Small, membrane-bound, alternatively spliced forms of ankyrin 1 associated with the sarcoplasmic reticulum of mammalian skeletal muscle

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Obscurin is a ligand for small ankyrin 1 in skeletal muscle

The factors that organize the internal membranes of cells are still poorly understood. We have been addressing this question using striated muscle cells, which have regular arrays of membranes that associate with the contractile apparatus in stereotypic patterns. Here we examine links between contractile structures and the sarcoplasmic reticulum (SR) established by small ankyrin 1 (sAnk1), a approximately 17.5-kDa integral protein of network SR. We used yeast two-hybrid to identify obscurin, a giant Rho-GEF protein, as the major cytoplasmic ligand for sAnk1. The binding of obscurin to the cytoplasmic sequence of sAnk1 is mediated by a sequence of obscurin that is C-terminal to its last Ig-like domain. Binding was confirmed in two in vitro assays. In one, GST-obscurin, bound to glutathione-matrix, specifically adsorbed native sAnk1 from muscle homogenates. In the second, MBP-obscurin bound recombinant GST-sAnk1 in nitrocellulose blots. Kinetic studies using surface plasmon resonance yielded a K(D) = 130 nM. On subcellular fractionation, obscurin was concentrated in the myofibrillar fraction, consistent with its identification as sarcomeric protein. Nevertheless, obscurin, like sAnk1, concentrated around Z-disks and M-lines of striated muscle. Our findings suggest that obscurin binds sAnk1, and are the first to document a specific and direct interaction between proteins of the sarcomere and the SR.

Regulation of erythrocyte membrane protein gene expression

Proteins of the erythrocyte membrane have served as the prototypes of homologous families of multifunctional proteins in erythroid and nonerythroid cells. These proteins demonstrate many different cell type, tissue-specific, and developmental stage-specific functions. This complex pattern of functional diversity appears to have evolved from the cell type, tissue-specific, developmentally regulated expression of multiple protein isoforms. Isoform diversity arises from different gene products from related genes; from differential, alternate splicing of the same gene product; from the use of tissue-specific promoters; and from alternate polyadenylation. The identification and characterization of the regulatory elements that control erythrocyte membrane protein gene expression have important implications for several biologic processes. These include disease pathogenesis, membrane assembly, hematopoiesis, gene regulation, and direction of other erythroid-specific genes in transgenic mouse and gene therapy applications.

The hydrophilic domain of small ankyrin-1 interacts with the two N-terminal immunoglobulin domains of titin

Little is known about the mechanisms that organize the internal membrane systems in eukaryotic cells. We are addressing this question in striated muscle, which contains two novel systems of internal membranes, the transverse tubules and the sarcoplasmic reticulum (SR). Small ankyrin-1 (sAnk1) is an approximately 17-kDa transmembrane protein of the SR that concentrates around the Z-disks and M-lines of each sarcomere. We used the yeast two-hybrid assay to determine whether sAnk1 interacts with titin, a giant myofibrillar protein that organizes the sarcomere. We found that the hydrophilic cytoplasmic domain of sAnk1 interacted with the two most N-terminal Ig domains of titin, ZIg1 and ZIg2, which are present at the Z-line in situ. Both ZIg1 and ZIg2 were required for binding activity. sAnk1 did not interact with other sequences of titin that span the Z-disk or with Ig domains of titin near the M-line. Titin ZIg1/2 also bound T-cap/telethonin, a 19-kDa protein of the Z-line. We show that titin ZIg1/2 could form a three-way complex with sAnk1 and T-cap. Our results indicate that titin ZIg1/2 can bind sAnk1 in muscle homogenates and suggest a role for these proteins in organizing the SR around the contractile apparatus at the Z-line.

