Ankyrin is fatty acid acylated in erythrocytes

Palmitoylation modification of Galpha(o) depresses its susceptibility to GAP-43 activation

Interaction between GAP-43 (growth associated protein-43) and Galpha(o) (alpha subunit of Go protein) influences the signal transduction pathways leading to differentiation of neural cells. GAP-43 is known to increase guanine nucleotide exchange by Galpha(o), which is a major component of neuronal growth cone membranes. However, it is not clear whether GAP-43 stimulation is related to the Galpha(o) palmitoylation or the conversion of Galpha(o) from oligmers to monomers, which was shown to be a necessary regulatory factor in GDP/GTP exchange of Galpha(o). Here we expressed and purified GAP-43, GST-GAP-43 and Galpha(o) proteins, detected their stimulatory effect on [(35)S]-GTPgammaS binding of Galpha(o). It was found that the EC(50) of both GAP-43 and GST-GAP-43 activation were tenfold lower in case of depalmitoylated Galpha(o) than palmitoylated Galpha(o). Non-denaturing gel electrophoresis and p-PDM cross-linking analysis revealed that addition of GST-GAP-43 induced disassociation of depalmitoylated Galpha(o) from oligomers to monomers, but did not influence the oligomeric state of palmitoylated Galpha(o), which suggests that palmitoylation is a key regulatory factor in GAP-43 stimulation on Galpha(o). These results indicated the interaction of GAP-43 and Galpha(o) could accelerate conversion of depalmitoylated Galpha(o) but not palmitoylated Galpha(o) from oligomers to monomers, so as to increase the GTPgammaS binding activity of Galpha(o). Results here provide new evidence about how signaling protein palmitoylation is involved in the G-protein-coupled signal transduction cascade, and give a useful clue on the participation of GAP-43 in G-protein cycle by its preferential activation of depalmitoylated Galpha(o).

Molecular epitopes of the ankyrin-spectrin interaction

Isoforms of ankyrin and its binding partner spectrin are responsible for a number of interactions in a variety of human cells. Conflicting evidence, however, had identified two different, non-overlapping human erythroid ankyrin subdomains, Zu5 and 272, as the minimum binding region for beta-spectrin. Complementary studies on the ankyrin-binding domain of spectrin have been somewhat more conclusive yet have not presented binding in terms of well-phased, integral numbers of spectrin repeats. Thus, the objective of this study was to clearly define and characterize the minimal ankyrin-spectrin binding epitopes. Circular dichroism (CD) wavelength spectra of the aforementioned ankyrin subdomains show that these fragments are 30-60% unstructured. In contrast, human erythroid beta-spectrin repeats 13, 14, 15, and 16 (prepared in all combinations of two adjacent repeats) demonstrated proper folding and stability as determined by CD and tryptophan wavelength and heat denaturation scans. Native polyacrylamide gel electrophoresis (PAGE) gel shifts as well as affinity pull-down assays implicated Zu5 and beta-spectrin repeats 14-15 as the minimum binding epitopes. These results were confirmed by analytical ultracentrifugation to sedimentation equilibrium by which a 1:1 complex was obtained if and only if Zu5 was mixed with beta-spectrin constructs containing repeats 14 and 15 in tandem. Surface plasmon resonance yielded a K D of 15.2 nM for binding of beta-spectrin fragments to the ankyrin subdomain Zu5, accounting for all of the binding observed between the intact molecules. Collectively, these results show the 14th and 15th beta-spectrin repeats comprise the minimal, phased region of beta-spectrin, which binds ankyrin at the Zu5 subdomain with high affinity.

Dematin and adducin provide a novel link between the spectrin cytoskeleton and human erythrocyte membrane by directly interacting with glucose transporter-1

Dematin and adducin are actin-binding proteins located at the spectrin-actin junctions, also called the junctional complex, in the erythrocyte membrane. Here we propose a new model whereby dematin and adducin link the junctional complex to human erythrocyte plasma membrane. Using a combination of surface labeling, immunoprecipitation, and vesicle proteomics approaches, we have identified glucose transporter-1 as the receptor for dematin and adducin in the human erythrocyte membrane. This finding is the first description of a transmembrane protein that binds to dematin and adducin, thus providing a rationale for the attachment of the junctional complex to the lipid bilayer. Because homologues of dematin, adducin, and glucose transporter-1 exist in many non-erythroid cells, we propose that a conserved mechanism may exist that couples sugar and other related transporters to the actin cytoskeleton.

