Specific 33-residue repeat(s) of erythrocyte ankyrin associate with the anion exchanger

Trichomonas vaginalis: Monolayer and Cluster Formation-Ultrastructural Aspects Using High-Resolution Scanning Electron Microscopy

Trichomonas vaginalis is an extracellular protozoan parasite that causes human trichomoniasis, a sexually transmitted infection (STI) that affects approximately 270 million people worldwide. The phenomenon of T. vaginalis adhesion to inert substrates has been described in several reports. Still, very few studies on cluster formation have been conducted, and more detailed analyses of the contact regions between the parasites' membranes in these aggregate formations have not been carried out. The present study aims to show that T. vaginalis forms a tight monolayer, similar to an epithelium, with parasites firmly adhered to the culture flask bottom by interdigitations and in the absence of host cells. In addition, we analyzed and compared the formation of the clusters, focusing on parasite aggregates that float in the culture flasks. We employed various imaging techniques, including high-resolution scanning electron microscopy, transmission electron microscopy, cytochemistry, TEM tomography, and dye injection. We analyzed whether the monolayer behaves as an epithelium, analyzing cell junctions, cell communication, and ultrastructural aspects, and concluded that monolayer formation differs from cluster formation in many aspects. The monolayers form strong adhesion, whereas the clusters have fragile attachments. We did not find fusion or the passage of molecules between neighbor-attached cells; there is no need for different strains to form filopodia, cytonemes, and extracellular vesicles during cluster and monolayer formation.

Giant ankyrin-B suppresses stochastic collateral axon branching through direct interaction with microtubules

Giant ankyrin-B (gAnkB) is a 440-kD neurospecific ankyrin-B isoform and a high-confidence target for autism mutations. gAnkB suppresses axon branching through coordination of cortical microtubules, and autism-related mutation of gAnkB results in ectopic neuronal connectivity. We identified a bipartite motif from gAnkB, which bundles and avidly binds to microtubules in vitro. This motif is composed of a module of 15 tandem repeats followed by a short, conserved fragment also found in giant ankyrin-G (BG-box). Combination of these two parts synergistically increases microtubule-binding avidity. Transfection of astrocytes (which lack gAnkB) with WT gAnkB resulted in prominent bundling of microtubules, which did not occur with mutant gAnkB with impaired microtubule-binding activity. Similarly, rescue of gAnkB-deficient neurons with WT gAnkB suppressed axonal branching and invasion of EB3-tagged microtubules into filopodia, which did not occur with the same mutant gAnkB. Together, these findings demonstrate that gAnkB suppresses axon collateral branching and prevents microtubule invasion of nascent axon branches through direct interaction with microtubules.

Comparison of pathological clotting using haematological, functional and morphological investigations in HIV-positive and HIV-negative patients with deep vein thrombosis

BACKGROUND

Patients infected with the human immunodeficiency virus (HIV) are more prone to systemic inflammation and pathological clotting, and many may develop deep vein thrombosis (DVT) as a result of this dysregulated inflammatory profile. Coagulation tests are not routinely performed unless there is a specific reason.

METHODS

We recruited ten healthy control subjects, 35 HIV negative patients with deep vein thrombosis (HIV negative-DVT), and 13 HIV patients with DVT (HIV positive-DVT) on the primary antiretroviral therapy (ARV) regimen-emtricitabine, tenofovir and efavirenz. Serum inflammatory markers, haematological results, viscoelastic properties using thromboelastography (TEG) and scanning electron microscopy (SEM) of whole blood (WB) were used to compare the groups.

RESULTS

The DVT patients (HIV positive and HIV negative) had raised inflammatory markers. The HIV positive-DVT group had anaemia in keeping with anaemia of chronic disorders. DVT patients had a hypercoagulable profile on the TEG but no significant difference between HIV negative-DVT and HIV positive-DVT groups. The TEG analysis compared well and supported our ultrastructural results. Scanning electron microscopy of DVT patient's red blood cells (RBCs) and platelets demonstrated inflammatory changes including abnormal cell shapes, irregular membranes and microparticle formation. All the ultrastructural changes were more prominent in the HIV positive-DVT patients.

CONCLUSIONS

Although there were trends that HIV-positive patients were more hypercoagulable on functional tests (viscoelastic profile) compared to HIV-negative patients, there were no significant differences between the 2 groups. The sample size was, however, small in number. Morphologically there were inflammatory changes in patients with DVT. These ultrastructural changes, specifically with regard to platelets, appear more pronounced in HIV-positive patients which may contribute to increased risk for hypercoagulability and deep vein thrombosis.

Modular Diversity of the BLUF Proteins and Their Potential for the Development of Diverse Optogenetic Tools

Organisms can respond to varying light conditions using a wide range of sensory photoreceptors. These photoreceptors can be standalone proteins or represent a module in multidomain proteins, where one or more modules sense light as an input signal which is converted into an output response via structural rearrangements in these receptors. The output signals are utilized downstream by effector proteins or multiprotein clusters to modulate their activity, which could further affect specific interactions, gene regulation or enzymatic catalysis. The blue-light using flavin (BLUF) photosensory module is an autonomous unit that is naturally distributed among functionally distinct proteins. In this study, we identified 34 BLUF photoreceptors of prokaryotic and eukaryotic origin from available bioinformatics sequence databases. Interestingly, our analysis shows diverse BLUF-effector arrangements with a functional association that was previously unknown or thought to be rare among the BLUF class of sensory proteins, such as endonucleases, tet repressor family (tetR), regulators of G-protein signaling, GAL4 transcription family and several other previously unidentified effectors, such as RhoGEF, Phosphatidyl-Ethanolamine Binding protein (PBP), ankyrin and leucine-rich repeats. Interaction studies and the indexing of BLUF domains further show the diversity of BLUF-effector combinations. These diverse modular architectures highlight how the organism’s behaviour, cellular processes, and distinct cellular outputs are regulated by integrating BLUF sensing modules in combination with a plethora of diverse signatures. Our analysis highlights the modular diversity of BLUF containing proteins and opens the possibility of creating a rational design of novel functional chimeras using a BLUF architecture with relevant cellular effectors. Thus, the BLUF domain could be a potential candidate for the development of powerful novel optogenetic tools for its application in modulating diverse cell signaling.

