Calpactins: calcium-regulated membrane-skeletal proteins

S100 family proteins in inflammation and beyond

The S100 family proteins possess a variety of intracellular and extracellular functions. They interact with multiple receptors and signal transducers to regulate pathways that govern inflammation, cell differentiation, proliferation, energy metabolism, apoptosis, calcium homeostasis, cell cytoskeleton and microbial resistance. S100 proteins are also emerging as novel diagnostic markers for identifying and monitoring various diseases. Strategies aimed at targeting S100-mediated signaling pathways hold a great potential in developing novel therapeutics for multiple diseases. In this chapter, we aim to summarize the current knowledge about the role of S100 family proteins in health and disease with a major focus on their role in inflammatory conditions.

Joining S100 proteins and migration: for better or for worse, in sickness and in health

The vast diversity of S100 proteins has demonstrated a multitude of biological correlations with cell growth, cell differentiation and cell survival in numerous physiological and pathological conditions in all cells of the body. This review summarises some of the reported regulatory functions of S100 proteins (namely S100A1, S100A2, S100A4, S100A6, S100A7, S100A8/S100A9, S100A10, S100A11, S100A12, S100B and S100P) on cellular migration and invasion, established in both culture and animal model systems and the possible mechanisms that have been proposed to be responsible. These mechanisms involve intracellular events and components of the cytoskeletal organisation (actin/myosin filaments, intermediate filaments and microtubules) as well as extracellular signalling at different cell surface receptors (RAGE and integrins). Finally, we shall attempt to demonstrate how aberrant expression of the S100 proteins may lead to pathological events and human disorders and furthermore provide a rationale to possibly explain why the expression of some of the S100 proteins (mainly S100A4 and S100P) has led to conflicting results on motility, depending on the cells used.

Annexin A8 displays unique phospholipid and F-actin binding properties

Annexin A8 is a poorly characterized member of the annexin family of Ca2+-regulated membrane binding proteins. Initially only identified at the cDNA level it had been tentatively linked to acute promyelocytic leukaemia (APL) due to its high and regulated expression in APL-derived cells. Here we identify unique properties of the annexin A8 protein. We show that it binds Ca2+-dependently and with high specificity to phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2) and is also capable of interacting with F-actin. In line with these characteristics annexin A8 is recruited to F-actin-associated PtdIns(4,5)P2-rich membrane domains formed in HeLa cells upon infection with non-invading enteropathogenic Escherichia coli. These properties suggest a role of annexin A8 in the organization of certain actin-associated membrane domains.

Regulation of actin dynamics by annexin 2

Annexin 2 is a ubiquitous Ca(2+)-binding protein that is essential for actin-dependent vesicle transport. Here, we show that in spontaneously motile cells annexin 2 is concentrated in dynamic actin-rich protrusions, and that depletion of annexin 2 using siRNA leads to the accumulation of stress fibres and loss of protrusive and retractile activity. Cells co-expressing annexin 2-CFP and actin-YFP exhibit Ca(2+)-dependent fluorescense resonance energy transfer throughout the cytoplasm and in membrane ruffles and protrusions, suggesting that annexin 2 may directly interact with actin. This notion was supported by biochemical studies, in which we show that annexin 2 reduces the polymerisation rate of actin monomers in a dose-dependent manner. By measuring actin polymerisation rates in the presence of barbed-end and pointed-end cappers, we further demonstrate that annexin 2 specifically inhibits filament elongation at the barbed ends. These results show that annexin 2 has an essential role in maintaining the plasticity of the dynamic membrane-associated actin cytoskeleton, and that its activity in this context may be at least partly explained through direct interactions with polymerised and monomeric actin.

Annexin-Actin Interactions

The actin cytoskeleton is a malleable framework of polymerised actin monomers that may be rapidly restructured to enable diverse cellular activities such as motility, endocytosis and cytokinesis. The regulation of actin dynamics involves the coordinated activity of numerous proteins, among which members of the annexin family of Ca2+- and phospholipid-binding proteins play an important role. Although the roles of annexins in actin dynamics are not understood at a mechanistic level, annexins have the requisite properties to integrate Ca2+-signaling with actin dynamics at membrane contact sites. In this review we discuss the current state of knowledge on this topic, and consider how and where annexins may fit into the complex molecular machinery that regulates the actin cytoskeleton.