Evolutionary Relationships and Protein Domain Architecture in an Expanded Calpain Superfamily in Kinetoplastid Parasites

Employing whole-genome analysis we have characterized a large family of genes coding for calpain-related proteins in three kinetoplastid parasites. We have defined a total of 18 calpain-like sequences in Trypanosoma brucei, 27 in Leishmania major, and 24 in Trypanosoma cruzi. Sequence characterization revealed a well-conserved protease domain in most proteins, although residues critical for catalytic activity were frequently altered. Many of the proteins contain a novel N-terminal sequence motif unique to kinetoplastids. Furthermore, 24 of the sequences contain N-terminal fatty acid acylation motifs indicating association of these proteins with intracellular membranes. This extended family of proteins also includes a group of sequences that completely lack a protease domain but is specifically related to other kinetoplastid calpain-related proteins by a highly conserved N-terminal domain and by genomic organization. All sequences lack the C-terminal calmodulin-related calcium-binding domain typical of most mammalian calpains. Our analysis emphasizes the highly modular structure of calpains and calpain-like proteins, suggesting that they are involved in diverse cellular functions. The discovery of this surprisingly large family of calpain-like proteins in lower eukaryotes that combines novel and conserved sequence modules contributes to our understanding of the evolution of this abundant protein family.

Palmitoylation is not required for trafficking of human anion exchanger 1 to the cell surface

AE1 (anion exchanger 1) is a glycoprotein found in the plasma membrane of erythrocytes, where it mediates the electroneutral exchange of chloride and bicarbonate, a process important in CO2 removal from tissues. It had been previously shown that human AE1 purified from erythrocytes is covalently modified at Cys-843 in the membrane domain with palmitic acid. In this study, the role of Cys-843 in human AE1 trafficking was investigated by expressing various AE1 and Cys-843Ala (C843A) mutant constructs in transiently transfected HEK-293 cells. The AE1 C843A mutant was expressed to a similar level to AE1. The rate of N-glycan conversion from high-mannose into complex form in a glycosylation mutant (N555) of AE1 C843A, and thus the rate of trafficking from the endoplasmic reticulum to the Golgi, were comparable with that of AE1 (N555). Like AE1, AE1 C843A could be biotinylated at the cell surface, indicating that a cysteine residue at position 843 is not required for cell-surface expression of the protein. The turnover rate of AE1 C843A was not significantly different from AE1. While other proteins could be palmitoylated, labelling of transiently transfected HEK-293 cells or COS7 cells with [3H]palmitic acid failed to produce any detectable AE1 palmitoylation. These results suggest that AE1 is not palmitoylated in HEK-293 or COS7 cells and can traffic to the plasma membrane.

CAP5.5, a life-cycle-regulated, cytoskeleton-associated protein is a member of a novel family of calpain-related proteins in Trypanosoma brucei

The cell shape of African trypanosomes is determined by the presence of an extensive subpellicular microtubule cytoskeleton. Other possible functions of the cytoskeleton, such as providing a potential framework for signalling proteins transducing information from the intracellular and extracellular environment, have not yet been investigated in trypanosomes. In this study, we have identified a novel cytoskeleton-associated protein in Trypanosoma brucei. CAP5.5 is the first member of a new family of proteins in trypanosomes, characterised by their similarity to the catalytic region of calpain-type proteases. CAP5.5 is only expressed in procyclic, but not in bloodstream, trypanosomes. Furthermore, CAP5.5 has been shown to be both myristoylated and palmitoylated, suggesting a stable interaction with the cell membrane. A bioinformatics analysis of the trypanosome genome revealed a diverse family of calpain-related proteins with primary structures similar to CAP5.5, but of varying length. We suggest a nomenclature for this new family of proteins in T. brucei.

Signalling functions of protein palmitoylation

Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.