Erythrocyte-platelet interaction in uncomplicated pregnancy

Maternal and fetal requirements during uncomplicated pregnancy are associated with changes in the hematopoietic system. Platelets and erythrocytes [red blood cells (RBCs)], and especially their membranes, are involved in coagulation, and their interactions may provide reasons for the changed hematopoietic system during uncomplicated pregnancy. We review literature regarding RBC and platelet membrane structure and interactions during hypercoagulability and hormonal changes. We then study interactions between RBCs and platelets in uncomplicated pregnancy, as their interactions may be one of the reasons for increased hypercoagulability during uncomplicated pregnancy. Scanning electron microscopy was used to study whole blood smears from 90 pregnant females in different phases of pregnancy. Pregnancy-specific interaction was seen between RBCs and platelets. Typically, one or more platelets interacted through platelet spreading and pseudopodia formation with a single RBC. However, multiple interactions with RBCs were also shown for a single platelet. Specific RBC-platelet interaction seen during uncomplicated pregnancy may be caused by increased estrogen and/or increased fibrinogen concentrations. This interaction may contribute to the hypercoagulable state associated with healthy and uncomplicated pregnancy and may also play a fundamental role in gestational thrombocytopenia.

Identification of contact sites between ankyrin and band 3 in the human erythrocyte membrane

The red cell membrane is stabilized by a spectrin/actin-based cortical cytoskeleton connected to the phospholipid bilayer via multiple protein bridges. By virtue of its interaction with ankyrin and adducin, the anion transporter, band 3 (AE1), contributes prominently to these bridges. In a previous study, we demonstrated that an exposed loop comprising residues 175-185 of the cytoplasmic domain of band 3 (cdB3) constitutes a critical docking site for ankyrin on band 3. In this paper, we demonstrate that an adjacent loop, comprising residues 63-73 of cdB3, is also essential for ankyrin binding. Data that support this hypothesis include the following. (1) Deletion or mutation of residues within the latter loop abrogates ankyrin binding without affecting cdB3 structure or its other functions. (2) Association of cdB3 with ankyrin is inhibited by competition with the loop peptide. (3) Resealing of the loop peptide into erythrocyte ghosts alters membrane morphology and stability. To characterize cdB3-ankyrin interaction further, we identified their interfacial contact sites using molecular docking software and the crystal structures of D(3)D(4)-ankyrin and cdB3. The best fit for the interaction reveals multiple salt bridges and hydrophobic contacts between the two proteins. The most important ion pair interactions are (i) cdB3 K69-ankyrin E645, (ii) cdB3 E72-ankyrin K611, and (iii) cdB3 D183-ankyrin N601 and Q634. Mutation of these four residues on ankyrin yielded an ankyrin with a native CD spectrum but little or no affinity for cdB3. These data define the docking interface between cdB3 and ankyrin in greater detail.

Novel FixL homologues in Chlamydomonas reinhardtii bind heme and O(2)

Genome inspection revealed nine putative heme-binding, FixL-homologous proteins in Chlamydomonas reinhardtii. The heme-binding domains from two of these proteins, FXL1 and FXL5 were cloned, expressed in Escherichia coli, purified and characterized. The recombinant FXL1 and FXL5 domains stained positively for heme, while mutations in the putative ligand-binding histidine FXL1-H200S and FXL5-H200S resulted in loss of heme binding. The FXL1 and FXL5 [Fe(II), bound O(2)] had Soret absorption maxima around 415 nm, and weaker absorptions at longer wavelengths, in concurrence with the literature. Ligand-binding measurements showed that FXL1 and FXL5 bind O(2) with moderate affinity, 135 and 222 μM, respectively. This suggests that Chlamydomonas may use the FXL proteins in O(2)-sensing mechanisms analogous to that reported in nitrogen-fixing bacteria to regulate gene expression.

Novel structural components of the ventral disc and lateral crest in Giardia intestinalis

Giardia intestinalis is a ubiquitous parasitic protist that is the causative agent of giardiasis, one of the most common protozoan diarrheal diseases in the world. Giardia trophozoites attach to the intestinal epithelium using a specialized and elaborate microtubule structure, the ventral disc. Surrounding the ventral disc is a less characterized putatively contractile structure, the lateral crest, which forms a continuous perimeter seal with the substrate. A better understanding of ventral disc and lateral crest structure, conformational dynamics, and biogenesis is critical for understanding the mechanism of giardial attachment to the host. To determine the components comprising the ventral disc and lateral crest, we used shotgun proteomics to identify proteins in a preparation of isolated ventral discs. Candidate disc-associated proteins, or DAPs, were GFP-tagged using a ligation-independent high-throughput cloning method. Based on disc localization, we identified eighteen novel DAPs, which more than doubles the number of known disc-associated proteins. Ten of the novel DAPs are associated with the lateral crest or outer edge of the disc, and are the first confirmed components of this structure. Using Fluorescence Recovery After Photobleaching (FRAP) with representative novel DAP::GFP strains we found that the newly identified DAPs tested did not recover after photobleaching and are therefore structural components of the ventral disc or lateral crest. Functional analyses of the novel DAPs will be central toward understanding the mechanism of ventral disc-mediated attachment and the mechanism of disc biogenesis during cell division. Since attachment of Giardia to the intestine via the ventral disc is essential for pathogenesis, it is possible that some proteins comprising the disc could be potential drug targets if their loss or disruption interfered with disc biogenesis or function, preventing attachment.