Induction of endogenous genes following infection of human endothelial cells with an E1(-) E4(+) adenovirus gene transfer vector

Recombinant adenovirus (Ad) gene transfer vectors are effective at transferring exogenous genes to a variety of cells and tissue types both in vitro and in vivo. However, in the process of gene transfer, the Ad vectors induce the expression of target cell genes, some of which may modify the function of the target cell and/or alter the local milieu. To develop a broader understanding of Ad vector-mediated induction of endogenous gene expression, genes induced by first-generation E1(-) E4(+) Ad vectors in primary human umbilical vein endothelial cells were identified by cDNA subtraction cloning. The identified cDNAs included signaling molecules (lymphoid blast crisis [LBC], guanine nucleotide binding protein alpha type S [Galpha-S], and mitogen kinase [MEK5]), calcium-regulated/cytoskeletal proteins (calpactin p11 and p36 subunits, vinculin, and spinocerebellar ataxia [SCA1]), growth factors (insulin-like growth factor binding protein 4 and transforming growth factor beta2), glyceraldehyde-6-phosphate dehydrogenase, an expressed sequence tag, and a novel cDNA showing homology to a LIM domain sequence. Two- to sevenfold induction of the endogenous gene expression was observed at 24 h postinfection, and induction continued up to 72 h, although the timing of gene expression varied among the identified genes. In contrast to that observed in endothelial cells, the Ad vector-mediated induction of gene expression was not found following Ad vector infection of primary human dermal fibroblasts or human alveolar macrophages. Empty Ad capsids did not induce endogenous gene expression in endothelial cells. Interestingly, additional deletion of the E4 gene obviated the upregulation of genes in endothelial cells by the E1(-) E3(-) Ad vector, suggesting that genes carried by the E4 region play a central role in modifying target cell gene expression. These findings are consistent with the notion that efficient transfer of exogenous genes to endothelial cells by first-generation Ad vectors comes with the price that these vectors also induce the expression of a variety of cellular genes.

Localization of five annexins in J774 macrophages and on isolated phagosomes

Annexins are a family of structurally related proteins which bind phospholipids in a calcium-dependent manner. Although the precise functions of annexins are unknown, there is an accumulating set of data arguing for a role for some of them in vesicular transport and, specifically, in membrane-membrane or membrane-cytoskeletal interactions during these processes. Here we describe our qualitative and quantitative analysis of the localization of annexins I-V in J774 macrophages that had internalized latex beads, both with and without IgG opsonization. Our results show that whereas all these annexins are present on both the plasma membrane and on phagosomes, the localization on other organelles differs. Annexins I, II, III and V were detected on early endosomes, while only annexin V was seen on late endocytic organelles and mitochondria. Annexins I and II distributed along the plasma membrane non-uniformly and co-localized with F-actin at the sites of membrane protrusions. We also investigated by western blot analysis the association of annexins with purified phagosomes isolated at different time-points after latex bead internalization. While the amounts of annexins I, II, III and V associated with phagosomes were similar at all times after their formation, the level of annexin IV was significantly higher on older phagosomes. Whereas annexins I, II, IV and V could be removed from phagosome membranes with a Ca2+ chelator they remained membrane bound under low calcium conditions. In contrast, annexin III was removed under these conditions and needed a relatively high Ca2+ concentration to remain phagosome bound. Because of their purity and ease of preparation we suggest that phagosomes are a powerful system to study the potential role of annexins in membrane traffic.