Binding of an ankyrin-1 isoform to obscurin suggests a molecular link between the sarcoplasmic reticulum and myofibrils in striated muscles

Assembly of specialized membrane domains, both of the plasma membrane and of the ER, is necessary for the physiological activity of striated muscle cells. The mechanisms that mediate the structural organization of the sarcoplasmic reticulum with respect to the myofibrils are, however, not known. We report here that ank1.5, a small splice variant of the ank1 gene localized on the sarcoplasmic reticulum membrane, is capable of interacting with a sequence of 25 aa located at the COOH terminus of obscurin. Obscurin is a giant sarcomeric protein of approximately 800 kD that binds to titin and has been proposed to mediate interactions between myofibrils and other cellular structures. The binding sites and the critical aa required in the interaction between ank1.5 and obscurin were characterized using the yeast two-hybrid system, in in vitro pull-down assays and in experiments in heterologous cells. In differentiated skeletal muscle cells, a transfected myc-tagged ank1.5 was found to be selectively restricted near the M line region where it colocalized with endogenous obscurin. The M line localization of ank1.5 required a functional obscurin-binding site, because mutations of this domain resulted in a diffused distribution of the mutant ank1.5 protein in skeletal muscle cells. The interaction between ank1.5 and obscurin represents the first direct evidence of two proteins that may provide a direct link between the sarcoplasmic reticulum and myofibrils. In keeping with the proposed role of obscurin in mediating an interaction with ankyrins and sarcoplasmic reticulum, we have also found that a sequence with homology to the obscurin-binding site of ank1.5 is present in the ank2.2 isoform, which in striated muscles has been also shown to associate with the sarcoplasmic reticulum. Accordingly, a peptide containing the COOH terminus of ank2.2 fused with GST was found to bind to obscurin. Based on reported evidence showing that the COOH terminus of ank2.2 is necessary for the localization of ryanodine receptors and InsP3 receptors in the sarcoplasmic reticulum, we propose that obscurin, through multiple interactions with ank1.5 and ank2.2 isoforms, may assemble a large protein complex that, in addition to a structural function, may play a role in the organization of specific subdomains in the sarcoplasmic reticulum.

Identification of Ank(G107), a muscle-specific ankyrin-G isoform

We previously showed that alternatively spliced ankyrins-G, the Ank3 gene products, are expressed in skeletal muscle and localize to the postsynaptic folds and to the sarcoplasmic reticulum. Here we report the molecular cloning, tissue expression, and subcellular targeting of Ank(G107), a novel ankyrin-G from rat skeletal muscle. Ank(G107) lacks the entire ANK repeat domain and contains a 76-residue sequence near the COOH terminus. This sequence shares homology with COOH-terminal sequences of ankyrins-R and ankyrins-B, including the muscle-specific skAnk1. Despite widespread tissue expression of Ank3, the 76-residue sequence is predominantly detected in transcripts of skeletal muscle and heart, including both major 8- and 5.6-kb mRNAs of skeletal muscle. In 15-day-old rat skeletal muscle, antibodies against the 76-residue sequence localized to the sarcolemma and to the postsynaptic membrane and cross-reacted with three endogenous ankyrins-G, including one 130-kDa polypeptide that comigrated with in vitro translated Ank(G107). In adult muscle, these polypeptides appeared significantly decreased, and immunofluorescence labeling was no more detectable. Green fluorescent protein-tagged Ank(G107) transfected in primary cultures of rat myotubes was targeted to the plasma membrane. Deletion of the 76-residue insert resulted in additional cytoplasmic labeling suggestive of a reduced stability of Ank(G107) at the membrane. Recruitment of the COOH-terminal domain to the membrane was much less efficient but still possible only in the presence of the 76-residue insert. We conclude that the 76-residue sequence contributes to the localization and is essential to the stabilization of Ank(G107) at the membrane. These results suggest that tissue-dependent and developmentally regulated alternative processing of ankyrins generates isoforms with distinct sequences, potentially involved in specific protein-protein interactions during differentiation of the sarcolemma and, in particular, of the postsynaptic membrane.