Determination of structural models of the complex between the cytoplasmic domain of erythrocyte band 3 and ankyrin-R repeats 13-24

The adaptor protein ankyrin-R interacts via its membrane binding domain with the cytoplasmic domain of the anion exchange protein (AE1) and via its spectrin binding domain with the spectrin-based membrane skeleton in human erythrocytes. This set of interactions provides a bridge between the lipid bilayer and the membrane skeleton, thereby stabilizing the membrane. Crystal structures for the dimeric cytoplasmic domain of AE1 (cdb3) and for a 12-ankyrin repeat segment (repeats 13-24) from the membrane binding domain of ankyrin-R (AnkD34) have been reported. However, structural data on how these proteins assemble to form a stable complex have not been reported. In the current studies, site-directed spin labeling, in combination with electron paramagnetic resonance (EPR) and double electron-electron resonance, has been utilized to map the binding interfaces of the two proteins in the complex and to obtain inter-protein distance constraints. These data have been utilized to construct a family of structural models that are consistent with the full range of experimental data. These models indicate that an extensive area on the peripheral domain of cdb3 binds to ankyrin repeats 18-20 on the top loop surface of AnkD34 primarily through hydrophobic interactions. This is a previously uncharacterized surface for binding of cdb3 to AnkD34. Because a second dimer of cdb3 is known to bind to ankyrin repeats 7-12 of the membrane binding domain of ankyrin-R, the current models have significant implications regarding the structural nature of a tetrameric form of AE1 that is hypothesized to be involved in binding to full-length ankyrin-R in the erythrocyte membrane.

Characterization of band 3-ankyrin-Protein 4.2 complex by biochemical and mass spectrometry approaches

The elastic property of red blood cell is supported by interaction between red cell membrane and the intricate cytoskeleton network underlying the membrane bilayer cytoplasmic face. One of the major scaffold protein linkers is band 3-ankyrin complex. Defects occurring in this complex have been found in many inherited diseases, causing red blood cell abnormalities. Here we combined the power of mass spectrometry with conventional biochemical purification methods in order to study the native interactions among band 3, ankyrin and Protein 4.2. This approach provided in vivo evidence for the association between band 3 and N-terminal ankyrin purified directly from the cell membrane. The C-terminal regions of ankyrin were not found to be a stable partner of the band 3 complex. Protein 4.2 was shown here to be an integral part of the complex. Its association to the band 3-ankyrin complex could withstand harsh purification conditions. Our findings lend additional support to the interaction between band 3 and ankyrin N-terminal domain previously shown by in vitro binding assays and provide evidence for a band 3 core complex comprising of band 3, ankyrin and Protein 4.2.

Ankyrin-B interactions with spectrin and dynactin-4 are required for dystrophin-based protection of skeletal muscle from exercise injury

Costameres are cellular sites of mechanotransduction in heart and skeletal muscle where dystrophin and its membrane-spanning partner dystroglycan distribute intracellular contractile forces into the surrounding extracellular matrix. Resolution of a functional costamere interactome is still limited but likely to be critical for understanding forms of muscular dystrophy and cardiomyopathy. Dystrophin binds a set of membrane-associated proteins (the dystrophin-glycoprotein complex) as well as γ-actin and microtubules and also is required to align sarcolemmal microtubules with costameres. Ankyrin-B binds to dystrophin, dynactin-4, and microtubules and is required for sarcolemmal association of these proteins as well as dystroglycan. We report here that ankyrin-B interactions with β2 spectrin and dynactin-4 are required for localization of dystrophin, dystroglycan, and microtubules at costameres as well as protection of muscle from exercise-induced injury. Knockdown of dynactin-4 in adult mouse skeletal muscle phenocopied depletion of ankyrin-B and resulted in loss of sarcolemmal dystrophin, dystroglycan, and microtubules. Moreover, mutations of ankyrin-B and of dynactin-4 that selectively impaired binary interactions between these proteins resulted in loss of their costamere-localizing activity and increased muscle fiber fragility as a result of loss of costamere-associated dystrophin and dystroglycan. In addition, costamere-association of dynactin-4 did not require dystrophin but did depend on β2 spectrin and ankyrin-B, whereas costamere association of ankyrin-B required β2 spectrin. Together, these results are consistent with a functional hierarchy beginning with β2 spectrin recruitment of ankyrin-B to costameres. Ankyrin-B then interacts with dynactin-4 and dystrophin, whereas dynactin-4 collaborates with dystrophin in coordinating costamere-aligned microtubules.

Ankyrin protein networks in membrane formation and stabilization

In eukaryotic cells, ankyrins serve as adaptor proteins that link membrane proteins to the underlying cytoskeleton. These adaptor proteins form protein complexes consisting of integral membrane proteins, signalling molecules and cytoskeletal components. With their modular architecture and ability to interact with many proteins, ankyrins organize and stabilize these protein networks, thereby establishing the infrastructure of membrane domains with specialized functions. To this end, ankyrin collaborates with a number of proteins including cytoskeletal proteins, cell adhesion molecules and large structural proteins. This review addresses the targeting and stabilization of protein networks related to ankyrin interactions with the cytoskeletal protein β-spectrin, L1-cell adhesion molecules and the large myofibrillar protein obscurin. The significance of these interactions for differential targeting of cardiac proteins and neuronal membrane formation is also presented. Finally, this review concludes with a discussion about ankyrin dysfunction in human diseases such as haemolytic anaemia, cardiac arrhythmia and neurological disorders.

The 22.5 kDa spectrin-binding domain of ankyrinR binds spectrin with high affinity and changes the spectrin distribution in cells in vivo

It was previously shown that ankyrins play a crucial role in the membrane skeleton arrangement. Purifying ankyrinR obtained from erythrocytes is a time-consuming process. Therefore, cloned and bacterially expressed ankyrinR-spectrin-binding domain (AnkSBD) is a demanded tool for studying spectrin-ankyrin interactions. In this communication, we report on the cloning and purification of AnkSBD and describe the results of binding experiments, in which we showed high-affinity interactions between the AnkSBD construct and isolated erythrocyte or non-erythroid spectrins. pEGFP-AnkSBD-transfected cells co-localised with non-erythroid spectrin in HeLa cells. The functional interactions of the AnkSBD construct in vivo and in vitro open many possibilities to study the structure and function of this domain, which has not yet been as extensively studied when compared to the aminoterminal domain of this protein.