Kx, a quantitatively minor protein from human erythrocytes, is palmitoylated in vivo

Kx is a quantitatively minor blood group protein of human erythrocytes which is thought to be a membrane transporter. In the red cell membrane, Kx forms a complex stabilized by a disulfide bond with the Kell blood group membrane protein which might function as a metalloprotease. The palmitoylation status of these proteins was studied by incubating red cells with [3H] palmitic acid. Purification of the Kell-Kx complex, by immunochromatography on an immobilized human monoclonal antibody of Kell blood group specificity demonstrated that the Kx but not the Kell protein is palmitoylated. Six cysteines in Kx are predicted to be intracytoplasmic and might be targets for palmitoylation. Three of these cysteines are present in a portion of sequence which is predicted to form an amphipathic alpha helix. Palmitoylation of one or several of these cysteines might contribute to anchor the cytoplasmic portion of the Kx protein to the inner surface of red cell membrane.

Thioesterification of platelet proteins with saturated and polyunsaturated fatty acids

We have demonstrated that more than 20 platelet proteins can be acylated with fatty acids via thioester linkages. These include the glycoprotein IX beta chain of glycoprotein Ib, components of the von Willebrand factor receptor on the platelet surface, P-selectin, and alpha subunits of Gz, Gq, and Gi. Our studies have shown that platelet proteins can be posttranslationally acylated in thioester linkages not only with palmitate but with myristate and also with the eicosanoid precursor fatty acids arachidonate and eicosapentaenoate. Thioesterification of platelet proteins with fatty acids other than palmitate may have significant functional consequences for reversible binding of proteins to membranes.

Human erythrocyte membrane protein 4.2 is palmitoylated

Protein 4.2 is a major protein of the human erythrocyte membrane. It has previously been shown to be N-myristoylated. After labeling of intact human erythrocytes with [3H]palmitic acid, radioactivity was found to be associated with protein 4.2 by immunoprecipitation of peripheral membrane proteins extracted at pH 11 from ghosts with anti-(4.2) sera, followed by SDS/PAGE and fluorography. The fatty acid linked to protein 4.2 was identified as palmitic acid after hydrolysis of protein and thin-layer chromatography of the fatty acid extracted in the organic phase. Protein 4.2 could be depalmitoylated with hydroxylamine, suggesting a thioester linkage. Depalmitoylated protein 4.2 showed significantly decreased binding to protein-4.2-depleted membranes, compared to native protein 4.2.

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.

Keratinocyte transglutaminase membrane anchorage: analysis of site-directed mutants

Keratinocyte transglutaminase is anchored on the cytosolic side of the plasma membrane by fatty acid thioesterification near the amino terminus, a process which is seen to occur within 30 min of synthesis. The importance of a cluster of five cysteines (residues 47, 48, 50, 51, and 53) where acylation was presumed to occur is now demonstrated by site-directed mutagenesis. Transglutaminase mutants in which the cluster is deleted or the cysteines are all converted to alanine or serine are cytosolic. Partial replacement of the cluster, leaving two contiguous cysteines, is sufficient to confer membrane anchorage, while a single cysteine is only partially effective. As demonstrated with a soluble transglutaminase mutant, membrane anchorage confers susceptibility of the amino-terminal region to phorbol ester-stimulated phosphorylation. Attachment of 105 residues from the transglutaminase amino terminus to involucrin, a highly soluble protein, results in membrane anchorage of the hybrid protein. Attachment of the cysteine cluster alone does not result in membrane attachment of involucrin, but a 32-residue segment containing this cluster is sufficient. Stable transfectants of the human transglutaminase in mouse 3T3 cells are membrane-bound, indicating the fatty acid transacylation is not keratinocyte-specific.

Antagonism of [3H]fatty acid incorporation into vimentin by sodium pyruvate: pitfalls of protein acylation