Modification of annexin II expression in PC12 cell lines does not affect Ca(2+)-dependent exocytosis

The Ca2+/phospholipid/cytoskeletal-binding protein annexin II has been proposed to play an important role in Ca(2+)-dependent exocytosis; however, the evidence for this role is inconclusive. More direct evidence obtained by manipulating annexin II levels in cells is still required. We have attempted to do this by generating stably transfected PC12 cell lines expressing proteins which elevate or lower functional annexin II levels and using these cell lines to investigate Ca(2+)-dependent exocytosis. Three cell lines were generated: one expressing an annexin II mutant which aggregates annexin II in at least a proportion of the cells, thereby removing functional protein from the cell; a mixed clonal cell line constitutively overexpressing human annexin II; and a clonal cell line capable of over-expressing annexin II in the presence of sodium butyrate. After digitonin permeabilization, Ca(2+)-dependent dopamine release from these cell lines was compared with that from control nontransfected cells, and, in addition, release was compared in induced to uninduced cells. There were no significant differences in Ca(2+)-dependent exocytosis between any of the transfected cell lines before or after induction and the control cells. In addition, nontransfected PC12 cells treated with nerve growth factor, which elevates annexin II levels severalfold, failed to increase Ca(2+)-dependent exocytosis after digitonin permeabilization, compared with control cells. We conclude that annexin II is not an important regulator of Ca(2+)-dependent exocytosis in PC12 cells.

Annexin II modulates volume-activated chloride currents in vascular endothelial cells

The membrane-associated, microfilament-binding protein annexin II is abundantly expressed in endothelial cells from calf pulmonary artery (CPAE cells). We have analyzed its role in the regulation of volume-activated chloride currents (ICl, vol) by loading the cells via the patch pipette with a peptide comprising the N-terminal 14 residues of annexin II. This sequence harbors the binding site for the intracellular annexin II ligand, p11, and the peptide interferes with the annexin II-p11 complex formation. Loading of a CPAE cell with this peptide caused a gradual decrease in the amplitude of ICl, vol during repetitive stimulations with a 28% hypotonic extracellular solution. This run down of the current was virtually absent in untreated cells and in cells that were loaded with a mutated 14-amino acid peptide, which has a single amino acid replacement known to result in a more than 1000 times reduced affinity for binding to p11. We conclude that annexin II-p11 complex formation is either directly or indirectly involved in the activation of ICl, vol in endothelial cells.

The association of annexin I with early endosomes is regulated by Ca2+ and requires an intact N-terminal domain

Annexin I is a member of a multigene family of Ca2+/phospholipid-binding proteins and a major substrate for the epidermal growth factor (EGF) receptor kinase, which has been implicated in membrane-related events along the endocytotic pathway, in particular in the sorting of internalized EGF receptors occurring in the multivesicular body. We analyzed in detail the intracellular distribution of this annexin by cell fractionation and immunoelectron microscopy. These studies used polyclonal as well as a set of species-specific monoclonal antibodies, whose epitopes were mapped to the lateral surface of the molecule next to a region thought to be involved in vesicle aggregation. Unexpectedly, the majority of annexin I was identified on early and not on multivesicular endosomes in a form that required micromolar levels of Ca2+ for the association. The specific cofractionation with early endosomes was also observed in transfected baby hamster kidney cells when the intracellular fate of ectopically expressed porcine annexin I was analyzed by using the species-specific monoclonal antibodies in Western blots of subcellular fractions. Interestingly, a truncation of the N-terminal 26, but not the N-terminal 13 residues of annexin I altered its intracellular distribution, shifting it from fractions containing early to those containing late and multivesicular endosomes. These findings underscore the regulatory importance of the N-terminal domain and provide evidence for an involvement of annexin I in early endocytotic processes.

Annexin II up-regulates cellular levels of p11 protein by a post-translational mechanisms

Annexin II (p36) and p11, which belong to two different families of calcium-binding proteins, are able to form a heterotetrameric protein complex (p36)2(p11)2 called calpactin I. As these proteins were detectable only in the presence of each other in a variety of cell lines, we studied the mechanisms of regulation of cellular levels of annexin II and p11. In cells expressing p11 messenger RNA, p11 protein is undetectable unless annexin II is also expressed. As an example, the hepatoblastoma HepG2 cell line displays no detectable annexin II nor p11 protein, although it expresses p11 mRNA. The overexpression of annexin II by gene transfer into HepG2 cells leads to the up-regulation of the cellular levels of p11 by a post-translational mechanism. In the presence of annexin II, there is no major change in the p11 transcript levels, but the half-life of the p11 protein is increased more than 6-fold. Thus, the degree of expression of annexin II, which varies according to different states of cellular differentiation and transformation, is an essential factor in the regulation of cellular levels of p11.

