their presence about 20-30 microM-Ca2+. On the basis of (i) inhibition studies with antibodies to calelectrin and (ii) competition experiments with soluble phospholipids (both chemically defined as well as total homologous membrane lipids) we conclude that the AF-associated calpactin and plasma-membrane-bound phospholipid(s) are involved in cell-cell aggregation in sponges.     

Phospholipid-dependent Ca2+ binding by the 36-kDa tyrosine kinase substrate (calpactin) and its 33-kDa core

A 36-kDa protein, which is a component of the membrane skeleton, has been shown to co-localize with spectrin in addition to serving as a major substrate for tyrosine-protein kinases. This protein, which will be referred to as calpactin (for calcium-dependent phospholipid and actin binding protein), was isolated from bovine intestine as the complex with a 10-kilodalton light chain and the Ca2+ binding was analyzed by equilibrium dialysis with 45Ca2+ in the presence or absence of phospholipid. Although Ca2+ binding by calpactin alone was negligible at micromolar free Ca2+, it was greatly enhanced by liposomes containing phosphatidylserine or phosphatidylinositol. A proteolytic derivative of calpactin, termed the "core," which has lost the site of association with the light chain in addition to the site of tyrosine phosphorylation by pp60src, was also found to contain this high affinity phospholipid enhanced Ca2+-binding activity. Scatchard plots reveal that each calpactin monomer or core polypeptide bound 2 Ca2+ ions with a Kd of 4.5 X 10(-6) M at 200 micrograms of phosphatidylserine/ml. Liposome binding experiments confirmed that calpactin as a complex with light chain as well as calpactin monomer or the 33-kDa core interact with phosphatidylserine liposomes in a Ca2+-dependent manner.     

Characterization of the interaction between calpactin I and fodrin (non-erythroid spectrin)

Calpactin I, a calcium-binding protein associated with the membrane cytoskeleton, has been reported to bind to a calcium-dependent manner to fodrin, to certain phospholipids, and to F-actin. We have investigated the interaction between calpactin I and fodrin. Using a gel filtration assay, we observed one or more calpactin I molecules were bound calcium-dependently only at high concentrations of calpactin (greater than 1 microM), indicating that the interaction is of only moderate affinity. At higher concentrations of calpactin I, the calpactin coprecipitated with fodrin in a calcium-dependent manner. The molar ratio of calpactin to fodrin tetramer in the precipitate was greater than 25:1, indicating that the calpactin binds to a large number of sites. Moreover, the monomeric form of calpactin I (p36), which did not induce precipitation of fodrin, showed no evidence of saturation in its binding to fodrin even when more than 30 mol of p36 were bound per mole of fodrin tetramer. Several proteins other than fodrin, including clathrin, alpha-actinin, and neurofilament-H, also interacted calcium-dependently with calpactin I in the gel filtration assay. These results demonstrate that the interaction between calpactin and fodrin is not of high affinity, is not readily saturated, and is not specific for fodrin. Our results suggest that calpactin's interaction with fodrin is a particular example of a calcium-dependent, but promiscuous, binding of calpactin to proteins.     

Annexin II (calpactin I) in the mouse mammary gland: immunolocalisation by light- and electron microscopy

Summary To elucidate the putative role of annexin II (calpactin I) in the secretory function of mammary tissue its immunolocalisation in the mammary gland of pregnant and lactating mice was investigated by light- and electron microscopy using the immunoperoxidase technique. A low level of fairly uniform annexin II staining was evident throughout the gland despite its mixed composition during pregnancy. In lactating tissue it was revealed that apparently mature alveoli contained a concentration of annexin II staining outlining their epithelium. The staining was localised by immuno-electron microscopy to the apical membrane of these alveolar epithelial cells and their microvillar extentions. There was also an apparent association of annexin II with vesicles of a range of sizes located near, or actually fused with, the apical membrane. Many of the small, stained vasicles could clearly be identified as casein-containing vesicle while the large vesicles were apparently associated with either casein granules or possibly lipid. The appearance of a selective concentration of annexin II in apparently actively secreting mammary epithelial cells, as revealed in this study, is consistent with a possible structural and/or functional role for this protein at the membranes participating in the secretion of protein and possibly lipid from these secretory cells.     

