Transformation by Rous sarcoma virus: a cellular substrate for transformation-specific protein phosphorylation contains phosphotyrosine

Protein alterations associated with N-methyl-N-nitrosourea exposure of preimplantation mouse embryos transferred to surrogate mothers

Mouse preimplantation embryo functions have been shown to be disrupted by in vitro exposure to N-methyl-N-nitrosourea (MNU) with subsequent transfer to the uteri of pseudopregnant surrogate mothers. Increased gross malformations and decreased fetal body lengths in the midgestational period have been previously documented. Protein extracts were isolated from day 12 mouse fetuses developed from MNU- or solvent-exposed blastocysts and analyzed by two-dimensional electrophoresis. The electrophoretic patterns reveal six protein alterations in day 12 fetal tissue induced by MNU treatment at the blastocyst stage. Five of these alterations involve shifts in isoelectric point (pI) and the other alteration involves a quantitative increase in a protein. The possibility that two of the proteins which exhibit a shift in pI following MNU exposure represent the cell adhesion molecules, N-CAM and L-CAM (based on similar Mr values), was investigated by Western blot analysis. No pI alteration in L-CAM or N-CAM expression is seen after MNU exposure. These results demonstrate that in vitro MNU exposure of preimplantation embryos results in protein alterations in midgestational fetuses. Thus, the effects of MNU exposure on preimplantation embryos may be manifest long after exposure, and subtle, non-lethal mutations may play a role in poor fetal outcome after early chemical exposure.

Enhanced expression of the protein kinase substrate p36 in human hepatocellular carcinoma

A basic phosphoprotein defined by a monoclonal antibody named AF5 was found to be highly abundant in human hepatocellular carcinoma by Western immunoblotting. Under the same conditions, the levels of this phosphoprotein were low or undetectable in normal liver extracts. The AF5 antibody was used to screen a cDNA expression library of a human hepatoma cell line named FOCUS. A 960-base-pair cDNA was isolated and found to be a partial cDNA encoding the human protein-tyrosine kinase substrate p36, also known as lipocortin II. p36 expression was highly abundant in hepatocellular carcinomas at both the transcript and protein levels. Its expression was not induced significantly during rat liver regeneration following a partial hepatectomy. These results suggest that the induction of p36 expression is associated with malignant transformation of hepatocytes. p36 was previously shown to be phosphorylated upon transformation of normal fibroblasts by retroviral oncogenes without significant modulation of expression. We report here the initial description of the association of increased p36 expression with malignant transformation.

Enhanced expression of the protein kinase substrate p36 in human hepatocellular carcinoma

A basic phosphoprotein defined by a monoclonal antibody named AF5 was found to be highly abundant in human hepatocellular carcinoma by Western immunoblotting. Under the same conditions, the levels of this phosphoprotein were low or undetectable in normal liver extracts. The AF5 antibody was used to screen a cDNA expression library of a human hepatoma cell line named FOCUS. A 960-base-pair cDNA was isolated and found to be a partial cDNA encoding the human protein-tyrosine kinase substrate p36, also known as lipocortin II. p36 expression was highly abundant in hepatocellular carcinomas at both the transcript and protein levels. Its expression was not induced significantly during rat liver regeneration following a partial hepatectomy. These results suggest that the induction of p36 expression is associated with malignant transformation of hepatocytes. p36 was previously shown to be phosphorylated upon transformation of normal fibroblasts by retroviral oncogenes without significant modulation of expression. We report here the initial description of the association of increased p36 expression with malignant transformation.

Mitogenic signalling pathway of tumour necrosis factor involves the rapid tyrosine phosphorylation of 41,000-Mr and 43,000-Mr cytosol proteins

Tumour necrosis factor (TNF) is a potent mitogen for some fibroblast cell lines. Here we have examined the TNF-mediated changes in protein phosphorylation in Swiss 3T3 and human FS-4 fibroblasts, and compared them with changes observed after the treatment of cells with other mitogens, such as platelet-derived growth factor (PDGF) and bombesin. TNF stimulated the rapid phosphorylation of two 41,000-Mr and two 43,000-Mr cytosol proteins on tyrosine, threonine and/or serine, as did PDGF, epidermal growth factor and fibroblast growth factor; the increased levels of this mitogen-induced protein-tyrosine phosphorylation correlated well with the extent of mitogen-induced DNA synthesis as determined by the percentage of labelled nuclei. In contrast, bombesin, which is an even better mitogen for Swiss 3T3 cells than TNF, stimulated the tyrosine phosphorylation of 41,000-Mr and 43,000-Mr proteins only to a limited extent. On the other hand, bombesin and PDGF stimulated the rapid serine phosphorylation of an 80,000-Mr acidic protein, a major substrate for protein kinase C; increased phosphorylation of the 80,000-Mr protein was not observed at all when cells were stimulated with TNF. These results suggest significant differences among the mitogenic signalling pathways of TNF, PDGF and bombesin as regards the involvement of protein kinases; the mitogenic signalling pathway of TNF involves the activation of tyrosine kinase, but not of protein kinase C, whereas bombesin seems to transduce its mitogenic signal mainly through the activation of protein kinase C, and the activation of both kinases seems to be involved in the mitogenic signalling pathway of PDGF.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