Dynamics of ankyrin-containing complexes in chicken embryonic erythroid cells: role of phosphorylation

Chicken erythroid ankyrin undergoes a fairly rapid cycle of cytoskeletal association, dissociation, and turnover. In addition, the cytoskeletal association of ankyrin is regulated by phosphorylation. Treatment of erythroid cells with serine and threonine phosphatase inhibitors stimulated the hyperphosphorylation of the 225- and 205-kDa ankyrin isoforms, and dissociated the bulk of these isoforms from cytoskeletal spectrin. In vitro binding studies have shown that this dissociation of ankyrin from spectrin in vivo can be attributed to a reduced ability of hyperphosphorylated ankyrin to bind spectrin. Interestingly, a significant fraction of detergent insoluble ankyrin accumulates in a spectrin-independent pool. At least some of this spectrin-independent pool of ankyrin is complexed with the AE1 anion exchanger, and the solubility properties of this pool are also regulated by phosphorylation. Treatment of cells with serine and threonine phosphatase inhibitors had no effect on ankyrin/AE1 complex formation. However, these inhibitors were sufficient to shift ankyrin/AE1 complexes from the detergent insoluble to the soluble pool. These analyses, which are the first to document the in vivo consequences of ankyrin phosphorylation, indicate that erythroid ankyrin-containing complexes can undergo dynamic rearrangements in response to changes in phosphorylation.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

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.

Regulation of alternative pre-mRNA splicing during erythroid differentiation

Although the mature enucleated erythrocyte is no longer active in nuclear processes such as pre-mRNA splicing, the function of many of its major structural proteins is dependent on alternative splicing choices made during the earlier stages of erythropoiesis. These splicing decisions fundamentally regulate many aspects of protein structure and function by governing the inclusion or exclusion of exons that encode protein interaction domains, regulatory signals, or translation initiation or termination sites. Alternative splicing events may be partially or entirely erythroid-specific, ie, distinct from the splicing patterns imposed on the same transcripts in nonerythroid cells. Moreover, differentiation stage-specific splicing "switches" may alter the structure and function of erythroid proteins in physiologically important ways as the cell is morphologically and functionally remodeled during normal differentiation. Derangements in the splicing of individual mutated pre-mRNAs can produce synthesis of truncated or unstable proteins that are responsible for numerous erythrocyte disorders. This review will summarize the salient features of regulated alternative splicing in general, review existing information concerning the widespread extent of alternative splicing among erythroid genes, and describe recent studies that are beginning to uncover the mechanisms that regulate an erythroid splicing switch in the protein 4.1R gene.

Na,K-ATPase in skeletal muscle: two populations of beta-spectrin control localization in the sarcolemma but not partitioning between the sarcolemma and the transverse tubules

We used immunological approaches to study the factors controlling the distribution of the Na,K-ATPase in fast twitch skeletal muscle of the rat. Both alpha subunits of the Na,K-ATPase colocalize with beta-spectrin and ankyrin 3 in costameres, structures at the sarcolemma that lie over Z and M-lines and in longitudinal strands. In immunoprecipitates, the alpha1 and alpha2 subunits of the Na,K-ATPase as well as ankyrin 3 associate with beta-spectrin/alpha- fodrin heteromers and with a pool of beta-spectrin at the sarcolemma that does not contain alpha-fodrin. Myofibers of mutant mice lacking beta-spectrin (ja/ja) have a more uniform distribution of both the alpha1 and alpha2 subunits of the Na,K-ATPase in the sarcolemma, supporting the idea that the rectilinear sarcomeric pattern assumed by the Na,K-ATPase in wild-type muscle requires beta-spectrin. The Na,K-ATPase and beta-spectrin are distributed normally in muscle fibers of the nb/nb mouse, which lacks ankyrin 1, suggesting that this isoform of ankyrin is not necessary to link the Na,K-ATPase to the spectrin-based membrane skeleton. In immunofluorescence and subcellular fractionation experiments, the alpha2 but not the alpha1 subunit of the Na,K-ATPase is present in transverse (t-) tubules. The alpha1 subunit of the pump is not detected in increased amounts in the t-tubules of muscle from the ja/ja mouse, however. Our results suggest that the spectrin-based membrane skeleton, including ankyrin 3, concentrates both isoforms of the Na,K-ATPase in costameres, but that it does not play a significant role in restricting the entry of the alpha1 subunit into the t-tubules.

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.