In the course of studying possible fatty acid acylation of vimentin by cultured bovine lens epithelial cells, several potential pitfalls of protein-fatty acid acylation were recognized. Even exhaustive delipidation of vimentin with organic solvents failed to remove all noncovalently associated [3H]palmitate and [3H]myristate. Hydroxylamine treatment of vimentin, separated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), failed to remove either palmitate or myristate derived radiolabel. Hydroxylamine treatment did remove palmitate label from a group of lower molecular weight proteins. The myristate radiolabel associated with vimentin recovered after SDS-PAGE and subjected to acid hydrolysis was shown due to incorporated [3H]amino acids, mainly glutamic acid, generated from the fatty acid. Adding excess sodium pyruvate to labeling media has been used by others to reduce the metabolic conversion of fatty acids to amino acids; however, no direct evidence in support of this antagonism was presented. We observed that inclusion of sodium pyruvate at between 5 and 20 mM in the labeling medium produced a dramatic decrease in incorporation of myristic acid radiolabel into vimentin. However, inclusion of even 20 mM pyruvate did not completely antagonize the metabolic conversion of fatty acid label to amino acids. Furthermore, the sodium pyruvate antagonism could be totally obscured if the exposure of X-ray film by fluorography was even slightly prolonged. The results illustrate the danger in assuming that solvent extraction totally delipidates proteins and that adding sodium pyruvate to labeling media prevents the transfer of fatty acid label to amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Molecular identification of a major palmitoylated erythrocyte membrane protein containing the src homology 3 motif

The complete amino acid sequence of a 55-kDa erythrocyte membrane protein was deduced from cDNA clones isolated from a human reticulocyte library. This protein, p55, is copurified during the isolation of dematin, an actin-bundling protein of the erythrocyte membrane cytoskeleton. Fractions enriched in p55 also contain protein kinase activity that completely abolishes the actin-bundling property of purified dematin in vitro. The predicted amino acid sequence of p55 does not contain any consensus sequence corresponding to the catalytic domains of protein kinases but does contain a conserved sequence found in the noncatalytic domains of oncogene-encoded tyrosine kinases. This conserved src homology 3 (SH-3) motif appears to suppress the tyrosine kinase activity of various oncoproteins and has also been found in several plasma membrane associated proteins involved in signal transduction. Northern blot analysis indicated that p55 mRNA was constitutively expressed during erythropoiesis and underwent 2-fold amplification after induction of K562 erythroleukemia cells toward the erythropoietic lineage. The abundant expression of p55 mRNA, along with protein 4.1 mRNA, was evident in terminally differentiated human reticulocytes. Although p55 has many features consistent with known peripheral membrane proteins, its tight association with the plasma membrane is reminiscent of an integral membrane protein. This fact may be partly explained by the observation that p55 is the most extensively palmitoylated protein of the erythrocyte membrane.

Chorea-acanthocytosis: abnormal composition of covalently bound fatty acids of erythrocyte membrane proteins

Phospholipid class, peak profile of each phospholipid class, loosely bound fatty acids, covalently (tightly) bound fatty acids of the erythrocyte membranes, and plasma fatty acids were investigated using high-performance liquid chromatography in six patients with chorea-acanthocytosis and 14 age- and sex-matched normal control subjects. Additionally, six patients with Huntington's disease were included as disease control subjects in the study of covalently bound fatty acids. Study of covalently (tightly) bound fatty acids in erythrocyte membrane proteins after alkaline hydrolysis, hitherto undescribed in chorea-acanthocytosis, revealed that palmitic acid (C16:0) was significantly increased and stearic acid (C18:0) was decreased in the patients with chorea-acanthocytosis. Analyses for total covalently bound fatty acids disclosed that palmitic and docosahexaenoic (C22:6) acids were increased and stearic acid was decreased in chorea-acanthocytosis. Phospholipid class (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and phosphatidylserine) and peak profile of each phospholipid class from the erythrocyte membranes did not differ between the patients with chorea-acanthocytosis and the control subjects. Of the loosely bound fatty acids, linoleic acid (C18:2) was significantly decreased in those with chorea-acanthocytosis, which seemed to be nonspecific.

Synthesis of myristoyl-carba(dethia)-coenzyme A and S-(3-oxohexadecyl)-coenzyme A, two potent inhibitors of myristoyl-CoA:protein N-myristoyltransferase

1. Two non-hydrolysable analogues of myristoyl-coenzyme A were synthesised and spectroscopically characterized. Myristoyl-carba(dethia)coenzyme A was prepared in a multistep synthesis starting from tridecyl vinyl ketone. S-(3-Oxohexadecyl)-coenzyme A was synthesised from 3-oxohexadecyl chloride by direct condensation with coenzyme A. 2. Both analogues were strong competitive inhibitors of N-myristoyltransferase from yeast. Ki values of 0.3 and 0.25 microM were determined for myristoyl-carba(dethia)-coenzyme A and S-(3-oxohexadecyl)-coenzyme A, respectively.