Structure and chromosome assignment of the murine p36 (calpactin I heavy chain) gene

p36 is a major substrate of both viral and growth factor receptor associated protein kinases. This protein has recently been named calpactin I heavy chain since it is the large subunit of a Ca2(+)-dependent phospholipid and actin binding heterotetramer. The primary structure of p36 has been determined from analysis of cloned cDNA. The protein contains 338 amino acids, has an approximate molecular weight of 39,000, and is comprised of several distinct domains, including four 75 amino acid repeats. From two overlapping cosmid clones isolated from different mouse genomic liver libraries, the complete intron/exon structure of the p36 gene was determined and the 5' and 3' noncoding regions of the gene were analyzed. The coding and 3' untranslated region of the p36 gene contains 12 exons which range in size from 48 to 322 base pairs (bp) with an average size of 107 bp. The repeat structures found at the protein level are not delineated by single exons, but the N-terminal p11-binding domain is encoded by a single exon. Structural mapping of the gene demonstrated that the lengths of the first two introns in the coding region are together approximately 6 kilobases (kb), while the other introns range in size from 600 to 3600 bp with an average size of 1650 bp. The p36 gene is at least 22 kb in length and has a coding sequence of approximately 1 kb, representing only 4.5% of the gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-binding proteins 33 kDa, 35 kDa, and 65/67 kDa in normal rat and Morris hepatoma tissues. A biochemical and immunohistochemical study

Polyclonal antibodies were raised against membrane-associated calcium-binding proteins (apparent molecular masses 65000 and 67000 (CBP 65/67) and 33000 and 35000 (CBP 33 and CBP 35)), which were isolated from rat liver and Morris hepatoma. Using immunoblotting, various amounts of CBP 33 and CBP 35 as well as CBP 65/67 were detected in most rat organs. Using alkaline phosphatase and monoclonal-anti-alkaline phosphatase antibodies (APAAP), all the calcium-binding proteins were detected by immunohistochemical techniques in the plasma membranes of many cells, such as vascular endothelial cells, lymphocytes, epididymal principal cells, secretory and excretory duct cells of certain exocrine glands, straight distal tubular cells of the kidney, and in the cytoplasm of muscle cells and fibres as well as nerve cells and chondrocytes, and in connective tissue elements. Immunohistochemical analysis also showed that in polarized epithelial cells, e.g., renal tubular cells, epididymal principal cells or excretory duct cells, these calcium-binding proteins are present exclusively or mostly in the luminal plasma membrane.

A glycoprotein in the plasma membrane matrix as a major potential substrate of p60v-src

A potential substrate of p60v-src in Rous sarcoma virus-transformed cells was found to be a 130-kilodalton (kDa) glycoprotein which binds to lectin-Sepharose and can be immunoprecipitated by an anti-phosphotyrosine antibody. This glycoprotein was shown to be distinct from the fibronectin receptor and a cellular protein phosphorylated in p60v-src immune complexes. The protein was a transmembrane protein localized in the plasma membrane and resistant to extraction with Triton X-100. The 130-kDa protein was also highly phosphorylated in cells transformed by Fujinami sarcoma virus or Y73 but not in cells infected with Rous sarcoma virus mutants that encode p60v-src lacking myristoylated N termini. Phosphorylation of this glycoprotein was temperature dependent in cells infected with temperature-sensitive mutants. The good correlation between its phosphorylation and morphological transformation, together with its relative abundance among phosphorylated proteins and its subcellular localization, suggests that phosphorylation of the 130-kDa glycoprotein is one of the primary events important for cell transformation by p60v-src and related oncogene products.

A tyrosine-specific protein kinase inhibitor, alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamide, blocks the phosphorylation of tyrosine kinase substrate in intact cells

Inhibition by alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamide (ST 638) of tyrosine-specific protein kinase was examined using epidermal growth factor (EGF)-treated A431 cells at the concentration of 25 to 100 microM. ST 638 had negligible effects on the growth and morphology of A431 cells and on EGF binding to its receptor, and subsequent down-regulation of the receptor. ST 638 specifically inhibited EGF-induced phosphorylation of tyrosine residues of whole cell proteins in a dose-dependent manner without affecting the phosphorylation of serine and threonine residues. ST 638 greatly inhibited the EGF-induced phosphorylation of lipocortin I at 25 microM, and yet had a negligible effect on the EGF-induced phosphorylation of EGF receptor. Neither the amount of [35S]methionine-labeled lipocortin I nor the serine/threonine phosphorylation level of fodrin beta-subunit was affected by the same concentration of ST 638. These results indicate that the phosphorylation of lipocortin I is not relevant to the transformation of A431 cells. In cell lines transformed by src or fgr oncogene encoding tyrosine kinase, ST 638 also inhibited phosphorylation of calpactin I (p36) without affecting that of the oncogene products. Two-dimensional polyacrylamide gel electrophoresis showed that ST 638 specifically inhibited the EGF-induced phosphorylation and dephosphorylation of cellular proteins in A431 cells.

The phosphorylation of p68, a calcium-binding protein associated with the human syncytiotrophoblast submembranous cytoskeleton, is modulated by growth factors, activators of protein kinase C and cyclic AMP

The phosphorylation of the lipocortin-related protein, p68, found in Ca2+-dependent association with the submembranous cytoskeleton has been studied using isolated human placental syncytiotrophoblast plasma membrane vesicles. p68 undergoes rapid, cation-independent phosphorylation in unstimulated membrane vesicles which was inhibited, in a dose-dependent manner, by insulin, platelet-derived growth factor, macrophage colony stimulating factor, protein kinase C-activating phorbol esters and phosphatidylinositol-specific phospholipase C. Epidermal growth factor had no effect on overall p68 phosphorylation. Transferrin induced an increase in p68 phosphorylation. However, phosphotyrosine was detected in p68 after treatment with epidermal growth factor, macrophage colony stimulating factor or transferrin, whereas a reduction in p68 phosphorylation appeared to be restricted to serine. cAMP and both cholera and pertussis toxins inhibited p68 phosphorylation. Both toxins were synergistic with the effects of insulin and platelet-derived growth factor whilst being antagonistic to the effect of transferrin. Epidermal growth factor and both human and equine immunoglobulin G, all of which alone did not affect overall p68 phosphorylation, reduced cholera or pertussis toxin-induced inhibition of p68 phosphorylation. Several phosphatase inhibitors failed to prevent macrophage colony stimulating factor-induced reduction of p68 phosphorylation. These results indicate that (i) p68 is a potential substrate of receptor tyrosyl kinases, (ii) p68 is not phosphorylated by protein kinase C or cAMP-dependent kinase and (iii) p68 phosphorylation is inhibited by activation of multiple pathways including those employing diacylglycerol or cAMP as second messengers.