Protein 4.2 Komatsu (D175Y) associated with the lack of interaction with ankyrin in human red blood cells

Membrane skeletal proteins play an important role in regulating the shape and function of the human red blood cell. Protein 4.2 interacts with cytoplasmic domain of band 3 (CDB3) and ankyrin for association between the skeleton network and the membrane. The deficiency of protein 4.2 may result in hereditary spherocytosis. In order to explore the molecular mechanism of the linkage of protein 4.2 Komatsu (D175Y) and protein 4.2 Nippon (A142T) with hereditary spherocytosis, a series of protein 4.2-derived mutants were designed and expressed in Escherichia coli. Their interactions with ankyrin and CDB3 were investigated by Far Western blot and pull-down assay in vitro. The results showed that the mutant D175Y of protein 4.2 cannot interact with ankyrin while mutant A142T, just like normal protein 4.2, can bind to ankyrin directly and can associate with CDB3 in the presence of ankyrin. Based on comparing the binding abilities of the protein 4.2 mutants D175F, D175A, D175K and D175Y with ankyrin and CDB3, we suggested that defective binding of protein 4.2 Komatsu to ankyrin is resulted from the charge effect of amino acid residue 175 substitution (D-->Y), which leads to significant structural change in protein 4.2 function domain.

Associations of protein 4.2 with band 3 and ankyrin

Protein-protein and protein-lipid interactions are thought to play the vital role in maintenance and deformation of red blood cell (RBC) membrane. Protein 4.2, a 76-KDa peripheral protein, binds to the cytoplasmic domain of band 3 (CDB3) and also interacts with ankyrin in RBCs. In order to explore the characteristics of protein 4.2-CDB3-ankyrin interactions, three protein 4.2-derived recombinant proteins encompassing amino acid residues 31-200, 1-300, and 187-260 respectively were expressed in Escherichia coli. Their interactions with CDB3 and ankyrin were investigated by using Far-Western blot and pull-down assay. The results showed that the CDB3-binding site of protein 4.2 is located in the region of residues 200-211 and the ankyrin-binding site is located in the region of residues 187-200 of protein 4.2. Our findings also suggested that the ankyrin D34 domain can interact directly with protein 4.2. The proper tertiary structures of these protein 4.2 fragments are essential for protein 4.2-ankyrin interaction. Meanwhile, ankyrin can enhance the interaction between protein 4.2 and CDB3.

Examination of a novel head-stalk protein family in Giardia lamblia characterised by the pairing of ankyrin repeats and coiled-coil domains

The intestinal pathogen Giardia lamblia possesses several unusual organelle features, including two equivalent nuclei, no mitochondria or peroxisomes, and a developmentally regulated rough endoplasmic reticulum and Golgi. Giardia also possesses a number of complex and unique cytoskeleton structures that dictate cell shape, motility and attachment. Our investigations of cytoskeletal proteins have revealed the presence of a new protein family. Proteins in this family contain both ankyrin repeats and coiled-coil domains; although these are common protein motifs, their pairing is unique, thus establishing a new class of head-stalk proteins. Examination of the G. lamblia genome shows evidence for at least 18 genes coding for proteins with a series of ankyrin repeats followed by a lengthy coiled-coil domain and at least an additional 14 genes coding for proteins with a prominent coiled-coil domain flanked by two series of ankyrin repeats. We have examined one of these proteins, Giardia Axoneme Associated Protein (GASP-180), in detail. GASP-180 is a 180 kDa protein containing five ankyrin repeats in a 200 amino acid N-terminal domain separated by a short spacer from an approximately 1375 amino acid coiled-coil domain. Using anti-peptide antibodies raised against a unique 20 amino acid sequence found at the C-terminus, we have determined that GASP-180 is present in cytoskeleton extractions of the parasite and localises to the proximal base of the anterior flagellar axonemes. The combination of the localisation and the structural and functional motifs of GASP-180 make it a strong candidate to participate in control of flagellar activity.

The ankyrin repeat as molecular architecture for protein recognition

The ankyrin repeat is one of the most frequently observed amino acid motifs in protein databases. This protein-protein interaction module is involved in a diverse set of cellular functions, and consequently, defects in ankyrin repeat proteins have been found in a number of human diseases. Recent biophysical, crystallographic, and NMR studies have been used to measure the stability and define the various topological features of this motif in an effort to understand the structural basis of ankyrin repeat-mediated protein-protein interactions. Characterization of the folding and assembly pathways suggests that ankyrin repeat domains generally undergo a two-state folding transition despite their modular structure. Also, the large number of available sequences has allowed the ankyrin repeat to be used as a template for consensus-based protein design. Such projects have been successful in revealing positions responsible for structure and function in the ankyrin repeat as well as creating a potential universal scaffold for molecular recognition.

The ANK repeat: a ubiquitous motif involved in macromolecular recognition

Many proteins rely on stable, noncovalent interactions with other macromolecules to perform their function. The identification of a repeated sequence motif, the ANK repeat, in diverse proteins whose common function involves binding to other proteins indicates one way nature may achieve a wide range of protein-protein interactions. In this article, we describe evidence that these ANK repeats are involved in the specific recognition of proteins and possibly DNA, and present a model for the folding of the motif.

Carbonic anhydrase II binds to and enhances activity of the Na+/H+ exchanger

We examined the ability of carbonic anhydrase II to bind to and affect the transport efficiency of the NHE1 isoform of the mammalian Na(+)/H(+) exchanger. The C-terminal region of NHE1 was expressed in Escherichia coli fused with an N-terminal glutathionine S-transferase or with a C-terminal polyhistidine tag. Using a microtiter plate binding assay we showed that the C-terminal region of NHE1 binds carbonic anhydrase II (CAII) and binding was stimulated by low pH and blocked by antibodies against the C-terminal of NHE1. The binding to NHE1 was confirmed by demonstrating protein-protein interaction using affinity blotting with CAII and immobilized NHE1 fusion proteins. CAII co-immunoprecipitated with NHE1 from CHO cells suggesting the proteins form a complex in vivo. In cells expressing CAII and NHE1, the H(+) transport rate was almost 2-fold greater than in cells expressing NHE1 alone. The CAII inhibitor acetazolamide significantly decreased the H(+) transport rate of NHE1 and transfection with a dominant negative CAII inhibited NHE1 activity. Phosphorylation of the C-terminal of NHE1 greatly increased the binding of CAII. Our study suggests that NHE1 transport efficiency is influenced by CAII, likely through a direct interaction at the C-terminal region. Regulation of NHE1 activity by phosphorylation could involve modulation of CAII binding.