Thiol-fatty acylation of the glucose transport protein of human erythrocytes

Incubation of intact human erythrocytes with [3H]palmitate labeled a protein with electrophoretic characteristics of the glucose transporter. This labeling occurred via a thioester linkage, since it was unaffected by organic solvent extraction, but was substantially removed as the hydroxamate upon treatment with neutral hydroxylamine. Immunoprecipitation of the labeled protein with a monoclonal antibody to the glucose transporter confirmed its identity.

Asymmetric distribution of dystrophin in developing and adult Torpedo marmorata electrocyte: evidence for its association with the acetylcholine receptor-rich membrane

Dystrophin has been shown to occur in Torpedo electrocyte [Chang, H. W., Bock, E. & Bonilla, E. (1989) J. Biol. Chem. 264, 20831-20834], a highly polarized syncytium that is embryologically derived from skeletal muscle and displays functionally distinct plasma membrane domains on its innervated and noninnervated faces. In the present study, we investigated the subcellular distribution of dystrophin in the adult electrocyte from Torpedo marmorata and the evolution of its distribution during embryogenesis. Immunofluorescence experiments performed on adult electrocytes with a polyclonal antibody directed against chicken dystrophin revealed that dystrophin immunoreactivity codistributed exclusively with the acetylcholine receptor along the innervated membrane. At the ultrastructural level, dystrophin immunoreactivity appears confined to the face of the subsynaptic membrane exposed to the cytoplasm. In developing electrocytes (45-mm embryo), dystrophin is already detectable at the acetylcholine receptor-rich ventral pole of the cells before the entry of the electromotor axons. Furthermore, we show that dystrophin represents a major component of purified membrane fractions rich in acetylcholine receptor. A putative role of dystrophin in the organization and stabilization of the subsynaptic membrane domain of the electrocyte is discussed.

A 39-kD plasma membrane protein (IP39) is an anchor for the unusual membrane skeleton of Euglena gracilis

The major integral plasma membrane protein (IP39) of Euglena gracilis was radiolabeled, peptide mapped, and dissected with proteases to identify cytoplasmic domains that bind and anchor proteins of the cell surface. When plasma membranes were radioiodinated and extracted with octyl glucoside, 98% of the extracted label was found in IP39 or the 68- and 110-kD oligomers of IP39. The octyl glucoside extracts were incubated with unlabeled cell surface proteins immobilized on nitrocellulose (overlays). Radiolabel from the membrane extract bound one (80 kD) of the two (80 and 86 kD) major membrane skeletal protein bands. Resolubilization of the bound label yielded a radiolabeled polypeptide identical in Mr to IP39. Intact plasma membranes were also digested with papain before or after radioiodination, thereby producing a cytoplasmically truncated IP39. The octyl glucoside extract of truncated IP39 no longer bound to the 80-kD membrane skeletal protein in the nitrocellulose overlays. EM of intact or trypsin digested plasma membranes incubated with membrane skeletal proteins under stringent conditions similar to those used in the nitrocellulose overlays revealed a partially reformed membrane skeletal layer. Little evidence of a membrane skeletal layer was found, however, when plasma membranes were predigested with papain before reassociation. A candidate 80-kD binding domain of IP39 has been tentatively identified as a peptide fragment that was present after trypsin digestion of plasma membranes, but was absent after papain digestion in two-dimensional peptide maps of IP39. Together, these data suggest that the unique peripheral membrane skeleton of Euglena binds to the plasma membrane through noncovalent interactions between the major 80-kD membrane skeletal protein and a small, papain sensitive cytoplasmic domain of IP39. Other (62, 51, and 25 kD) quantitatively minor peripheral proteins also interact with IP39 on the nitrocellulose overlays, and the possible significance of this binding is discussed.