Biological and biochemical properties of the c-src+ gene product overexpressed in chicken embryo fibroblasts

The c-src protein isolated from neuronal cells (pp60c-src+) displays a higher level of protein kinase activity than does pp60c-src from nonneural tissues. There are two structural alterations present in the amino-terminal half of pp60c-src+ expressed in neurons which could contribute to the enhanced activity of this form of pp60c-src: (i) a hexapeptide insert located at amino acid 114 of avian pp60c-src+ and (ii) a novel site(s) of serine phosphorylation. We characterized pp60c-src+ expressed in a nonneuronal cell type to identify factors that regulate the activity of the c-src+ protein and the importance of the neuronal environment on this regulation. The c-src+ protein overexpressed in chicken embryo fibroblasts (CEFs) displayed higher kinase activity than did pp60c-src. The major sites of phosphorylation of the c-src+ protein were Ser-17 and Tyr-527. The unique site(s) of serine phosphorylation originally identified in pp60c-src+ expressed in neurons was not detected in the c-src+ protein overexpressed in CEFs. Therefore, the hexapeptide insert is sufficient to cause an elevation in the tyrosine protein kinase activity of pp60c-src+. Our data also indicate that CEFs infected with the Rous sarcoma virus (RSV)c-src+ display phenotypic changes that distinguish them from cultures producing pp60c-src and that pp60c-src+-expressing cells are better able to grow in an anchorage-independent manner. The level of total cellular tyrosine phosphorylation in RSVc-src+-infected cultures was moderately higher than the level observed in cultures infected with RSVc-src. This level was not as pronounced as that observed in cells infected with RSVv-src or oncogenic variants of RSVc-src. Thus, pp60c-src+ could be considered a partially activated c-src variant protein much like other c-src proteins that contain mutations in the amino-terminal domain.

Purification, characterization, and localization of 70 kDa calcium-sensitive protein (calelectrin) from mammary glands

Mammary glands contain a group of calcium-sensitive proteins that bind to membranes in a calcium-dependent manner. Using the calcium-dependent binding to hydrophobic surfaces in combination with conventional techniques, we have purified the 70 kDa mammary calcium-binding protein (70 kDa M-CBP) to homogeneity. Antisera prepared to the 70 kDa M-CBP or to bovine liver 67 kDa calelectrin reacted in immunoblot analysis with the 70 kDa M-CBP antigen and with several additional mammary CBP species in crude tissue homogenates. Limited proteolysis of the 70 kDa M-CBP produced smaller immunoreactive species; extensive proteolysis resulted in more complete degradation of the protein. Identical data were obtained with digestion of 67 kDa calelectrin. The pl for the 70 kDa M-CBP was determined to be approximately 5.8; the same value reported for 67 kDa calelectrin. Phosphorylation of 70 kDa M-CBP was not detected in epithelial cell culture metabolic labeling. Immunohistochemical localization showed the protein to be located in ductal epithelia of virgin mouse mammary glands with a pattern of increased staining of the basal portions of the cells. Some stromal cells were also reactive. Apparently, the 70 kDa M-CBP and 67 kDa calelectrin are the same protein. Furthermore, like the 32.5 calelectrin (endonexin) and calpactin I/p36/lipocortin II, the 70 kDa protein appears to be a ductal epithelial cell associated protein in the mammary gland.

Biological and biochemical properties of the c-src+ gene product overexpressed in chicken embryo fibroblasts

The c-src protein isolated from neuronal cells (pp60c-src+) displays a higher level of protein kinase activity than does pp60c-src from nonneural tissues. There are two structural alterations present in the amino-terminal half of pp60c-src+ expressed in neurons which could contribute to the enhanced activity of this form of pp60c-src: (i) a hexapeptide insert located at amino acid 114 of avian pp60c-src+ and (ii) a novel site(s) of serine phosphorylation. We characterized pp60c-src+ expressed in a nonneuronal cell type to identify factors that regulate the activity of the c-src+ protein and the importance of the neuronal environment on this regulation. The c-src+ protein overexpressed in chicken embryo fibroblasts (CEFs) displayed higher kinase activity than did pp60c-src. The major sites of phosphorylation of the c-src+ protein were Ser-17 and Tyr-527. The unique site(s) of serine phosphorylation originally identified in pp60c-src+ expressed in neurons was not detected in the c-src+ protein overexpressed in CEFs. Therefore, the hexapeptide insert is sufficient to cause an elevation in the tyrosine protein kinase activity of pp60c-src+. Our data also indicate that CEFs infected with the Rous sarcoma virus (RSV)c-src+ display phenotypic changes that distinguish them from cultures producing pp60c-src and that pp60c-src+-expressing cells are better able to grow in an anchorage-independent manner. The level of total cellular tyrosine phosphorylation in RSVc-src+-infected cultures was moderately higher than the level observed in cultures infected with RSVc-src. This level was not as pronounced as that observed in cells infected with RSVv-src or oncogenic variants of RSVc-src. Thus, pp60c-src+ could be considered a partially activated c-src variant protein much like other c-src proteins that contain mutations in the amino-terminal domain.