Spectrin and ankyrin-based pathways: metazoan inventions for integrating cells into tissues

The spectrin-based membrane skeleton of the humble mammalian erythrocyte has provided biologists with a set of interacting proteins with diverse roles in organization and survival of cells in metazoan organisms. This review deals with the molecular physiology of spectrin, ankyrin, which links spectrin to the anion exchanger, and two spectrin-associated proteins that promote spectrin interactions with actin: adducin and protein 4.1. The lack of essential functions for these proteins in generic cells grown in culture and the absence of their genes in the yeast genome have, until recently, limited advances in understanding their roles outside of erythrocytes. However, completion of the genomes of simple metazoans and application of homologous recombination in mice now are providing the first glimpses of the full scope of physiological roles for spectrin, ankyrin, and their associated proteins. These functions now include targeting of ion channels and cell adhesion molecules to specialized compartments within the plasma membrane and endoplasmic reticulum of striated muscle and the nervous system, mechanical stabilization at the tissue level based on transcellular protein assemblies, participation in epithelial morphogenesis, and orientation of mitotic spindles in asymmetric cell divisions. These studies, in addition to stretching the erythrocyte paradigm beyond recognition, also are revealing novel cellular pathways essential for metazoan life. Examples are ankyrin-dependent targeting of proteins to excitable membrane domains in the plasma membrane and the Ca(2+) homeostasis compartment of the endoplasmic reticulum. Exciting questions for the future relate to the molecular basis for these pathways and their roles in a clinical context, either as the basis for disease or more positively as therapeutic targets.

alpha -Catenin binds directly to spectrin and facilitates spectrin-membrane assembly in vivo

The anchorage of spectrin to biological membranes is mediated by protein and phosphoinositol phospholipid interactions. In epithelial cells, a nascent spectrin skeleton assembles in regions of cadherin-mediated cell-cell contact, and conversely, cytoskeletal assembly is required to complete the cell-adhesion process. The molecular interactions guiding these processes remain incompletely understood. We have examined the interaction of spectrin with alpha-catenin, a component of the adhesion complex. Spectrin (alphaIIbetaII) and alpha-catenin coprecipitate from extracts of confluent Madin-Darby canine kidney, HT29, and Clone A cells and from solutions of purified spectrin and alpha-catenin in vitro. By surface plasmon resonance and in vitro binding assays, we find that alpha-catenin binds alphaIIbetaII spectrin with an apparent K(d) of approximately 20-100 nm. By gel-overlay assay, alpha-catenin binds recombinant betaII-spectrin peptides that include the first 313 residues of spectrin but not to peptides that lack this region. Similarly, the binding activity of alpha-catenin is fully accounted for in recombinant peptides encompassing the NH(2)-terminal 228 amino acid region of alpha-catenin. An in vivo role for the interaction of spectrin with alpha-catenin is suggested by the impaired membrane assembly of spectrin and its enhanced detergent solubility in Clone A cells that harbor a defective alpha-catenin. Transfection of these cells with wild-type alpha-catenin reestablishes alpha-catenin at the plasma membrane and coincidentally recruits spectrin to the membrane. We propose that ankyrin-independent interactions of modest affinity between alpha-catenin and the amino-terminal domain of beta-spectrin augment the interaction between alpha-catenin and actin, and together they provide a polyvalent linkage directing the topographic assembly of a nascent spectrin-actin skeleton to membrane regions enriched in E-cadherin.

Ankyrins and cellular targeting of diverse membrane proteins to physiological sites

Ankyrins are spectrin-binding proteins that associate via ANK repeats with a variety of ion channels/pumps, calcium release channels and cell adhesion molecules. Recent studies in mice indicate that ankyrins have a physiological role in restricting voltage-gated sodium channels and members of the L1 CAM family of cell adhesion molecules to excitable membranes in the central nervous system and in targeting calcium-release channels to the calcium homeostasis compartment of striated muscle.

Gene transfer to ankyrin-deficient bone marrow corrects spherocytosis in vitro

OBJECTIVE

The goal of this study was to transfer by retroviral vector the cDNA for ankyrin to progenitors from normal bone marrow and from the nb/nb spherocytosis mutant deficient in expression of full-length ankyrin to achieve erythroid expression of functional ankyrin protein.

MATERIALS AND METHODS

A minigene composed of the human ankyrin promoter, murine ankyrin cDNA, and the 3' human domain corresponding to the ankyrin 2.2 isoform was assembled in the retroviral vector, pG1. Murine erythroleukemia (MEL) cells, normal murine bone marrow cells, 3T3 fibroblasts, and nb/nb mutant bone marrow and spleen cells were transduced with the retroviral supernatant. Transduced mutant cells were induced to differentiate in liquid culture. Gene transfer was assessed by colony polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR, immunofluorescence, and Southern, Northern, and Western blot analysis.

RESULTS

MEL cells, normal bone marrow progenitors, and nb/nb cells were all successfully transduced and expressed ankyrin by RT-PCR and Western blot. Transduced murine 3T3 fibroblasts and MEL cells exhibited cell membrane staining by immunofluorescence. Colony RT-PCR demonstrated dependence of expression on erythropoietin. In vitro, the transduced nb/nb cells matured to polychromatophils, whereas nontransduced nb/nb cells matured to microspherocytes.

CONCLUSION

Retroviral transfer of ankyrin corrected the defect leading to formation of microspherocytes in erythroid differentiation cultures from the nb/nb mutant. The human ankyrin promoter conferred erythropoietin-dependent expression in normal and mutant erythroid progenitors, which could have implications for the gene therapy of human hemolytic anemias.