The association between murine beta 2-microglobulin and HLA class I heavy chains results in serologically detectable conformational changes of both chains

HLA-A3-, HLA-B7-, and HLA-CW3-transfected L cells, maintained in medium supplemented with murine serum so as to ensure that the human heavy chains were associated with murine beta 2-microglobulin, were subjected to a systematic serologic analysis for an evaluation of the structural consequences of such an heterologous association. The hybrid molecules exhibited alterations of their serologic reactivities that suggest the occurrence of structural modifications of both light and heavy chains. Thus, reactivity of HLA-A3-, HLA-B7-, and HLA-Cw3-transfected L cells with a monoclonal antibody (B1.1G6) directed at a human beta 2-microglobulin specific antigenic determinant was observed; this implies structural modifications of murine beta 2-microglobulin after its association with HLA class I heavy chains. Conversely, a profound reduction of the reactivity of the same transfectants with a monoclonal antibody (W6/32) directed at a monomorphic heavy chain related epitope was observed. The W6/32 reactivity was restored after replacement of the murine by the human light chain, indicating that the conformation adopted by the HLA class I heavy chain depends on the origin of the beta 2-microglobulin associated. Therefore it appears that the complex interactions that develop between the extracellular domains (including the one formed by the light chain) markedly influence the overall structure and the antigenic properties of HLA class I molecules.     

Biochemical characterization of a Dictyostelium myosin II heavy-chain phosphatase that promotes filament assembly.

In Dictyostelium cells, myosin II is found as cytosolic nonassembled monomers and cytoskeletal bipolar filaments. It is thought that the phosphorylation state of three threonine residues in the tail of myosin II heavy chain regulates the molecular motor's assembly state and localization. Phosphorylation of the myosin heavy chain at threonine residues 1823, 1833 and 2029 is responsible for maintaining myosin in the nonassembled state, and subsequent dephosphorylation of these residues is a prerequisite for assembly into the cytoskeleton. We report here the characterization of myosin heavy-chain phosphatase activities in Dictyostelium utilizing myosin II phosphorylated by myosin heavy-chain kinase A as a substrate. One of the myosin heavy-chain phosphatase activities was identified as protein phosphatase 2A and the purified holoenzyme was composed of a 37-kDa catalytic subunit, a 65-kDa A subunit and a 55-kDa B subunit. The protein phosphatase 2A holoenzyme displays two orders of magnitude higher activity towards myosin phosphorylated on the heavy chains than it does towards myosin phosphorylated on the regulatory light chains, consistent with a role in the control of filament assembly. The purified myosin heavy-chain phosphatase activity promotes bipolar filament assembly in vitro via dephosphorylation of the myosin heavy chain. This system should provide a valuable model for studying the regulation and localization of protein phosphatase 2A in the context of cytoskeletal reorganization.     

Epidermal-growth-factor-stimulated phosphorylation of calpactin II in membrane vesicles shed from cultured A-431 cells.

Membrane vesicles shed from intact A-431 epidermoid carcinoma cells and harvested in the presence of Ca2+ contained epidermal-growth-factor (EGF) receptor/kinase substrates of apparent molecular masses 185, 85, 70, 55, 38 and 27 kDa. The 38 kDa substrate (p38) was recognized by an antibody that had been raised against the human placental EGF receptor/kinase substrate calpactin II (lipocortin I). The A-431 and placental substrates, isolated by immunoprecipitation after phosphorylation in situ, yielded identical phosphopeptide maps upon limited proteolytic digestion with each of five different enzymes. The A-431-cell vesicular p38 is therefore calpactin II. EGF treatment of the intact A-431 cells before inducing vesiculation was not necessary for the substrate to be present within the vesicles. Our data thus indicate that receptor internalization is not a prerequisite for receptor-mediated phosphorylation of calpactin II. The ability of the protein to function as a substrate for the receptor/kinase depended upon the continued presence of Ca2+ during the vesicle-isolation procedure. EGF-stimulated phosphorylation of calpactin II was much less pronounced in vesicles prepared from A-431 cells in the absence of Ca2+, although comparable amounts of the protein were detectable by immunoblotting. Calpactin II therefore appears to be sequestered in a Ca2+-modulated manner within shed vesicles, along with at least four other major targets for the EGF receptor/kinase. The vesicle preparation may be a useful model system in which to study the phosphorylation and function of potentially important membrane-associated substrates for the receptor.     