Novel tyrosine kinase substrates from Rous sarcoma virus-transformed cells are present in the membrane skeleton

We have previously reported the production of monoclonal antibodies directed against phosphotyrosine, which is the modification induced by many oncogene products and growth factor receptors. One of these antiphosphotyrosine antibodies (py20) was used in affinity chromatography to isolate phosphotyrosine (PY)-containing proteins from Rous sarcoma virus-transformed chick embryo fibroblasts (RSV-CEFs). Mice were immunized with these PY-proteins for the production of monoclonal antibodies to individual substrates. Fifteen antibodies were generated in this way to antigens with molecular masses of 215, 76, 60, and 22 kD. Antibodies to individual substrates were used to analyze the subcellular location in normal and RSV-CEFs. Antibodies to the 215- and 76-kD antigen stained focal contacts when used in immunofluorescence microscopy while anti-22-kD protein antibodies resulted in punctate staining concentrated in the margins of cells and in parallel arrays. Both distributions were altered in transformed cells. When cells were extracted with nonionic detergent under conditions that stabilize the cytoskeleton, 50% of the 76-kD protein and greater than 90% of the 22-kD protein fractionated with the cytoskeleton. This study offers a new approach to both the identification of membrane skeletal proteins in fibroblasts and changes that occur upon transformation by an activated tyrosine kinase.

Synthesis of p36 and p35 is increased when U-937 cells differentiate in culture but expression is not inducible by glucocorticoids

p36 and p35 are distinct but related proteins that share many structural and biochemical features which were first identified as major substrates for protein-tyrosine kinases. Subsequently, both proteins have been shown to be Ca2+-, phospholipid-, and F-actin-binding proteins that underlie the plasma membrane and are associated with the cortical cytoskeleton. Recent reports have claimed that these proteins function as lipocortins, i.e., phospholipase A2 inhibitors that mediate the anti-inflammatory action of glucocorticoids. To investigate this possibility and to learn more about the functions of p36 and p35, we used human-specific anti-p36 and anti-p35 monoclonal antibodies to determine whether the expression or secretion of either protein was inducible by dexamethasone in the human U-937 myeloid cell line and in other human cell types. Additionally, we examined the levels of mRNA for both proteins. No effect of dexamethasone was observed on p36 or p35 expression at either the mRNA or protein level, nor were these proteins secreted under any of the culture conditions investigated. However, it was observed that in these cells the rate of synthesis and accumulation of both proteins was increased when the U-937 cells were induced to differentiate in culture to adherent macrophagelike cells. This offers a model system with which to study the control of p36 and p35 expression.

Synthesis of calelectrins and calpactin I during cytochalasin mediated cell spreading inhibition

Mammary epithelial cell spreading on collagen gels has previously been shown to be correlated with the synthesis of a group of calcium-binding proteins (CBPs) which we have identified as the calcium-binding proteins termed calelectrins and calpactin I monomer/p36. To determine whether cell spreading per se is required for CBP synthesis, we examined the effect of cytochalasin D on these two events. Concentrations of cytochalasin D that did not reduce total protein synthesis, caused inhibition of cell spreading in a dose-dependent manner, but did not cause inhibition of CBP synthesis. Synthesis of collagen also continued during cytochalasin inhibition of cell spreading. Removal of the inhibitor from the cultures initiated cell spreading and CBP synthesis continued. Membrane-cytoskeleton complexes from control and CD treated cells were identical in regard to binding CBPs in a calcium-dependent manner. Colchicine, which inhibited cell spreading, was shown to be toxic to general protein synthesis at 75 nM. The data clearly indicate that mere inhibition of epithelial cell spreading does not automatically suppress CBP synthesis.

Developmental regulation of calcium-binding proteins (calelectrins and calpactin I) in mammary glands

We recently showed that mammary glands contain a novel class of calcium-binding proteins (CBPs) that bind to membranes in a calcium-dependent manner. We have also established that these mammary CBPs are equivalent to the calelectrins and calpactin I/p36. Since it has been suggested that these proteins might be involved in exocytosis, we examined mammary glands for these CBPs during secretory differentiation. Immunohistochemical examination showed glands from virgin animals to be rich in calelectrins and calpactin I/p36, while glands from lactating animals contained little immunoreactive material. In addition, silver-staining and immunoblot estimation of the CBPs in lysates from collagenase harvested secretory epithelia showed these proteins to be significantly reduced compared to nonsecretory epithelia. Close examination of the CBP immunoreactive cells of the mammary gland shows that ductal cells are prominent in their staining and that the immunoreactive material is associated with the cell surface. Also, in juvenile glands the myoepithelial stem cells (cap cells) of the elongating end bud are devoid of the CBPs. In contrast to the in vivo data, epithelia cultivated on collagen gels demonstrate comparable levels of the CBPs in both nonsecretory and secretory monolayers. The in vivo data indicate that the CBPs are developmentally regulated during mammary gland differentiation such that secretory epithelia are essentially devoid of these novel proteins. Furthermore, a role for calelectrin and calpactin I/p36 in exocytotic casein secretion is questioned.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

