Diversity in Membrane Binding Sites of Ankyrins

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

The ankyrin-B C-terminal domain determines activity of ankyrin-B/G chimeras in rescue of abnormal inositol 1,4,5-trisphosphate and ryanodine receptor distribution in ankyrin-B (-/-) neonatal cardiomyocytes

Ankyrins are a closely related family of membrane adaptor proteins that are believed to participate in targeting diverse membrane proteins to specialized domains in the plasma membrane and endoplasmic reticulum. This study addresses the question of how individual ankyrin isoforms achieve functional specificity when co-expressed in the same cell. Cardiomyocytes from ankyrin-B (-/-) mice display mis-localization of inositol 1,4,5-trisphosphate receptors and ryanodine receptors along with reduced contraction rates that can be rescued by expression of green fluorescent protein (GFP)-ankyrin-B but not GFP-ankyrin-G. We developed chimeric GFP expression constructs containing all combinations of the three major domains of ankyrin-B and ankyrin-G to determine which domain(s) of ankyrin-B are required for ankyrin-B-specific functions. The death/C-terminal domain of ankyrin-B determined activity of ankyrin-B/G chimeras in localization in a striated pattern in cardiomyocytes and in restoration of a normal striated distribution of both ryanodine and inositol 1,4,5-trisphosphate receptors as well as normal beat frequency of contracting cardiomyocytes. Further deletions within the death/C-terminal domain demonstrated that the C-terminal domain determines ankyrin-B activity, whereas deletion of the death domain had no effect. C-terminal domains are the most divergent between ankyrin isoforms and are candidates to encode the signal(s) that enable ankyrins to selectively target proteins to diverse cellular sites.

Identification of a novel ankyrin isoform (AnkG190) in kidney and lung that associates with the plasma membrane and binds alpha-Na, K-ATPase

Ankyrins are a family of adapter molecules that mediate linkages between integral membrane and cytoskeletal proteins. Such interactions are crucial to the polarized distribution of membrane proteins in transporting epithelia. We have cloned and characterized a novel 190-kDa member of this family from a rat kidney cDNA library, which we term AnkG190 based on the predicted size and homology with the larger neuronal AnkG isoform. AnkG190 displays a unique 31-residue amino terminus, a repeats domain consisting of 24 repetitive 33-residue motifs, a spectrin binding domain, and a truncated regulatory domain. Probes derived from the unique amino terminus hybridize to an 8-kilobase message exclusively in kidney and lung and specifically to the kidney outer medullary collecting ducts by in situ hybridization. Transfections of Madin-Darby canine kidney and COS-7 epithelial cell lines with a full-length AnkG190 construct result in (a) expression at the lateral plasma membrane, (b) functional assembly with the cytoskeleton, and (c) interaction with at least one membrane protein, the Na,K-ATPase. Two independent Na,K-ATPase binding domains on AnkG190 are demonstrated as follows: one within the distal 12 ankyrin repeats, and a second site within the spectrin binding domain. Thus, ankyrins may interact with integral membrane proteins in a pleiotropic manner that may involve complex tertiary structural determinants.

Distribution of epithelial ankyrin (Ank3) spliceoforms in renal proximal and distal tubules

In diverse cell types, ankyrin tethers a variety of ion transport and cell adhesion molecules to the spectrin-based membrane skeleton. In the whole kidney, epithelial ankyrin (Ank3) is the predominantly expressed ankyrin and is expressed as distinct spliceoforms. Antibodies against a portion of the Ank3 regulatory domain detected four major spliceoforms at 215, 200, 170, and 120 kDa. Immunoblotting of the renal cortex, which is 80% proximal tubule (PT), detected all four spliceoforms but showed significantly diminished Ank3(200/215). To determine the Ank3 spliceoforms present in the mouse PT cells, PT fragments were purified to 100% from the renal cortex. Isolation was performed by incubating cortical tubule segments with fluorescein and isolating the fluorescein-laden PT fragments or fluorescein-deplete non-PT (distal) fragments under fluorescence microscopy. Distal tubule (DT) fragments displayed abundance of the Ank3(200/215) but no Ank3(170) or Ank3(120). Isolated PT segments contained all four spliceoforms but dramatically diminished Ank3(200/215). These larger spliceoforms bind Na-K-ATPase in diverse cell types. Densitometric analysis of Ank3(200/215) and Na-K-ATPase abundance measured a lower Ank3(200/215)-to-Na-K-ATPase ratio in the PT vs. the renal cortex. These proximal vs. distal differences in Ank3 spliceoforms were displayed in LLC-PK1 cells, a proximal cell line, and MDCK cells, a distal cell line. The lower PT content of Ank3(200/215) suggests Na-K-ATPase in PT may be organized differently than in DT. Likely reflecting their cell-specific organization, regulation, and function, these studies indicate the different renal cell types express distinct Ank3 spliceoforms.