Monoclonal IgM antiphosphatidylserine antibody reacts against cytoskeleton-like structures in cultured human umbilical cord endothelial cells

PROBLEM

It has been proposed that antibodies against phospholipid-dependent antigens (aPLs), induce recurrent pregnancy loss and thrombosis through modulation of endothelial cell function, yet aPLs have not been conclusively shown to bind with endothelial cells.

METHOD

Using indirect immunofluorescence we investigated the anti-endothelial cell reactivity of three monoclonal antibodies that differentiate between the phospholipids cardiolipin (CL) and phosphatidylserine (PS): BA3B5C4 (CL+/PS+); 3SB9b (CL-/PS+); and D11A4 (CL+/PS-). Cultured umbilical cord endothelial cells were prepared without fixation or with cold acetone fixation.

RESULTS

None of the aPLs reacted with endothelial cells prepared without fixation. 3SB9B reacted strongly with cytoskeletal-like components in acetone-fixed cells, whereas BA3B5C4 and D11A4 were unreactive. The cytoskeletal-like binding of 3SB9b was completely blocked by a monoclonal antibody against vimentin, whereas antibodies against tubulin or actin were not inhibitory. Lipid extraction of the cells destroyed the 3SB9b reactive antigen without affecting the reactivity of anti-vimentin.

CONCLUSION

These results suggest that phospholipid-dependent antigenic determinants are not expressed on the surface of resting endothelial cells but that a PS-dependent antigenic determinant is associated with endothelial cell intermediate filaments.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Ultrastructural localization of the S-100-like proteins MRP8 and MRP14 in monocytes is calcium-dependent

MRP8 and MRP14 are members of the S-100 family of Ca(2+)-binding proteins and are expressed by granulocytes and monocytes. Members of this family have been described to be involved in membrane and cytoskeleton interactions; we therefore studied the subcellular distribution of MRP8/MRP14 in cultured human monocytes at the ultrastructural level. Monospecific rabbit antisera against MRP8 and MRP14 and a monoclonal antibody (moAb 27E10), which exclusively recognizes the MRP8/MRP14 heterodimer but not the monomers, were used in both immunoperoxidase/preembedding- and immunogold/cryotechniques. Comparing non-stimulated monocytes with Ca2+ ionophore A23187-treated cells, we could demonstrate that MRP8 and MRP14 associate with membrane and cytoskeletal structures in a Ca(2+)-dependent manner. Employing moAb 27E10, MRP8/MRP14 complexes were shown to be translocated to these cellular components. In addition, immunogold double-labelling experiments revealed a clear co-localization of MRP8/MRP14 complexes with the type III intermediate filament vimentin. Analysis of immunogold-labelled cryosections of renal allografts after acute vascular rejection demonstrated that a subpopulation of infiltrating macrophages showed a similar association of MRP8/MRP14 to the cytoskeleton in situ; this finding emphasizes the in vivo relevance of our observations. We conclude that Ca(2+)-dependent translocation of MRP8/MRP14 occurs to distinct subcellular components suggesting a role of these proteins for the modulation of cytoskeletal and membrane interactions.