Synthesis of p36 and p35 is increased when U-937 cells differentiate in culture but expression is not inducible by glucocorticoids

p36 and p35 are distinct but related proteins that share many structural and biochemical features which were first identified as major substrates for protein-tyrosine kinases. Subsequently, both proteins have been shown to be Ca2+-, phospholipid-, and F-actin-binding proteins that underlie the plasma membrane and are associated with the cortical cytoskeleton. Recent reports have claimed that these proteins function as lipocortins, i.e., phospholipase A2 inhibitors that mediate the anti-inflammatory action of glucocorticoids. To investigate this possibility and to learn more about the functions of p36 and p35, we used human-specific anti-p36 and anti-p35 monoclonal antibodies to determine whether the expression or secretion of either protein was inducible by dexamethasone in the human U-937 myeloid cell line and in other human cell types. Additionally, we examined the levels of mRNA for both proteins. No effect of dexamethasone was observed on p36 or p35 expression at either the mRNA or protein level, nor were these proteins secreted under any of the culture conditions investigated. However, it was observed that in these cells the rate of synthesis and accumulation of both proteins was increased when the U-937 cells were induced to differentiate in culture to adherent macrophagelike cells. This offers a model system with which to study the control of p36 and p35 expression.

Functional diversity among spectrin isoforms

The purpose of this review on spectrin is to examine the functional properties of this ubiquitous family of membrane skeletal proteins. Major topics include spectrin-membrane linkages, spectrin-filament linkages, the subcellular localization of spectrins in various cell types and a discussion of major functional differences between erythroid and nonerythroid spectrins. This includes a summary of studies from our own laboratories on the functional and structural comparison of avian spectrin isoforms which are comprised of a common alpha subunit and a tissue-specific beta subunit. Consequently, the observed differences among these spectrins can be assigned to differences in the properties of the beta subunits.

p59v-rel, the transforming protein of reticuloendotheliosis virus, is complexed with at least four other proteins in transformed chicken lymphoid cells

Previous studies have identified the protein product of v-rel, the oncogene carried by reticuloendotheliosis virus (REV), as a 59,000-dalton phosphoprotein located predominantly in the cytosol of transformed chicken lymphoid cells. In immune precipitates of p59v-rel, there is a closely associated protein kinase activity. In chicken lymphoid cells that do not contain REV, p68c-rel is found free in the cytosol not associated with other proteins and not detectably phosphorylated. In this study, we found that immune precipitates of 59v-rel from REV-transformed cells contain at least four other proteins, of approximate molecular weights 124, 115, 68, and 36 kilodaltons (kDa). The 124-, 115-, and 36-kDa proteins are apparently unrelated to p59v-rel in sequence, and their coprecipitation suggests that they are complexed with p59v-rel. The coprecipitating 68-kDa protein was found to be p68c-rel, which, like the other three proteins, precipitates by virtue of its association with p59v-rel. Glycerol gradient analysis suggested the presence of more than one type of complex: one containing p115, p68c-rel, p59v-rel, and p36, and another containing p124, p115, p59v-rel, and possibly p68c-rel. In vitro kinase activity was found in all size classes, coinciding with the distribution of p115 and p59v-rel. The complex(es) was stable under a variety of conditions, including a wide range of ionic strengths, chelators, and detergents, and through multiple cycles of immune precipitation and elution. This suggests a specific and functionally significant interaction among the members that may be of direct relevance to the mechanism of REV-induced transformation.

p60c-src is complexed with a cellular protein in subcellular compartments involved in exocytosis

We found high levels of the c-src gene product in neuroendocrine tissues from adult animals. To understand the role of this proto-oncogene product, the subcellular localization of p60c-src was studied in neuroendocrine tissue from adrenal medulla. The results indicate that p60c-src was highly enriched in chromaffin granule membranes, in stable association with a protein of 38 kD. The complex with the 38-kD protein was also detected in brain, a tissue known to carry high levels of p60c-src. The 38-kD protein is not calpactin I, II, or synaptophysin. Comparison of its peptide map showed a high degree of conservation among the different species and tissues examined. The interaction between p60c-src and the 38-kD protein involves disulphide bonds that are stable even when the cell fractionation is performed in the presence of a reducing agent. Since the presence of disulphide bonds among cytoplasmic proteins is very unlikely, the possibility of a noncovalent association between p60c-src and the 38-kD protein in vivo is discussed. The 38-kD protein may be involved in a function of p60c-src related to secretory organelles.

Molecular cloning of murine p68, a Ca2+-binding protein of the lipocortin family

A plasmid cDNA library prepared from a T-lymphocyte clone of murine strain B10.A origin was screened by cross-species DNA hybridisation using a partial human p68 cDNA clone, identified as containing coding sequences for previously determined amino-acid sequences. The longest p68 cDNA insert from this library and a full-length cDNA insert from a second similar library were fully sequenced. A comparison of the derived amino-acid sequence with that of human p68 revealed extensive homology (95% overall). Homology at the nucleotide level was 89% in the open reading frame and 85% and 50% in the 5' (33 nucleotides) and 3' (347 nucleotides) non-coding regions respectively. Eight segments of internal homology were observed, each containing a highly conserved consensus region of 17 amino acids correlating with that described for several membrane associated calcium-binding proteins [Geisow, M. J., Fritsche, U., Hexham, J. M. & Johnson, T. (1986) Nature (Lond.) 320, 636-638]. These results provide further evidence that p68 is a member of the same gene family as p32,p36 and lipocortin I and demonstrate an unusually high level of inter-species sequence conservation of p68 between mouse and human.