Immunoreactive analogues of erythrocyte ankyrin in human epidermal keratinocytes

Using immunoblot and immunofluorescence analysis, we demonstrated the presence and localization of an immunoreactive form of erythrocyte ankyrin in human epidermal keratinocytes. Immunoblot analysis revealed that both human epidermis and cultured epidermal keratinocytes contained ankyrin-like proteins of molecular mass 210 kDa that crossreacted with antihuman erythrocyte ankyrin antibodies. Immunofluorescence microscopy revealed that the plasma membrane of epidermal keratinocytes was stained. Eccrine sweat gland cells and ductal cells were also stained. These results indicate that in human epidermal keratinocytes, eccrine sweat gland cells and ductal cells, an ankyrin-like protein is present as one of the membrane proteins. The present findings and our recent previous studies showing the presence of a spectrin-like protein (fodrin) and 4.1-like proteins in these cells enable us to suggest that a membrane skeletal protein lattice may exist in these cells.

Identification of an M(r) 60,000 polypeptide unique to the meiotic spindle of the mouse oocyte

The mouse oocyte expresses an M(r) 60,000 (p60) polypeptide that is associated with the first and second meiotic spindles. Immunoreactive p60 was not detectable in the meiotic spindles of male germ cells or in mitotic spindles. P60 was identified with a polyclonal antibody whose predominant activity is directed against ankyrin. However, immunoadsorption experiments demonstrated that p60 is not an ankyrin isoform and represents a secondary activity of the polyclonal antibody. Circumstantial evidence suggest that p60 may be a microtubule-associated protein. Since the most obvious difference between the female meiotic spindle and other spindles is the long half-life of the former, we hypothesize that p60 may function in the maintenance of the long-lived female meiotic apparatus.

The polarity of the membrane skeleton in retinal pigment epithelial cells of developing chicken embryos and in primary culture

We studied the morphogenesis and the membrane skeleton in the retinal pigment epithelium during chicken embryogenesis and in culture, by using immunofluorescence and electron microscopy. During embryogenesis two distinct membrane skeletal structures were formed, an apical and a basolateral one. The former was seen in the apical surface already in the 10-day-old embryos. It was comprised of ankyrin and alpha-fodrin and showed a codistribution with Na+,K(+)-ATPase and an as yet uncharacterized cadherin-like molecule. The basolateral membrane skeleton was seen in the lateral walls already in the 10-day-old embryos, and later, between the 13th and 17th embryonic days, it also appeared at the basal membrane, coincidentally with the formation of the basal infoldings. It consisted of ankyrin and alpha-fodrin, but did not codistribute with any of the integral membrane proteins studied (Na+,K(+)-ATPase and cadherins). In culture, the retinal pigment epithelial cells retained their polarized morphology. Compared with the situation in vivo, however, there was a distinct translocation of the membrane skeletal components fodrin and ankyrin from the apical surface to the lateral walls, accompanied by a similar redistribution of Na+,K(+)-ATPase and the cadherin-like molecule. The results suggest that (1) there is, in the retinal pigment epithelium, an apical Na+,K(+)-ATPase-membrane skeleton structure stabilized by contacts between the retinal pigment epithelium and the neural retina, possibly mediated by a cadherin-like molecule, and that (2) there is another fodrin/ankyrin-based membrane skeleton in the basolateral walls that is important for the maintenance of the extensive folding of these surface areas.

Antidiuretic hormone and exocytosis: lessons from neurosecretion

Many cells, both single and epithelial, are programmed for exocytosis. In most cases, the contents of cytoplasmic vesicles are delivered rapidly and directly to the extracellular fluid. The process has been intensively studied in the chromaffin cell and the nerve terminal, where, as in other cells, exocytosis is under a complex type of cytoskeletal control. An array of vesicle-associated proteins mediates attachment of the vesicles to the cytoskeleton, their release, and their fusion with the plasma membrane. Two functional pools of vesicles, the releasable and reserve pool, carry out immediate and long-term secretory activity. Some of the mediators of neurotransmitter vesicle fusion, originally thought to be restricted to neurosecretory cells, have now been found in nonneuronal cells. The mammalian collecting duct and the amphibian bladder are also engaged in exocytosis. In both epithelia, antidiuretic hormone (ADH) induces the transfer of water channels from cytoplasmic vesicles to the apical cell membrane. The process is slower than in the nerve terminal and ends with channel placement rather than the extrusion of vesicular contents. Nevertheless, there are several respects in which cytoskeletal control, vesicle positioning in the cell, docking, and fusion may prove to resemble the events in neurosecretion. This review begins with a survey of cytoskeletal structure and function in the erythrocyte, the chromaffin cell, and the nerve terminal and then presents current studies of ADH-induced exocytosis, emphasizing common themes in cytoskeletal control.