An alternate promoter directs expression of a truncated, muscle-specific isoform of the human ankyrin 1 gene

Ankyrin 1, an erythrocyte membrane protein that links the underlying cytoskeleton to the plasma membrane, is also expressed in brain and muscle. We cloned a truncated, muscle-specific ankyrin 1 cDNA composed of novel 5' sequences and 3' sequences previously identified in the last 3 exons of the human ankyrin 1 erythroid gene. Northern blot analysis revealed expression restricted to cardiac and skeletal muscle tissues. Deduced amino acid sequence of this muscle cDNA predicted a peptide of 155 amino acids in length with a hydrophobic NH2 terminus. Cloning of the corresponding chromosomal gene revealed that the ankyrin 1 muscle transcript is composed of four exons spread over approximately 10 kilobase pairs of DNA. Reverse transcriptase-polymerase chain reaction of skeletal muscle cDNA identified multiple cDNA isoforms created by alternative splicing. The ankyrin 1 muscle promoter was identified as a (G + C)-rich promoter located > 200 kilobase pairs from the ankyrin 1 erythroid promoter. An ankyrin 1 muscle promoter fragment directed high level expression of a reporter gene in cultured C2C12 muscle cells, but not in HeLa or K562 (erythroid) cells. DNA-protein interactions were identified in vitro at a single Sp1 and two E box consensus binding sites contained within the promoter. A MyoD cDNA expression plasmid transactivated an ankyrin 1 muscle promoter fragment/reporter gene plasmid in a dose-dependent fashion in both HeLa and K562 cells. A polyclonal antibody raised to human ankyrin 1 muscle-specific sequences reacted with peptides of 28 and 30 kDa on immunoblots of human skeletal muscle.

Red cell membranes of ankyrin-deficient nb/nb mice lack band 3 tetramers but contain normal membrane skeletons

The role of ankyrin in the formation and stabilization of the spectrin-based skeletal meshwork and of band 3 oligomers was studied by characterizing, in nb/nb mouse red cells, the effect of ankyrin deficiency on skeletal ultrastructure, band 3-skeleton associations, and band 3 oligomeric states. Despite severe ankyrin deficiency, nb/nb mouse red cell skeletal components formed a relatively uniform two-dimensional hexagonal array of junctional complexes cross-linked by spectrin tetramers. Treatment of nb/nb ghosts with the nonionic detergent C12E8 (octaethylene glycol n-dodecyl monoether) resulted in nearly complete extraction of band 3. The extracted band 3 was present exclusively as band 3 dimers. Fluorescence photobleaching recovery and polarized fluorescence depletion measurements showed increases in the laterally (33% vs 10%) and rotationally (90% vs 76%) mobile fractions of band 3 in intact nb/nb compared to control red cells. The rotational correlation time of the major fraction of band 3 molecules was 10-fold shorter in nb/nb compared to control red cells, indicating a significant relaxation of rotational constraints in nb/nb cells. These data suggest that, although ankyrin plays a major role in strengthening the attachment of the skeleton to the membrane bilayer, ankyrin is not required for the formation of a stable two-dimensional spectrin-based skeleton. The absence of band 3 tetramers in the membrane of ankyrin-deficient red cells suggests that ankyrin is required for the formation of stable band 3 tetramers.

Structure and organization of the human ankyrin-1 gene. Basis for complexity of pre-mRNA processing

Ankyrin-1 (ANK-1) is an erythrocyte membrane protein that is defective in many patients with hereditary spherocytosis, a common hemolytic anemia. In the red cell, ankyrin-1 provides the primary linkage between the membrane skeleton and the plasma membrane. To gain additional insight into the structure and function of this protein and to provide the necessary tools for further genetic studies of hereditary spherocytosis patients, we cloned the human ANK-1 chromosomal gene. Characterization of the ANK-1 gene genomic structure revealed that the erythroid transcript is composed of 42 exons distributed over approximately 160 kilobase pairs of DNA. Comparison of the genomic structure with the protein domains reveals a near-absolute correlation between the tandem repeats encoding the membrane-binding domain of ankyrin with the location of the intron/exon boundaries in the corresponding part of the gene. Erythroid stage-specific, complex patterns of alternative splicing were identified in the region encoding the regulatory domain of ankyrin-1. Novel brain-specific transcripts were also identified in this region, as well as in the "hinge" region between the membrane-binding and spectrin-binding domains. Utilization of alternative polyadenylation signals was found to be the basis for the previously described, stage-specific 9.0- and 7.2-kilobase pair transcripts of the ANK-1 gene.

Cloning and structure of delta-latroinsectotoxin, a novel insect-specific member of the latrotoxin family: functional expression requires C-terminal truncation

The venom of the black widow spider (BWSV) (Latrodectus mactans tredecimguttatus) contains several potent, high molecular mass (>110 kDa) neurotoxins that cause neurotransmitter release in a phylum-specific manner. The molecular mechanism of action of these proteins is poorly understood because their structures are largely unknown, and they have not been functionally expressed. This study reports on the primary structure of delta-latroinsectotoxin (delta-LIT), a novel insect-specific toxin from BWSV, that contains 1214 amino acids. delta-LIT comprises four structural domains: a signal peptide followed by an N-terminal domain that exhibits the highest degree of identity with other latrotoxins, a central region composed of 15 ankyrin-like repeats, and a C-terminal domain. The domain organization of delta-LIT is similar to that of other latrotoxins, suggesting that these toxins are a family of related proteins. The predicted molecular mass and apparent mobility of the protein (approximately 130 kDa) encoded in the delta-LIT gene differs from that of native delta-LIT purified from BWSV (approximately 100 kDa), suggesting that the toxin is produced by proteolytic processing of a precursor. MALDI-MS of purified native delta-LIT revealed a molecular ion with m/z+ of 110916 +/- 100, indicating that the native delta-LIT is 991 amino acids in length. When the full-length delta-LIT cDNA was expressed in bacteria the protein product was inactive, but expression of a C-terminally truncated protein containing 991 residues produced a protein that caused massive neurotransmitter release at the locust neuromuscular junction at nanomolar concentrations. Channels formed in locust muscle membrane and artificial lipid bilayers by the native delta-LIT have a high Ca2+ permeability, whereas those formed by truncated, recombinant protein do not.