Phosphorylation of cellular proteins in Rous sarcoma virus-infected cells: analysis by use of anti-phosphotyrosine antibodies

The protein substrates for the tyrosine protein kinases in cells transformed by avian sarcoma viruses were analyzed by gel electrophoresis in combination with immunoblotting or immunoprecipitation by antibodies against phosphotyrosine. We found that greater than 90% of phosphotyrosine-containing cellular proteins can be immunoprecipitated by these antibodies. The level of phosphotyrosine-containing cellular proteins detectable by this method markedly increased upon transformation with Rous sarcoma virus, and more than 20 distinct bands of such proteins were found in lysates of Rous sarcoma virus-transformed cells. Most of these phosphotyrosine-containing proteins had not been identified by other methods, and their presence appeared to correlate with morphological transformation in cells infected with various Rous sarcoma virus mutants and Y73, PRCII, and Fujinami sarcoma viruses. However, considerably different patterns were obtained with cells infected with nontransforming Rous sarcoma virus mutants that encode nonmyristylated src kinases, indicating that most substrates that correlate with transformation can only be recognized by p60v-src associated with the plasma membrane.

Biosynthesis, secretion and extracellular localization of anchorin CII, a collagen-binding protein of the calpactin family

The amino acid sequence of anchorin CII, a collagen-binding protein isolated originally from chondrocyte membranes, was previously determined by sequencing of cDNA and proteolytic fragments of the protein. Computer analysis of the protein sequence revealed four internal repeats of approximately 70-80 residues, each containing a highly conserved consensus sequence of 17 residues. These repeats show considerable homology with sequences in human and bovine calpactin, lipocortin, endonexin and protein II, which are members of a family of Ca2+- and phospholipid-binding proteins, as well as major substrates of tyrosine kinases. While these proteins have been located at the inner side of the plasma membrane of fibroblasts and epithelial cells, here we present experimental evidence that anchorin CII is at least partially released from cells and binds to the outer cell surface. Biosynthesis studies in cell-free systems and in cell culture indicate that anchorin CII is not processed, which is consistent with the absence of signal sequences from the protein. Yet, pulse-chase experiments show that anchorin is released into the culture medium of fibroblasts after 30 min, and in chondrocyte cultures after 20 h. Anchorin CII was located to the outer cell surface of chondrocytes by lactoperoxidase-catalyzed cell surface iodination as well as by antibody labeling both at light- and electron-microscopical level. The pericellular localization of anchorin CII is consistent with the notion that this protein is involved in the interaction of chondrocytes and fibroblasts with extracellular collagen.

Mammary gland Ca2+-binding (-dependent) proteins: identification as calelectrins and calpactin I/p36

Calcium-binding (-dependent) proteins (CBPs) associated with the spreading of mammary epithelial cell cultures have been identified as various calelectrins and calpactins (p36). In immunoblot analysis, the CBPs of 30-36 kD and 68-70 kD variously react with different calelectrin and calpactin I monomer/p36 antisera. The same immunoreactive proteins were shown to be present in virgin mammary glands and collagen gel mouse mammary epithelial cell cultures. The mammary CBPs show extensive immunochemical relatedness; however, they fail to show cross-reaction with antiserum to calpactin II (lipocortin) antiserum. These immunoreactive CBPs comigrate in electrophoresis with 35S-methionine-labeled CBPs isolated from mammary epithelial cell cultures. Unlike calmodulin, the mammary CBPs that correspond to calelectrins and calpactin I monomer/p36 are not stable to thermal denaturation. The mammary CBPs bind to epithelial cell membranes in a Ca2+-dependent manner and are differentially released from ruptured cells, compared with calmodulin, suggesting subcellular localization. Phenothiazine-agarose and phenylagarose are equivalent in their ability to bind the mammary CBPs. Thus, mammary gland CBPs of 30-36 kD and 68-70 kD have been shown to be related or equivalent to the calelectrins and to calpactin I monomer/p36. Since these proteins are known to bind Ca2+, we conclude that the mammary gland CBPs are also Ca2+-binding proteins. The mammary gland CBPs are immunologically related and probably represent members of a larger family of related proteins.

The submembranous location of p11 and its interaction with the p36 substrate of pp60 src kinase in situ

p36, a major cytoplasmic substrate of pp60 src kinase, is present beneath the plasma membrane. It can be isolated either as a monomer or as a heterotetramer (protein I) containing two copies each of p36 and a unique p11 polypeptide. To compare the expression rules of p36 and p11 as well as their cellular distributions, monoclonal antibodies to the two porcine proteins were isolated. In tissue culture cells p11-specific antibodies decorated the same submembranous compartment previously seen with antibodies to p36 and fodrin or spectrin and followed the p36 images under all fixation/extraction conditions tested. Immunofluorescence microscopy on tissue sections showed coincident expression patterns of both proteins confirming and extending previous results with p36 antibodies. Antibodies with limited cross-species reaction have been used to trace the fate of porcine p11 and p36 injected into cultured cells. Both proteins are incorporated in the submembranous compartment, where they remain in Triton cytoskeletons prepared in the presence but not in the absence of Ca2+. The incorporation of p36 in vivo conforms with its Ca2+-dependent binding to actin, fodrin, and certain phospholipids in vitro. In contrast, the incorporation of p11 seems to depend on an in situ interaction with p36 or an exchange with endogenous p11 present on p36. The combined results indicate a strong coupling of p11 and p36 in cellular compartmentalization and tissue differentiation.

Changes in protein phosphorylation in Rous sarcoma virus-transformed chicken embryo cells

Rous sarcoma virus encodes a tyrosine-specific protein kinase (p60src) which is necessary for cell transformation. To identify substrates for this kinase, we set out to detect phosphotyrosine-containing proteins in Rous sarcoma virus-transformed chicken embryo cells, making use of the known alkali stability of phosphotyrosine. 32P-labeled phosphoproteins were separated by isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The gels were then incubated in alkali. Using this procedure with normal cells, we detected a total of about 190 alkali-resistant phosphoproteins. In Rous sarcoma virus-transformed cells, five phosphoproteins were found which were not detectable in normal cells. Two of these are probably structural proteins of the virus. The other three transformation-dependent phosphoproteins, and four other phosphoproteins which were elevated by transformation, all contained phosphotyrosine. Increased phosphorylation of these proteins did not occur with cells infected with a mutant Rous sarcoma virus, temperature sensitive for transformation, grown at the restrictive temperature. We conclude that these seven proteins are probably substrates of p60src, although they may be substrates for other tyrosine-specific protein kinases activated by p60src.