Regulation of annexin I-dependent aggregation of phospholipid vesicles by protein kinase C

Annexin I is a member of the annexin family of Ca(2+)- and phospholipid-binding proteins. The ability of this protein to aggregate and to mediate the fusion of various types of vesicles has supported the hypothesis that this protein might be involved in intracellular membrane fusion processes such as exocytosis. Although annexin I has been described as a major in vitro substrate of both protein kinase C and the epidermal-growth-factor-receptor protein tyrosine kinase, the functional consequences of these phosphorylation events have not been investigated. In this paper we examine the effect of the phosphorylation of annexin I by protein kinase C on the phospholipid aggregation activity of the protein. The stoichiometry of phosphorylation of the protein was affected by the method of preparation of the phospholipid. Under optimal assay conditions protein kinase C catalysed the incorporation of 2.83 +/- 0.23 mol of phosphate/mol of annexin I (mean +/- S.E.M., n = 21). Studies with the Ca(2+)- and phospholipid-independent form of protein kinase C suggested that the phosphorylation of annexin I was stimulated by phospholipid in the absence of Ca2+, although maximal phosphorylation was achieved in the presence of both phospholipid and Ca2+. Phosphorylation of annexin I resulted in a dramatic decrease in the rate and extent of phospholipid vesicle aggregation, without significantly disrupting the binding of the protein to the phospholipid vesicles. The phosphorylation of annexin I increased the EC50 (Ca2+) of phospholipid vesicle aggregation from 19 +/- 10 microM (mean +/- S.D., n = 7) for the native protein to 290 +/- 95 microM (mean +/- S.D., n = 5) for the phosphorylated protein. These results suggest that protein kinase C may act to inhibit the phospholipid vesicle aggregation activity of annexin I.

Annexin II is a major component of fusogenic endosomal vesicles

We have used an in vitro assay to follow the proteins transferred from a donor to an acceptor upon fusion of early endosomes. The acceptor was a purified early endosomal fraction immunoisolated on beads and the donor was a metabolically-labeled early endosomal fraction in suspension. In the assay, both fractions were mixed in the presence of unlabeled cytosol, and then the beads were retrieved and washed. The donor proteins transferred to the acceptor were identified by two-dimensional gel electrophoresis and autoradiography. Approximately 50 major proteins were transferred and this transfer fulfilled all criteria established for endosome fusion in vitro. However, only a small subset of proteins was efficiently transferred, if donor endosomes were briefly sonicated to generate small (0.1 micron diam) vesicles before the assay. These include two acidic membrane proteins, and three alkaline peripheral proteins exposed on the cytoplasmic face of the membrane. Partial sequencing and Western blotting indicated that one of the latter components is annexin II, a protein known to mediate membrane-membrane interactions. Immunogold labeling of cryosections confirmed that annexin II is present on early endosomes in vivo. These data demonstrate that annexin II, together with the other four proteins we have identified, is a major component of fusogenic endosomal vesicles, suggesting that these proteins are involved in the binding and/or fusion process.

Identification of calcium-dependent phospholipid-binding proteins (annexins) from guinea pig alveolar type II cells

A new group of calcium-regulating proteins, called annexins or Ca(++)-dependent phospholipid-binding proteins (PLBP), have been detected in different species, organs and cell types. In the present study, we have identified and quantitated PLBP from guinea pig lung, lavage fluid and alveolar type II cells to elucidate the possible role of PLBP in lung surfactant biogenesis and secretion. Lungs were lavaged and type II cells from lavaged lung were isolated by elastase digestion and purified by centrifugal elutriation. For the quantitative identification of PLBP, we performed ELISA assays and Western blot analysis by using an antiserum raised in guinea pigs against a pure rabbit lung 36 kDa PLBP. The lavage fluid, cytosol from lung and type II cells contained 784, 167 and 435 ng per mg protein, respectively, of PLBP. The SDS-PAGE electrophoretic pattern and Western blot confirmed that all lung samples have band corresponding to a 36 kDa protein. This indicates that both alveolar type II cells and lavage fluid have higher levels of PLBP than whole lung cytosol.

The pattern of plant annexin gene expression

Peptide sequence data derived from a plant annexin, P34 [Smallwood, Keen & Bowles (1990) Biochem. J. 270, 157-161] was used to design amplimers for PCR. A unique fragment of 95 bp, amplified from tomato (Lycopersicon esculertum) genomic DNA, was used in Northern analyses and demonstrated a differential pattern of expression in vegetative tissues of tomato, potato (Solanum tuberosum) and barley (Hordeum vulgare). The tissue-specific abundance of the annexin transcript was found to correlate closely with abundance of annexin protein as revealed by their partial purification and analysis with antisera specific for annexins isolated from tomato suspension-culture cells.