A nonerythroid isoform of protein 4.1R interacts with components of the contractile apparatus in skeletal myofibers

The approximately 80-kDa erythroid 4.1R protein is a major component of the erythrocyte cytoskeleton, where it links transmembrane proteins to the underlying spectrin/actin complexes. A diverse collection of 4.1R isoforms has been described in nonerythroid cells, ranging from approximately 30 to approximately 210 kDa. In the current study, we identified the number and primary structure of 4.1R isoforms expressed in adult skeletal muscle and characterized the localization patterns of 4.1R message and protein. Skeletal muscle 4.1R appears to originate solely from the upstream translation initiation codon (AUG-1) residing in exon 2'. Combinations of alternatively spliced downstream exons generate an array of distinct 4.1R spliceoforms. Two major isoform classes of approximately 105/110 and approximately 135 kDa are present in muscle homogenates. 4.1R transcripts are distributed in highly ordered signal stripes, whereas 4.1R protein(s) decorate the sarcoplasm in transverse striations that are in register with A-bands. An approximately 105/110-kDa 4.1R isoform appears to occur in vivo in a supramolecular complex with major sarcomeric proteins, including myosin, alpha-actin, and alpha-tropomyosin. In vitro binding assays showed that 4.1R may interact directly with the aforementioned contractile proteins through its 10-kDa domain. All of these observations suggest a topological model whereby 4.1R may play a scaffolding role by anchoring the actomyosin myofilaments and possibly modulating their displacements during contraction/relaxation.

Human spectrin Src homology 3 domain binding protein 1 regulates macropinocytosis in NIH 3T3 cells

Macropinocytosis is an endocytic process that occurs through non-clathrin coated vesicles larger than 0.2 microm in diameter. Although macropinocytic vesicles are readily visualized in cultured cells by the introduction of fluorescent, water-soluble dyes into the culture medium, protein markers associated with this type of vesicles have not yet been well defined. Here, we report that human spectrin SH3 domain binding protein 1, or Hssh3bp1, associates with macropinosomes in NIH 3T3 fibroblasts. Hssh3bp1 macropinosomes are heterogeneous in morphology and size, do not endocytose transferrin and are resistant to brefeldin A treatment. Cytochalasin D, and wortmannin block endocytosis of fluorescent dyes into the Hssh3bp1 macropinosomes and dramatically affect their morphology. Overexpression of Hssh3bp1-green fluorescent protein abolished fusion of vesicles resulting in a decreased endocytosis of fluorescence dyes, thus suggesting a potential regulatory role of Hssh3bp1 in macropinocytosis. In the macropinosomes of NIH 3T3 cells, Hssh3bp1 associates with a 200-kDa protein that crossreacts with a monoclonal antibody to the erythroid alpha-spectrin SH3 domain. Thus macropinosomes in cells may contain a spectrin-like protein.

Ankyrins

This review is focused on ankyrin which is a protein linker between the integral membrane proteins and spectrin-based cytoskeleton. Structure and distribution of different ankyrin isoforms that are products of alternative-spliced genes are described. Interaction of ankyrins with various membranes is considered. Special attention is paid to ankyrin participation in signal transduction and in assembly of integral membrane proteins in specialized membrane domains.

A minimal ankyrin promoter linked to a human gamma-globin gene demonstrates erythroid specific copy number dependent expression with minimal position or enhancer dependence in transgenic mice

In red blood cells ankyrin (ANK-1) provides the primary linkage between the erythrocyte membrane skeleton and the plasma membrane. We have previously demonstrated that a 271-bp 5'-flanking region of the ANK-1 gene has promoter activity in erythroid, but not non-erythroid, cell lines. To determine whether the ankyrin promoter could direct erythroid-specific expression in vivo, we analyzed transgenic mice containing the ankyrin promoter fused to the human (A)gamma-globin gene. Sixteen of 17 lines expressed the transgene in erythroid cells indicating nearly position-independent expression. We also observed a significant correlation between the level of Ank/(A)gamma-globin mRNA and transgene copy number. The level of Ank/(A)gamma mRNA averaged 11% of mouse alpha-globin mRNA per gene copy at all developmental stages. The addition of the HS2 enhancer from the beta-globin locus control region to the Ank/(A)gamma-globin transgene resulted in Ank/(A)gamma-globin mRNA expression in embryonic and fetal erythroid cells in six of eight lines but resulted in absent or dramatically reduced levels of Ank/(A)gamma-globin mRNA in adult erythroid cells in eight of eight transgenic lines. These data indicate that the minimal ankyrin promoter contains all sequences necessary and sufficient for erythroid-specific, copy number-dependent, position-independent expression of the human (A)gamma-globin gene.