Four different classes of retroviruses induce phosphorylation of tyrosines present in similar cellular proteins

Chicken embryo cells transformed by the related avian sarcoma viruses PRC II and Fujinami sarcoma virus, or by the unrelated virus Y73, contain three phosphoproteins not observed in untransformed cells and increased levels of up to four other phosphoproteins. These same phosphoproteins are present in increased levels in cells transformed by Rous sarcoma virus, a virus which is apparently unrelated to the three aforementioned viruses. In all cases, the phosphoproteins contain phosphotyrosine and thus may be substrates for the tyrosine-specific protein kinases encoded by these viruses. In one case, the site(s) of tyrosine phosphorylation within the protein is the same for all four viruses. A homologous protein is also phosphorylated, at the same major site, in mouse 3T3 cells transformed by Rous sarcoma virus or by the further unrelated virus Abelson murine leukemia virus. A second phosphotyrosine-containing protein has been detected in both Rous sarcoma virus and Abelson murine leukemia virus-transformed 3T3 cells, but was absent from normal 3T3 cells and 3T3 cells transformed by various other viruses. We conclude that representatives of four apparently unrelated classes of transforming retroviruses all induce the phosphorylation of tyrosines present in the same set of cellular proteins.

Avian sarcoma viruses

Twelve independent isolates of avian sarcoma viruses (ASVs) can be divided into four groups according to the transforming genes harbored in the viral genomes. The first group is represented by viruses containing the transforming sequence, src, inserted in the viral genome as an independent gene; the other three groups of viruses contain transforming genes fps, yes or ros fused to various length of the truncated structural gene gag. These transforming sequences have been obtained by avian retroviruses from chicken cellular DNA by recombination. The src-containing viruses code for an independent polypeptide, p60src; and the representative fps, yes and ros-containing ASVs code for P140/130gag-fps, P90gag-yes and P68gag-ros fusion polypeptides respectively. All of these transforming proteins are associated with the tyrosine-specific protein kinase activity capable of autophosphorylation and phosphorylating certain foreign substrates. p60src and P68gag-ros are integral cellular membrane proteins and P140/130gag-fps and P90gag-yes are only loosely associated with the plasma membrane. Cells transformed by ASVs contain many newly phosphorylated proteins and in most cases have an elevated level of total phosphotyrosine. However, no definitive correlation between phosphorylation of a particular substrate and transformation has been established except that a marked increase of the tyrosine phosphorylation of a 34,000 to 37,000 dalton protein is observed in most ASV transformed cells. The kinase activity of ASV transforming proteins appears to be essential, but not sufficient for transformation. The N-terminal domain of p60src required for myristylation and membrane binding is also crucial for transformation. By contrast, the gag portion of the FSV P130gag-fps is dispensable for in vitro transformation and removal of it has only an attenuating effect on in vivo tumorigenicity. The products of cellular src, fps and yes proto-oncogenes have been identified and shown to also have tyrosine-specific protein kinase activity. The transforming potential of c-src and c-fps has been studied and shown that certain structural changes are necessary to convert them into transforming genes. Among the cellular proto-oncogenes related to the four ASV transforming genes, c-ros most likely codes for a growth factor receptor-like molecule. It is possible that the oncogene products of ASVs act through certain membrane receptor(s) or enzyme(s), such as protein kinase C, in the process of cell transformation.

Aggregation of chromaffin granules by calpactin at micromolar levels of calcium

Several cytosolic proteins bind to secretory granule membranes in a Ca2+-dependent manner and thus may be involved in the mediation of membrane interactions during exocytosis. One of these proteins, calpactin, is a tetramer consisting of two heavy chains of relative molecular mass (Mr) 36K (p36) and two light chains of 10K (p10). We report here that calpactin promotes the Ca2+-dependent aggregation and fatty acid-dependent fusion of chromaffin granule membranes at a level of Ca2+ that is lower than that reported for other granule-aggregating proteins, and which parallels the Ca2+ requirement for secretion from permeabilized chromaffin cells. We found subunits of calpactin to be inactive in promoting granule aggregation. Two distinct 33K proteolytic fragments of p36, differing at their N termini, also promote granule aggregation but with different Ca2+ sensitivities from calpactin. These differences suggest that the N-terminal portion of p36 modulates the Ca2+/lipid binding sites in the core portion of p36 (ref.5).

Membrane immunoglobulin: anti-immunoglobulin interactions mediate the phosphorylation of actin associated proteins in the B-lymphocyte

The binding of membrane immunoglobulin (mIg) by anti-Ig antibodies is known to initiate a mitogenic signal in B lymphocytes. Because in many instances growth control appears to be correlated with phosphokinase activity, as well as with alterations in cytoskeletal architecture, we asked the question whether antibodies binding to mIg would also lead to the specific phosphorylation of lymphocyte actin-associated proteins. Utilizing a myosin affinity technique, we directly examined phosphoproteins that were associated with actin in the chicken B cell. We found that in a few instances the level of phosphorylation was indeed modulated by mIg:anti-Ig interactions. These actin-binding phosphoproteins may be important control elements in the lymphocyte cytoskeleton.