Inhibition of tumor cell invasion by verapamil

Verapamil, a calcium channel antagonist, inhibits murine B16 melanoma and colon adenocarcinoma C26 tumor metastasis by altering platelet aggregation [Tsuruo, T., et al. (1985) Cancer Chemother. Pharmacol., 14:30-33]. However, the role of calcium homeostasis in regulating several biochemical pathways implicated in other steps of the metastatic cascade suggests that calcium channel antagonists could also inhibit metastasis by other mechanisms. In this report, non-toxic doses of verapamil reversibly decreased human A375M and C8161 melanoma cell invasion and metastasis in a dose-dependent manner. Verapamil reduced cellular invasion and metastases by up to 96% (range 78-96%). Concomitantly, verapamil disrupts microtubule and microfilament organization and inhibits unidirectional cell migration but does not affect cellular adhesion to endothelial monolayers or reconstituted basement membranes. In addition, tumor cells treated with verapamil have a decrease in mRNA of type IV collagenase, a proteinase important in tumor cell degradation of basement membranes. Collectively, these data offer additional evidence regarding the mechanisms of action of verapamil as an anti-metastatic agent.

Calcium signalling and the triggering of secretion in adrenal chromaffin cells

The pivotal intracellular message for triggering catecholamine release from bovine adrenal chromaffin cells is an elevation in the concentration of cytosolic free Ca2+ ([Ca2+]i). Studies using video-imaging techniques have shown that a rise in [Ca2+]i at the cell periphery, that is due to Ca2+ entry, is the major activating signal for exocytosis. The cytoskeleton has been identified as a major regulatory site of exocytosis, with Ca(2+)-induced disruption of the cortical actin network being required in order that previously restrained granules may have access to their exocytotic sites. The Ca(2+)- and phospholipid-dependent annexin protein, calpactin, has been strongly implicated in a late stage of interaction between granules and the plasma membrane by both ultrastructural and biochemical studies.

Identification and purification of calpactins from cardiac muscle and their effect on Na+/Ca2+ exchange activity

Calpactins were purified from bovine cardiac muscle by a slightly modified Glenney et al. procedure (J. Cell. Biol. 104, 503-511, 1987). Two major proteins (apparent MW of 36 and 68 kDa) able to bind phospholipids in a Ca2(+)-dependent manner were identified. These proteins completely reversed the inhibition of Na+/Ca2+ exchange activity of cardiac sarcolemmal vesicles consequent to EGTA-treatment. A modulation of cardiac Na+/Ca2+ exchange activity by calpactins is suggested.

Evidence that the inositol phospholipids are necessary for exocytosis. Loss of inositol phospholipids and inhibition of secretion in permeabilized cells caused by a bacterial phospholipase C and removal of ATP

We directly manipulated the levels of PtdIns, PtdInsP and PtdInsP2 in digitonin-treated adrenal chromaffin cells with a bacterial phospholipase C (PLC) from Bacillus thuringiensis and by removal of ATP. The PtdIns-PLC acted intracellularly to cause a large decrease in [3H]inositol- or [32P]phosphate-labelled PtdIns, but did not directly hydrolyse PtdInsP or PtdInsP2. [3H]PtdInsP and [3H]PtdInsP2 levels declined markedly, probably because of the action of phosphatases in the absence of synthesis. Removal of ATP also caused marked decreases in [3H]PtdInsP and [3H]PtdInsP2. The decrease in polyphosphoinositide levels by PtdIns-PLC treatment or ATP removal was reflected by the inhibition of the production of inositol phosphates upon subsequent activation of the endogenous PLC by Ca2(+)-dependent catecholamine secretion from permeabilized cells was strongly inhibited by PtdIns-PLC treatment and by ATP removal. Ca2(+)-dependent secretion was similarly correlated with the sum of PtdInsP and PtdInsP2 when the level of these lipids was changed by either manipulation. PtdIns-PLC inhibited only the ATP-dependent component of secretion and did not affect ATP-dependent secretion. Both PtdIns-PLC and ATP removal inhibited the late slow phase of secretion, but had little effect on the initial rapid phase. Although we found a tight correlation between polyphosphoinositide levels and secretion, endogenous phospholipase C activity (stimulated by Ca2+, guanine nucleotides and related agents) was not correlated with secretion. Additional experiments indicated that neither the products of the PtdIns-PLC reaction (diacylglycerol and InsP1) nor the inability to generate products by subsequent activation of the endogenous PLC is likely to account for the inhibition of secretion. Incubation of permeabilized cells with neomycin in the absence of ATP maintained the level of polyphosphoinositides and more than doubled subsequent Ca2(+)-dependent secretion. The data suggest that: (1) Ca2(+)-dependent secretion has a requirement for the presence of inositol phospholipids; (2) the enhancement of secretion by ATP results in part from increased polyphosphoinositide levels; and (3) the role for inositol phospholipids in secretion revealed in these experiments is independent of their being substrates for the generation of diacylglycerol and InsP3.