Isolation of an ion channel gene from Arabidopsis thaliana using the H5 signature sequence from voltage-dependent K+ channels

A degenerate oligonucleotide corresponding to the K+ channel signature sequence (TMTTVGYGD) was used to isolate the genomic and cDNA forms of a new channel gene, AKT3, from Arabidopsis thaliana. The deduced protein sequence has a predicted membrane topography similar to Shaker-like K+ channels. Three distinct modules comprise the carboxyl-terminal half: a nucleotide-binding motif, an ankyrin repeat domain, and a polyglutamate track. Xenopus oocytes injected with cRNA exhibited an inward-rectifying K+ current, demonstrating that the AKT3 polypeptide is a functional transport protein. Two other Arabidopsis K+ transporters (AKT1 and KAT1) share 60% homology with AKT3; together these proteins constitute a family of plant inward-rectifying K+ channels.

The ANK repeats of erythrocyte ankyrin form two distinct but cooperative binding sites for the erythrocyte anion exchanger

The 24 ANK repeats of the membrane-binding domain of ankyrin form four folded subdomains of six ANK repeats each. These four repeat subdomains mediate interactions with at least seven different families of membrane proteins. In the erythrocyte, the main membrane target of ankyrin is the Cl-/HCO3- anion exchanger. This report presents the first evidence that ankyrin contains two separate binding sites for anion exchanger dimers. One site utilizes repeat subdomain two (repeats 7-12) while the other requires both repeat subdomains three and four (repeats 13-24). The two sites are positively coupled with a Hill coefficient of 1.4. Since the anion exchanger exists as a dimer in the membrane, the presence of two binding sites on ankyrin allows ankyrin to interact with four anion exchangers simultaneously. These findings provide a direct demonstration of the versatility of ANK repeats in protein recognition, and have important implications for the organization of ankyrin-linked integral membrane proteins in erythrocytes as well as other cells.

Functional characterization of black widow spider neurotoxins synthesised in insect cells

alpha-latrotoxin, alpha-latroinsectotoxin and the low-molecular-mass protein from black widow spider venom were synthesised in insect cells using the baculovirus expression system. SDS/PAGE analysis of recombinant-virus-infected cells revealed novel proteins that migrated with sizes similar to those of the neurotoxins from spider venom. The identities of these proteins as alpha-latrotoxin, alpha-latroinsectotoxin or the low-molecular-mass protein were confirmed by immunoblot analysis of infected cells with anti-(alpha-latrotoxin), anti-(alpha-latroinsectotoxin) or anti-(low-molecular-mass protein) IgG. Neither the low-molecular-mass protein nor alpha-latrotoxin were toxic upon injection into Trichoplusia ni larvae or upon virus-derived synthesis directly in the cytoplasm of the target tissue. Analysis of the biological activity of the recombinant virus encoding alpha-latroinsectotoxin, however, revealed a strong toxic effect on the T. ni larvae. These data indicate that the toxic effect of the native insectotoxin may be promoted by the alpha-latroinsectotoxin subunit alone and provides evidence that the mechanism of action of alpha-latroinsectotoxin may be mediated by internalisation of part of the neurotoxin alpha-subunit molecule.

Cloning of a differentially expressed I kappa B-related protein

We have cloned a cDNA corresponding to a novel gene from a human epithelial cell line by subtractive hybridization and polymerase chain reaction techniques. This gene is expressed at the message level and at the protein level in a lung alveolar type II-like epithelial cell line but not in lung fibroblasts. In adult human tissues, the mRNA for this gene was detected only in the heart and the skeletal muscle, but not in the brain, placenta, whole lung, liver, or kidney. We have named this gene I kappa BR (for I kappa B-related) since its 52-kDa protein product has significant homology to the I kappa B family of proteins which function as inhibitory cytoplasmic retention proteins for the vertebrate rel/NF-kappa B transcription factors. Although the important role of NF-kappa B in gene activation in cells of the immune system is now well established, a similar role in other cell types or in vertebrate development is less clear. The deduced amino acid sequence of I kappa BR has the most significant homology to the Drosophila protein Cactus which inhibits the function of the NF-kappa B-like protein Dorsal. In electrophoretic mobility shift experiments, I kappa BR inhibited the ability of the p50:p65 NF-kappa B heterodimer to bind DNA. The DNA binding ability of the p50 homodimer but not the p65 homodimer was drastically inhibited by I kappa BR. In transfection experiments, overexpression of I kappa BR significantly inhibited NF-kappa B-dependent transcription from the Ig kappa enhancer. This new member of the I kappa B family of proteins, I kappa BR, may play an important role in regulation of NF-kappa B function in epithelial cells.

The role of cysteine residues in the erythrocyte plasma membrane anion exchange protein, AE1

AE1 (Band 3), a congruent to 110-kDa integral plasma membrane protein, facilitates the electroneutral movement of Cl- and HCO3- across the erythrocyte membrane and serves as the primary attachment site for the erythrocyte spectrin-actin cytoskeleton. In this investigation, we have characterized the role of native cysteines in the function of AE1. We have constructed a mutant version of human AE1 (AE1C-) in which all five cysteines of AE1 were replaced with serines. Wild-type and AE1C- cDNAs were expressed by transient transfection of human embryonic kidney cells. Two of the mutated cysteines in AE1C- are in a region involved in ankyrin binding, and ankyrin binding has previously been shown to be sensitive to the oxidation state of these cysteines. However, the KD values for ankyrin binding by AE1 and AE1C- were indistinguishable, suggesting that AE1 cysteines are not essential components of the ankyrin-binding site. Using size exclusion chromatography, both AE1 and AE1C- were found to associate as a mixture of dimers and high molecular mass complexes. The rate of anion exchange by AE1C-, as measured in a reconstituted microsome sulfate transport assay, was indistinguishable from that by AE1 and was inhibited by 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate. We conclude that the cysteines of AE1 are not required for the anion exchange or cytoskeletal binding roles of the protein.