Binding of Par-4 to the actin cytoskeleton is essential for Par-4/Dlk-mediated apoptosis

Prostate apoptosis response-4 (Par-4) is a 38-kDa protein originally identified as a gene product upregulated in prostate cancer cells undergoing apoptosis. Cell death mediated by Par-4 and its interaction partner DAP like kinase (Dlk) is characterized by dramatic changes of the cytoskeleton. To uncover the role of the cytoskeleton in Par-4/Dlk-mediated apoptosis, we analyzed Par-4 for a direct association with cytoskeletal structures. Confocal fluorescence microscopy revealed that endogenous Par-4 is specifically associated with stress fibers in rat fibroblasts. In vitro cosedimentation analyses and in vivo FRET analyses showed that Par-4 directly binds to F-actin. Actin binding is mediated by the N-terminal 266 amino acids, but does not require the C-terminal region of Par-4 containing the leucine zipper and the death domain. Furthermore, the interaction of Par-4 with actin filaments leads to the formation of actin bundles in vitro and in vivo. In rat fibroblasts, this microfilament association is essential for the pro-apoptotic function of Par-4, since both disruption of the actin cytoskeleton by cytochalasin D treatment and overexpression of Par-4 constructs impaired in actin binding result in a significant decrease of apoptosis induction by Par-4 and Dlk. We propose a model, in which Par-4 recruits Dlk to stress fibers, leading to enhanced phosphorylation of the regulatory light chain of myosin II (MLC) and to the induction of apoptosis.     

The cDNA sequence for the protein-tyrosine kinase substrate p36 (calpactin I heavy chain) reveals a multidomain protein with internal repeats

We have isolated and sequenced a full-length cDNA clone for the protein-tyrosine kinase substrate p36 (calpactin I heavy chain). This sequence predicts a 339 amino acid (Mr 38,493) protein containing an N-terminal region of 20 amino acids, known to interact with a 10 kd protein (light chain), and a C-terminal region, found to contain two Ca2+/phospholipid-binding sites, that can be aligned as four 70 amino acid repeats. A single p36 gene was detected in the mouse genome, and a major p36 mRNA of 1.6 kb was found to be expressed in different mouse tissues. Unexpectedly, p36 and the phospholipase A2 inhibitor lipocortin I were found to be 50% identical in sequence over the C-terminal 300 residues. The function of p36 and its relation to other proteins are discussed.     

Acquisition of HLA class I W6/32 defined antigenic determinant by heavy chains from different species following association with bovine beta 2-microglobulin

Murine, rat, rabbit and guinea pig class I heavy chains, which do not react with W6/32 monoclonal antibody when they are expressed in association with autologous beta 2-microglobulin (beta 2-m), can acquire such a reactivity once they are expressed at the surface of cells cultured in conditions which allow their association with bovine beta 2-m. Sequence comparison of beta 2-ms suggests that glutamine at position 89 might be critical for the induction of the W6/32 defined antigenic determinant. However, in the murine species, certain class I heavy chains, in spite of their association with bovine beta 2-m, do not express this determinant. Using genetically engineered hybrid class I molecules and selected congenic strains of mice this negative property was shown to be related to the presence of a cysteine residue at position 121 which allows covalent association of beta 2-m to class I heavy chains (Bushkin, Y., J-S. Tung, A. Pinter, J. Michaelson, and E. A. Boyse. 1986. Unusual association of beta 2-microglobulin with certain class I heavy chains of the murine major histocompatibility complex. Proc. Natl. Acad. Sci. USA 83:432). Therefore, expression of the W6/32 defined antigenic determinant implicates both the beta 2-m and the second domain of the heavy chain, but its expression (or exposure) is prevented by the covalent fixation on cysteine 121 of the light chain.