Effect of sodium dodecyl sulfate on polar lobe formation and function in Ilyanassa obsoleta embryos

Polar lobes, anucleate vegetal pole protrusions formed by Ilyanassa obsoleta embryos, serve as a mechanism for shunting morphogenetic determinants to one cell during the first two cleavages. Polar lobe material becomes segregated in the CD cell during first cleavage and in the D cell during second cleavage, resulting in a very unequal four-cell stage. Larval structures including external shell, foot, operculum, statocysts, and eyes develop only when polar lobe material is present. Treatment with the anionic detergent sodium dodecyl sulfate (SDS) before and during the first cleavage inhibited polar lobe formation and equalized cleavage, as the lobe material was distributed to two cells. No polar lobes formed during second clevage in SDS-equalized embryos, and the four-cell stage consisted of four equal cells with reduced cell contacts. SDS inrreversibly inhibited polar lobe formation without affecting cytokinesis. Although 27% of the larvae from SDS-equalized embryos had one or more lobe-dependent structures duplicated, morphogenesis was impaired: more than 40% of such larvae failed to form shell and/or statocysts. When cells were separated after equalized first cleavage and raised as pairs, the pairs of resulting larvae duplicated lobe-dependent structures with the same frequency as whole equalized embryos. Possible explanations for impaired morphogenesis in SDS-treated embryos are discussed.

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.

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.

The annexin family of calcium-binding proteins. Review article

The annexins are a family of calcium-binding proteins. Data from protein and cDNA sequencing have shown that at least five distinct but closely related mammalian annexins exist each of which possesses four or eight homologous internal repeats which may be calcium-and phospholipid-binding domains. The proteins are present within a wide range of tissues and cell types, with each cell type having all or a subset of the proteins. The proteins are localised on the inner surface of the plasma membrane associated with the cytoskeleton and in some cases also with intracellular structures. Some members of the family are major substrates for tyrosine and serine kinases. The precise functions of the proteins are unknown but they are likely to play important roles in cellular regulation. Previously suggested functions are inhibition of phospholipase A2, membrane-cytoskeletal linkage and control of membrane fusion events in exocytosis. It is also suggested that they may be involved in the regulation of cell surface receptors.

Isolation and characterization of two novel calcium-dependent phospholipid-binding proteins from bovine lung

Two calcium-dependent proteins of apparent Mr 32,000 and 34,000 were isolated from bovine lung. Approx. 70 mg/kg of each was obtained. Two-dimensional gel electrophoresis in the presence of 8 M urea showed their apparent p/values to be 5.1 and 5.0, respectively. Both proteins are related immunologically to calelectrin from Torpedo marmorata. They also have very similar amino acid compositions to calelectrin. Partial sequence information shows that both proteins contain the highly conserved sequence described for the annexins, a new family of calcium-dependent membrane-binding proteins. In common with other members of this family, the new proteins bind to acidic phospholipids in a calcium-dependent manner.