Fatty acid acylation of membrane skeletal proteins in human erythrocytes

Fatty acid acylation of membrane proteins was studied on human erythrocytes by measuring incorporation of [3H]palmitate at different specific radioactivities. A 55 kDa polypeptide within the band 4.5 region was the main acceptor protein for acylation by fatty acids (palmitate, stearate, oleate), while other polypeptides (80, 65, 48, 30 kDa) incorporated [3H]palmitate slowly, in substoichiometric amounts. Integral membrane proteins were preferentially fatty acid acylated. Skeletal membrane proteins were, however, poorly labeled. Neither purified ankyrin nor band 4.1 protein were fatty acid acylated in human erythrocytes. On the other hand, label associated with high molecular weight skeletal proteins resisted low and high ionic strength extractions, and was extracted selectively by urea [corrected] along with a small subpopulation of spectrin which was also tightly associated with the membrane.

Partial deduced sequence of the 110-kD-calmodulin complex of the avian intestinal microvillus shows that this mechanoenzyme is a member of the myosin I family

The actin bundle within each microvillus of the intestinal brush border is laterally tethered to the membrane by bridges composed of the protein complex, 110-kD-calmodulin. Previous studies have shown that avian 110-kD-calmodulin shares many properties with myosins including mechanochemical activity. In the present study, a cDNA molecule encoding 1,000 amino acids of the 110-kD protein has been sequenced, providing direct evidence that this protein is a vertebrate homologue of the tail-less, single-headed myosin I first described in amoeboid cells. The primary structure of the 110-kD protein (or brush border myosin I heavy chain) consists of two domains, an amino-terminal "head" domain and a 35-kD carboxy-terminal "tail" domain. The head domain is homologous to the S1 domain of other known myosins, with highest homology observed between that of Acanthamoeba myosin IB and the S1 domain of the protein encoded by bovine myosin I heavy chain gene (MIHC; Hoshimaru, M., and S. Nakanishi. 1987. J. Biol. Chem. 262:14625-14632). The carboxy-terminal domain shows no significant homology with any other known myosins except that of the bovine MIHC. This demonstrates that the bovine MIHC gene most probably encodes the heavy chain of bovine brush border myosin I (BBMI). A bacterially expressed fusion protein encoded by the brush border 110-kD cDNA binds calmodulin. Proteolytic removal of the carboxy-terminal domain of the fusion protein results in loss of calmodulin binding activity, a result consistent with previous studies on the domain structure of the 110-kD protein. No hydrophobic sequence is present in the molecule indicating that chicken BBMI heavy chain is probably not an integral membrane protein. Northern blot analysis of various chicken tissue indicates that BBMI heavy chain is preferentially expressed in the intestine.

[Is there the possibility of transmission of AIDS by blood-sucking insects?]

The question whether the possibility exists of transmission of HIV by hematophagous insects from infected to uninfected persons is a point of very intensive discussion. The solution of this problem could help to explain the spreading of the disease in human populations and could contribute to an understanding of the evolution of AIDS and the possible transfer from wild primates into human populations. The classical routes of pathogen transmission by blood-sucking arthropods are either "mechanical" or "biological". Both ways are rejected, the latter since no replication of the retro-virus in the vector exists and its survival in the arthropod is very limited. Based on long experimental experience with biting flies as well as with plant-sucking insects a third hitherto neglected way of transmission by regurgitation of gut content can be introduced. Since regurgitation is neither "mechanical" nor "biological", "regurgitative transmission" must be introduced as an additional term.

Acylation of viral and eukaryotic proteins

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A membrane-cytoskeletal complex containing Na+,K+-ATPase, ankyrin, and fodrin in Madin-Darby canine kidney (MDCK) cells: implications for the biogenesis of epithelial cell polarity