Cloning and structural analysis of alpha-latroinsectotoxin cDNA. Abundance of ankyrin-like repeats

alpha-Latroinsectotoxin (alpha-LIT), purified from venom glands of the black widow spider Latrodectus mactans tredecimguttatus, is a presynaptic neurotoxin selective only for insects. A cDNA encoding the putative alpha-LIT precursor was isolated from a spider venom gland cDNA library. The cDNA contains a 4236-base-pair open reading frame corresponding to a 157826-Da protein composed of 1411 amino acids. The mature alpha-LIT, with molecular mass approximately 130 kDa, is probably derived from double processing in the N-terminal and C-terminal regions of the primary translation product. The structure region, extending over residues 464-1176, is composed almost entirely of ankyrin-like repeats which represent a motif also found in the alpha-latrotoxin (alpha-LTX), which has selective action on vertebrates. Total alignment of the alpha-LIT and alpha-LTX amino acid sequences reveals an overall similarity of 34.1%. Strong sequence divergence is observed in analogous cysteine-rich regions situated within the ankyrin-repeat domains of both alpha-LIT and alpha-LTX.

From anemia to cerebellar dysfunction. A review of the ankyrin gene family

The focus of this review is on the ankyrin gene family, key elements in the interaction of the spectrin-based membrane skeleton with the plasma membrane in a variety of tissues and multicellular organisms. The structure/function relationships of ankyrin molecules are reviewed, illustrating how these proteins are uniquely suited to serve as adaptors between the membrane skeleton and a number of integral membrane proteins. Advances in the understanding of ankyrin biology in the brain are discussed and used to show how ankyrins may be involved in the establishment and/or maintenance of specialized plasma membrane domains. Finally, recent research in hematological and neurological disorders are reviewed, suggesting that ankyrins have a role in the development of human disease.

cactus, a gene involved in dorsoventral pattern formation of Drosophila, is related to the I kappa B gene family of vertebrates

Among the maternally active genes of Drosophila, cactus is the only one whose loss of function mutations specifically produce ventralized embryos. Its product inhibits nuclear translocation of the dorsal morphogen in the dorsal region of the embryo. Here we report the cloning of cactus and the sequencing of its maternal transcript. The identity of our clones was verified by induction of phenocopies with antisense RNA and rescue of the mutant phenotype with sense RNA. cactus is predicted to encode an acidic, cytoplasmic protein with seven ankyrin repeats. The sequence has similarity to the I kappa B proteins that inhibit the vertebrate transcription factor NF-kappa B. In analogy to results obtained with I kappa B and NF-kappa B, bacterially expressed cactus protein can inhibit DNA binding of dorsal protein in vitro.

Single amino acid substitutions in EGF-like elements of Notch and Delta modify Drosophila development and affect cell adhesion in vitro

Notch locus EGF-like element mutations spl, altering eye development, and AxE2, affecting wing and sensilla development, are modified by mutations at Delta. It is shown that two allele-specific suppressors of spl involve single amino acid substitutions in the 4th (Dlsup5) and 9th (Dlsup4) EGF-like elements of the Delta protein. Cultured cells producing spl or AxE2 aggregate with cells producing wild-type Delta or Dlsup5 protein, and Dlsup5-producing cells adhere to cells producing wild-type Notch protein. However, spl,AxE2, and Dlsup5 are each defective in promoting these cell affinities, as none of the mutant proteins can compete with the corresponding wild-type proteins for formation of cell aggregates. Thus, widely separated EGF-like elements of Notch and Delta appear to participate in functional molecular interactions between the proteins. Dlsup5 does not improve adhesiveness of spl in vitro, so suppression in vivo may involve altered developmental signaling by spl-Dlsup5 complexes, rather than modified cell adhesion.

Candidate proto-oncogene bcl-3 encodes a subunit-specific inhibitor of transcription factor NF-kappa B

The NF-kappa B subunits p50 and p65 and the product of the rel proto-oncogene are members of a growing class of transcription factors with a unique DNA-binding and dimerization domain. Nuclear transfer of each of these factors is controlled by cytoplasmic inhibitors, and regulated by specific stimuli. The inhibitors I kappa B-alpha and -beta and pp40 recognize either p65 or the c-rel protein. We show here that the proto-oncogene bcl-3, believed to be involved in certain human B-cell leukaemias, encodes a protein that functions as an I kappa B-like molecule for native NF-kappa B but is specific for the p50 subunit. The ankyrin repeat domain of the bcl-3 product is shown to mediate complex formation with NF-kappa B dimers by contracting the conserved dimerization domain of NF-kappa B.

A rat cerebellar protein containing the cdc10/SWI6 motif

Systematic analysis of soluble proteins in developing rat cerebellum by an automated two-dimensional liquid-chromatography system detected a number of proteins which increased transiently during the initial stage of postnatal development. One of the proteins, V-1, was isolated using a liquid-chromatography system, and its amino acid sequence was determined by analysis of the purified protein. The sequence showed that the V-1 protein consists of 117 amino acids with an acetylated N-terminus, and has 2.5 internal sequence repeats of 33 amino acids. Computer retrieval of the sequence indicated that the repeated sequences have a structural characteristics of the cdc10/SWI6 motif, which is found in a series of proteins, including those involved in cell-cycle control and cell-fate determination in yeast, Drosophila melanogaster and Caenorhabditis elegans. The structure of V-1, coupled with its controlled expression in early postnatal development, implies a potential role for V-1 in cerebellar morphogenesis.

Specific EGF repeats of Notch mediate interactions with Delta and Serrate: implications for Notch as a multifunctional receptor

The neurogenic loci Notch and Delta, which both encode EGF-homologous transmembrane proteins, appear to function together in mediating cell-cell communication and have been shown to interact at the cell surface in vitro. To examine the role of the EGF repeats in this interaction, we performed an extensive deletion mutagenesis of the extracellular domain of Notch. We find that of the 36 EGF repeats of Notch, only two, 11 and 12, are both necessary and sufficient to mediate interactions with Delta. Furthermore, this Delta binding ability is conserved in the corresponding two repeats from the Xenopus Notch homolog. We report a novel molecular interaction between Notch and Serrate, another EGF-homologous transmembrane protein containing a region of striking similarity to Delta, and show that the same two EGF repeats of Notch also constitute a Serrate binding domain. These results suggest that Notch may act as a multifunctional receptor whose 36 EGF repeats form a tandem array of discrete ligand-binding units, each of which may potentially interact with several different proteins during development.