An induced extracellular matrix protein reverses the polarity of band 3 in intercalated epithelial cells

The intercalated epithelial cell exists in two interconvertible forms in vivo, one where band 3 protein is apical and the other where it is basolateral. We seeded an immortalized clone of these cells at low density and found that band 3 was apical at confluence. There was little or no apical endocytosis. But when the cells were plated at high density, band 3 was basolateral, and there was vigorous apical endocytosis. Extracellular matrix produced by high density cells was able to retarget band 3 in low density cells and to induce apical endocytosis, as did a 230 kd protein partially purified from this matrix. Therefore, polarized targeting of some proteins is determined by external cues that might determine their polarity by reorganizing the cytoplasm.

Degradation of spectrin and ankyrin in the ischemic rat kidney

This study investigates ischemia-induced degradation of the spectrin-based cytoskeleton in rat brain, heart, and kidney. Spectrin, in conjunction with ankyrin, structurally supports the plasma membrane and sequesters integral membrane proteins. After 60 and 120 min of ischemia, brain tissue displayed both spectrin and ankyrin breakdown. The spectrin fragmentation pattern is similar to previously reported ischemia-induced calpain I proteolysis of spectrin in N-methyl-D-aspartate receptor-containing neurons. Ischemic heart tissue displayed no spectrin or ankyrin degradation. Ischemic renal tissue showed minimal breakdown of spectrin but a major loss of ankyrin (25%/30 min of ischemia) that was essentially complete after 120 min of ischemia. Interestingly, this profound loss of ankyrin in the intact ischemic kidney was not mimicked in three renal cell lines (MDCK, LLC-PK1, and JTC cell lines) exposed to chemical anoxia. Immunocytochemistry showed ankyrin was concentrated in thick ascending limb (cTAL) cells and, although delayed by 30 min, was lost at the same rate as measured by immunoblot analysis. Spectrin and Na(+)-K(+)-ATPase, which complex with ankyrin, were essentially unaffected by ischemia. Ankyrin degradation in cTAL cells correlated with the loss of basal infolding organization. In conclusion, the spectrin-based cytoskeleton is differentially targeted by ischemia-induced degradative processes in different in vivo tissues.

Gastric parietal cell H(+)-K(+)-ATPase microsomes are associated with isoforms of ankyrin and spectrin

Stimulation of HCl secretion by gastric parietal cells requires the fusion of cytoplasmic H(+)-K(+)-ATPase-bearing tubulovesicles with the apical membrane. This insertion of membrane results in a dramatic increase in apical surface area through the formation of microvilli. To elucidate the elements that may stabilize the newly inserted H(+)-K(+)-ATPase within the apical membrane, we searched for specific cytoskeletal proteins associating with the gastric enzyme. We document by immunoblot analysis that ankyrin, spectrin, and actin copurify with H(+)-K(+)-ATPase microsomes prepared from gastric parietal cells. Coprecipitation of 125I-labeled native erythrocyte ankyrin with the H(+)-K(+)-ATPase from gastric microsomes using anti-H(+)-K(+)-ATPase antibodies suggests that ankyrin associates with the H(+)-K(+)-ATPase. Indirect immunofluorescence and confocal microscopy show that ankyrin and H(+)-K(+)-ATPase cosegregate within resting and secreting parietal cells. Taken together, these data suggest that the association of the gastric H(+)-K(+)-ATPase with spectrin and actin is mediated by ankyrin and that this interaction contributes to the maintenance of the polarized distribution of the enzyme to the apical domain of gastric parietal cells during acid secretion.

Murine erythrocyte ankyrin cDNA: highly conserved regions of the regulatory domain

Ankyrin is an essential link between cytoskeletal proteins, such as spectrin, and membrane bound proteins, such as protein 3, the erythrocyte anion exchanger. Although the amino acid structure of human ankyrin is known, the functional regions have been only partially defined. Sequence comparisons between mouse and human ankyrin offer one mechanism of identifying highly conserved regions that probably have functional significance. We report the isolation and sequencing of a series of overlapping murine erythroid ankyrin (Ank-1) cDNAs from spleen and reticulocyte libraries (total span 6238 bp) and identify potentially important regions of murine-human reticulocyte ankyrin homology. Comparison of the predicted peptide sequences of mouse and human erythroid ankyrins shows that these ankyrins are highly conserved in both the N-terminal, protein 3 binding domain (96% amino acid identity) and in the central spectrin-binding domain (97% identity), but differ in the C-terminal regulatory domain (79% identity). However, the C-terminal regulatory domain contains two regions of peptide sequence that are perfectly conserved. We postulate these regions are important in the regulatory functions of this domain.