In polarized Madin-Darby canine kidney (MDCK) epithelial cells, ankyrin, and the alpha- and beta-subunits of fodrin are components of the basolateral membrane-cytoskeleton and are colocalized with the Na+,K+-ATPase, a marker protein of the basolateral plasma membrane. Recently, we showed with purified proteins that the Na+,K+-ATPase is competent to bind ankyrin with high affinity and specificity (Nelson, W. J., and P. J. Veshnock. 1987. Nature (Lond.). 328:533-536). In the present study we have sought biochemical evidence for interactions between these proteins in MDCK cells. Proteins were solubilized from MDCK cells with an isotonic buffer containing Triton X-100 and fractionated rapidly in sucrose density gradients. Complexes of cosedimenting proteins were detected by analysis of sucrose gradient fractions in nondenaturing polyacrylamide gels. The results showed that ankyrin and fodrin cosedimented in sucrose gradient. Analysis of the proteins from the sucrose gradient in nondenaturing polyacrylamide gels revealed two distinct ankyrin:fodrin complexes that differed in their relative electrophoretic mobilities; both complexes had electrophoretic mobilities slower than that of purified spectrin heterotetramers. Parallel analysis of the distribution of solubilized Na+,K+-ATPase in sucrose gradients showed that there was a significant overlap with the distribution of ankyrin and fodrin. Analysis by nondenaturing polyacrylamide gel electrophoresis showed that the alpha- and beta-subunits of the Na+,K+-ATPase colocalized with the slower migrating of the two ankyrin:fodrin complexes. The faster migrating ankyrin:fodrin complex did not contain Na+,K+-ATPase. These results indicate strongly that the Na+,K+-ATPase, ankyrin, and fodrin are coextracted from whole MDCK cells as a protein complex. We suggest that the solubilized complex containing these proteins reflects the interaction of the Na+,K+-ATPase, ankyrin, and fodrin in the cell. This interaction may play an important role in the spatial organization of the Na+,K+-ATPase to the basolateral plasma membrane in polarized epithelial cells.

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.

Impaired echinocytic transformation of ankyrin- and spectrin-deficient erythrocytes in mice

The membrane skeleton of the red blood cell plays an important role in the determination of cell deformability and cell shape. Under various in vitro conditions, red blood cells undergo an echinocytic or stomatocytic shape transformation. The mechanism of this fundamental process is not well understood. We have studied the red cell shape transformation in normoblastic anemia mice (nb/nb) and spherocytic anemia mice (sph/sph), which are deficient in ankyrin and spectrin, respectively. We found that both ankyrin-deficient cells (nb/nb) and spectrin-deficient cells (sph/sph) have a reduced capacity to undergo echinocytic transformation with various echinocytogenic treatments, that is, incubation with sodium salicylate (40 and 120 mM), calcium loading (50 microM A23187 + 2.2 mM Ca2+), or metabolic depletion (24 hr at 37 degrees C). These results suggest that the functional integrity of the membrane skeleton is essential for the maintenance and transformation of the red cell shape.

Critical temperatures for the interaction of free fatty acids with the erythrocyte membrane

Non-esterified long-chain fatty acids reduce the extent of hypotonic hemolysis at a certain low concentration range but cause hemolysis at higher concentrations. This biphasic behavior was investigated at different temperatures (0-37 degrees C) for lauric (12:0), myristic (14:0), palmitoleic (16:1), oleic (cis-18:1) and elaidic (trans-18:1) acids. The results are summarized as follows: (A) the fatty acids examined exhibit a high degree of specificity in their thermotropic behavior; (B) oleic acid protects against hypotonic hemolysis even at the highest concentrations, up to 15 degrees C, when it becomes hemolytic, but only in a limited concentration range; (C) elaidic acid does not affect the osmotic stability of erythrocytes up to 20 degrees C, when it starts protecting: above 30 degrees C, it becomes hemolytic at the highest concentrations; (D) palmitoleic acid is an excellent protecting agent at all temperatures in a certain concentration range, becoming hemolytic at higher concentrations; (E) lauric acid protects up to 30 degrees C and becomes hemolytic only above this temperature; (F) myristic acid exhibits an extremely unusual behavior at 30 and 37 degrees C by having alternating concentration ranges of protecting and hemolytic effects; (G) there is a common critical temperature for hemolysis at 30 degrees C for saturated and trans-unsaturated fatty acids; (H) the initial slope of Arrhenius plots of percent hemolysis at the concentration of maximum protection is negative for cis-unsaturated fatty acids and positive for saturated and trans-unsaturated fatty acids.