The calpactin light chain is tightly linked to the cytoskeletal form of calpactin I: studies using monoclonal antibodies to calpactin subunits

Calpactins are a family of related Ca++-regulated cytoskeletal proteins. To analyze the expression and cytoskeletal association of calpactins we raised monoclonal antibodies with specificity for the heavy or light chains of calpactin I or to calpactin II. Comparison of the tissue distribution of calpactin I heavy and light chains by Western blots revealed that these subunits are coordinately expressed. Both soluble and cytoskeletal forms of the heavy chain of calpactin I were detected in human fibroblasts whereas only a soluble pool of calpactin II was found. These two forms of the calpactin I heavy chain differed both in their state of association with the light chain and in their rate of turnover. Both the soluble pool of the calpactin I heavy chain and calpactin II turned over three to four times faster than the cytoskeletal pool of heavy and light chains. Immunofluorescence microscopy revealed that the calpactin I light chain was present exclusively in the cytoskeleton whereas the calpactin I heavy chain distribution was more diffuse. No difference in the amount of light chain or the cytoskeletal attachment of phosphorylated calpactin I heavy chain was found in Rous sarcoma virus-transformed chick embryo fibroblasts compared with their normal counterpart. The antibody to the light chain of calpactin I was microinjected into cultured fibroblasts and kidney epithelial cells. In many cases antibody clustering was observed with the concomitant aggregation of the associated calpactin I heavy chain. The distribution of fodrin and calpactin II in injected cells remained unchanged. These results are consistent with the existence of two functionally distinct pools of calpactin I which differ in their association with the cytoskeleton.

Identification of an amino terminal domain required for the transforming activity of the Rous sarcoma virus src protein

Transformation of chicken cells by Rous sarcoma virus (RSV) requires the functional expression of the viral src protein, a tyrosine protein kinase, pp60src. Variants of RSV containing deletions within the amino terminal one-third of the src protein have been identified that exhibit either temperature-sensitive or transformation-defective phenotype when used to infect chicken embryo cells. To define the regions within the amino terminal portion of pp60src that influence morphological transformation, a series of overlapping deletion mutations in the src gene of Prague A RSV (Pr A RSV) were constructed and their biological and biochemical properties were analyzed. Deletions within the src gene which remove amino acid residues 38 to 142 had minimal effects on the ability of the mutant viruses to induce cellular transformation. However, deletions, which impinged upon the region of the src gene encoding residues 142 to 169, inhibited cellular transformation. A variant containing a deletion of amino acid residues 169 to 225, was temperature sensitive for transformation. Structurally altered src proteins recovered from cells infected with transformation-defective variants exhibited a somewhat reduced tyrosine protein kinase activities when assayed in the immune complex kinase assay. Analysis of the in vivo phosphorylation of a pp60src substrate, the 36-kDa protein, revealed virtually wild-type levels of phosphorylation in cells infected with the transformation-defective mutants. These studies suggest that the region of the Pr A RSV src protein delineated by amino acid residues 142 to 169 is essential for initiation and maintenance of morphological transformation of chicken cells in culture.

Activation of the oncogenic potential of the avian cellular src protein by specific structural alteration of the carboxy terminus

The role of tyrosine phosphorylation in the regulation of tyrosine protein kinase activity was investigated using site-directed mutagenesis to alter the structure and environment of the three tyrosine residues present in the C terminus of avian pp60c-src. Mutations that change Tyr 527 to Phe or Ser activate in vivo tyrosine protein kinase activity and induce cellular transformation of chicken cells in culture. In contrast, alterations of tyrosine residues present at positions 511 or 519 in c-src do not induce transformation or in vivo tyrosine protein kinase activity. Amber mutations, which alter the structure of the pp60c-src C terminus by inducing premature termination of the c-src protein at either residue 518 or 523 also induce morphological transformation and increase in vivo tyrosine phosphorylation, whereas removal of the last four residues of c-src by chain termination at residue 530 does not alter the kinase activity or the biological activity of the resultant c-src protein. We conclude from these studies that C-terminal alterations which either remove or replace Tyr 527 serve to activate the c-src protein resulting in cellular transformation and increased in vivo tyrosine protein kinase activity.

Rous sarcoma virus mutant dlPA105 induces different transformed phenotypes in quail embryonic fibroblasts and neuroretina cells

dlPA105 is a spontaneous variant of Rous sarcoma virus, subgroup E, which carries a deletion in the N-terminal portion of the v-src gene coding sequence. This virus was isolated on the basis of its ability to induce proliferation of quiescent quail neuroretina cells. The altered v-src gene encodes a phosphoprotein of 45,000 daltons which possesses tyrosine kinase activity. DNA sequencing of the mutant v-src gene has shown that deletion extends from amino acid 33 to 126 of wild-type p60v-src. We investigated the tumorigenic and transforming properties of this mutant virus. dlPA105 induced fibrosarcomas in quails with an incidence identical to that induced by wild-type virus. Quail neuroretina cells infected with the mutant virus were morphologically transformed and formed colonies in soft agar. In contrast, dlPA105 induced only limited morphological alterations in quail fibroblasts and was defective in promoting anchorage-independent growth of these cells. Synthesis and tyrosine kinase activity of the mutant p45v-src were similar in both cell types. These data indicate that the portion of the v-src protein deleted in p45v-src is dispensable for the mitogenic and tumorigenic properties of wild-type p60v-src, whereas it is required for in vitro transformation of fibroblasts. The ability of dlPA105 to induce different transformation phenotypes in quail fibroblasts and quail neuroretina cells is a property unique to this Rous sarcoma virus mutant and provides evidence for the existence of cell-type-specific response to v-src proteins.