Isolation and chromosomal localization of a novel nonerythroid ankyrin gene

Immunoreactive isoforms of erythrocyte ankyrin have been shown to be present in a variety of nonerythroid tissues. Isolation of the genes that encode these isoforms will clarify their relationship to erythrocyte ankyrin. Using an erythrocyte ankyrin cDNA clone as a hybridization probe, we screened a human genomic library and isolated a clone that hybridizes with the probe at low stringency but not at high stringency. Partial nucleotide sequence of the clone revealed the presence of a 99-bp segment that is homologous to an exon of the erythrocyte ankyrin gene. Northern analysis showed that a labeled fragment of the clone hybridized to a 7-kb message in RNA of fetal brain but not of erythroid cells, suggesting that this clone is part of a novel gene that is expressed predominantly in nonerythroid tissue. Comparison of the sequence of the genomic clone with that of a recently isolated cDNA clone for brain ankyrin (Otto et al., 1989) showed identity of 96 of 99 bp between the putative exon and a segment of the cDNA clone (V. Bennett, personal communication, 1991), suggesting that the genomic clone is part of a gene for nonerythroid ankyrin, which we have designated ANK2. By analysis of somatic cell hybrids and fluorescence in situ hybridization, we assigned ANK2 to human chromosome 4 at a position equivalent to bands 4q25-q27.

Cellular intermediate filament networks and their derangement in alcoholic hepatitis

Intermediate filaments are major components of most eukaryotic cells that form from the polymerization of protein subunits that are expressed in tissue and development specific fashions. The interactions of intermediate filaments with a myriad of other cellular proteins and structures give rise to a complex overall cellular architecture that is likely responsible for cellular well-being. The mature 10-nm filaments are relatively stable cellular structures, but the intermediate filaments undergo major morphological and biochemical changes, especially during mitosis, differentiation, and in response to certain drugs. Evidence exists that hepatocyte intermediate filaments (keratin filaments) are deranged in alcoholic hepatitis, an inflammatory liver disease of alcoholics and heavy spree drinkers. The classical and characteristic pathological hepatocyte inclusion bodies of alcoholic hepatitis, Mallory bodies, are composed in part of normal keratins that likely derive from the pre-existing hepatocyte intermediate filament network. It is unclear if intermediate filament network derangement in alcoholic hepatitis is directly caused by the actions of ethanol or its metabolites on intermediate filaments or their associated structures, or whether alcohol causes a cellular insult or injury elsewhere and a subsequent response (e.g., immune) causes intermediate filament network derangement. The precise mechanisms responsible for intermediate filament derangement remain to be elucidated; however, experimental data exist that support and refute several hypotheses. Hopefully, further studies will help determine a better overall understanding of the abnormalities of intermediate filaments and their relationship to the pathophysiology of alcoholic hepatitis and other diseases.

Spectrin: a structural mediator between diverse plasma membrane proteins and the cytoplasm

The spectrin skeleton of non-erythroid cells is likely to interact with a variety of integral membrane proteins and participate both in stable linkages as well as dynamic structures capable of rapid disassembly and assembly. The basis for diversity of roles for spectrin includes multiple, functionally distinct isoforms of spectrin, ankyrin and other associated proteins, regulation of protein interactions through phosphorylation and calcium/calmodulin, as well as differential expression of accessory proteins that determine the organization and localization of spectrin in cells. Spectrin is highly conserved from Drosophila to man and is likely to be involved in fundamental aspects of membrane structure requiring long range order and organization. Spectrin is a candidate to interact with many integral membrane proteins in roles basic to metazoan cells which must associate into tissues. Organization of cells into tissues requires loss of cell motility, formation of specialized membrane domains and assembly of cell junctions, which are all activities potentially involving spectrin. Future challenges lie in devising direct experiments to evaluate the functions of spectrin in cells and tissues.

Morphogenesis of the polarized epithelial cell phenotype

Polarized epithelial cells play fundamental roles in the ontogeny and function of a variety of tissues and organs in mammals. The morphogenesis of a sheet of polarized epithelial cells (the trophectoderm) is the first overt sign of cellular differentiation in early embryonic development. In the adult, polarized epithelial cells line all body cavities and occur in tissues that carry out specialized vectorial transport functions of absorption and secretion. The generation of this phenotype is a multistage process requiring extracellular cues and the reorganization of proteins in the cytoplasm and on the plasma membrane; once established, the phenotype is maintained by the segregation and retention of specific proteins and lipids in distinct apical and basal-lateral plasma membrane domains.