Characterization of [3H]palmitate- and [3H]ethanolamine-labelled proteins in the multicellular parasitic trematode Schistosoma mansoni

Biosynthetic labelling experiments with cercariae and schistosomula of the multicellular parasitic trematode Schistosoma mansoni were performed to determine whether [3H]palmitate or [3H]ethanolamine was incorporated into proteins. Parasites incorporated [3H]palmitate into numerous proteins, as judged by SDS/polyacrylamide-gel electrophoresis and fluorography. The radiolabel was resistant to extraction with chloroform, but sensitive to alkaline hydrolysis, indicating the presence of an ester bond. Further investigation of the major 22 kDa [3H]palmitate-labelled species showed that the label could be recovered in a Pronase fragment which bound detergent and had an apparent molecular mass of 1200 Da as determined by gel filtration on Sephadex LH-20. Schistosomula incubated with [3H]ethanolamine for up to 24 h incorporated this precursor into several proteins; labelled Pronase fragments recovered from the three most intensely labelled proteins were hydrophilic and had a molecular mass of approx. 200 Da. Furthermore, reductive methylation of such fragments showed that the [3H]ethanolamine bears a free amino group, indicating the lack of an amide linkage. We also evaluated the effect of phosphatidylinositol-specific phospholipase C from Staphylococcus aureus: [3H]palmitate-labelled proteins of schistosomula and surface-iodinated proteins were resistant to hydrolysis with this enzyme. In conclusion, [3H]palmitate and [3H]ethanolamine are incorporated into distinct proteins of cercariae and schistosomula which do not bear glycophospholipid anchors. The [3H]ethanolamine-labelled proteins represent a novel variety of protein modification.

Acetylcholine receptor-associated 43K protein contains covalently bound myristate

Torpedo electroplaque and vertebrate neuromuscular junctions contain high levels of a nonactin, 43,000-Mr peripheral membrane protein referred to as the 43K protein. 43K protein is associated with the cytoplasmic face of postsynaptic membranes at areas of high acetylcholine receptor density and has been implicated in the establishment and/or maintenance of these receptor clusters. Cloning of cDNAs encoding Torpedo 43K protein revealed that its amino terminus contains a consensus sequence sufficient for the covalent attachment of the rare fatty acid myristate. To examine whether 43K protein is, in fact, myristoylated, mouse muscle BC3H1 cells were metabolically labeled with either [35S]cysteine or [3H]myristate and immunoprecipitated with a monospecific antiserum raised against isolated Torpedo 43K protein. In cells incubated with either precursor, a single labeled species was specifically recovered that comigrated on SDS-PAGE with 43K protein purified from Torpedo electric organ. Approximately 95% of the 3H labeled material released from [3H]myristate-43K protein by acid methanolysis was extractable in organic solvents and eluted from a C18 reverse-phase HPLC column exclusively at the position of the methyl myristate internal standard. Thus, 43K protein contains authentic myristic acid rather than an amino or fatty acid metabolite of [3H]myristate. Myristate appears to be added to 43K protein cotranslationally and cannot be released from it by prolonged incubation in SDS, 2-mercaptoethanol, or hydroxylamine (pH 7.0 or 10.0), characteristics consistent with amino terminal myristoylation. Covalently linked myristate may be responsible for the high affinity of purified 43K protein for lipid bilayers despite the absence of a notably hydrophobic amino acid sequence.

Ankyrin and spectrin associate with voltage-dependent sodium channels in brain

The segregation of voltage-dependent sodium channels to specialized regions of the neuron is crucial for propagation of an action potential. Studies of their lateral mobility indicate that sodium channels are freely mobile on the neuronal cell body but are immobile at the axon hillock, presynaptic terminal and at focal points along the axon. To elucidate the mechanisms that regulate sodium channel topography and mobility, we searched for specific proteins from the brain that associate with sodium channels. Here we show that sodium channels labelled with 3H-saxitoxin (STX) are precipitated in the presence of exogenous brain ankyrin by anti-ankyrin antibodies and that 125I-labelled ankyrin binds with high affinity to sodium channels reconstituted into lipid vesicles. The cytoplasmic domain of the erythrocyte anion transporter competes for the latter interaction. Neither the neuronal GABA (gamma-aminobutyric acid) receptor channel complex nor the dihydropyridine (DHP) receptor bind brain ankyrin. The results indicate that brain ankyrin links the voltage-dependent sodium channel to the underlying cytoskeleton and may help to maintain axolemmal membrane heterogeneity and control sodium channel mobility.