Green fluorescent protein-tagged sarco(endo)plasmic reticulum Ca2+-ATPase overexpression in Paramecium cells: isoforms, subcellular localization, biogenesis of cortical calcium stores and functional aspects

We have followed the time-dependent transfection of Paramecium cells with a vector containing the gene of green fluorescent protein (GFP) attached to the C-terminus of the PtSERCA1 gene. The outlines of alveolar sacs (ASs) are labelled, as is the endoplasmic reticulum (ER) throughout the cell. When GFP fluorescence is compared with previous anti-PtSERCA1 antibody labelling, the much wider distribution of GFP (ER+ASs) indicates that only a small amount of SERCA molecules is normally retained in the ER. A second isoform, PtSERCA2, also occurs and its C-terminal GFP-tagging results in the same distribution pattern. However, when GFP is inserted in the major cytoplasmic loop, PtSERCA1 and two fusion proteins are mostly retained in the ER, probably because of the presence of the overt C-terminal KKXX ER-retention signal and/or masking of a signal for transfer into ASs. On the overall cell surface, new SERCA molecules seem to be permanently delivered from the ER to ASs by vesicle transport, whereas in the fission zone of dividing cells ASs may form anew. In cells overexpressing PtSERCA1 (with C-terminal GFP) in ASs, [Ca2+]i regulation during exocytosis is not significantly different from controls, probably because their Ca2+ pump has to mediate only slow reuptake.

The human ankyrin-1 gene is selectively transcribed in erythroid cell lines despite the presence of a housekeeping-like promoter

To begin to study the sequence variations identified in the 5′ flanking genomic DNA of the ankyrin gene in ankyrin-deficient hereditary spherocytosis patients and to provide additional insight into our understanding of the regulation of genes encoding erythrocyte membrane proteins, we have identified and characterized the erythroid promoter of the human ankyrin-1 gene. This compact promoter has characteristics of a housekeeping gene promoter, including very high G+C content and enzyme restriction sites characteristic of an HTF-island, no TATA, InR, or CCAAT consensus sequences, and multiple transcription initiation sites. In vitro DNAseI footprinting analyses revealed binding sites for GATA-1, CACCC-binding, and CGCCC-binding proteins. Transfection of ankyrin promoter/reporter plasmids into tissue culture cell lines yielded expression in erythroid, but not muscle, neural, or HeLa cells. Electrophoretic mobility shift assays, including competition and antibody supershift experiments, demonstrated binding of GATA-1, BKLF, and Sp1 to core ankyrin promoter sequences. In transfection assays, mutation of the Sp1 site had no effect on reporter gene expression, mutation of the CACCC site decreased expression by half, and mutation of the GATA-1 site completely abolished activity. The ankyrin gene erythroid promoter was transactivated in heterologous cells by forced expression of GATA-1 and to a lesser degree BKLF.

The human ankyrin-1 gene is selectively transcribed in erythroid cell lines despite the presence of a housekeeping-like promoter

To begin to study the sequence variations identified in the 5′ flanking genomic DNA of the ankyrin gene in ankyrin-deficient hereditary spherocytosis patients and to provide additional insight into our understanding of the regulation of genes encoding erythrocyte membrane proteins, we have identified and characterized the erythroid promoter of the human ankyrin-1 gene. This compact promoter has characteristics of a housekeeping gene promoter, including very high G+C content and enzyme restriction sites characteristic of an HTF-island, no TATA, InR, or CCAAT consensus sequences, and multiple transcription initiation sites. In vitro DNAseI footprinting analyses revealed binding sites for GATA-1, CACCC-binding, and CGCCC-binding proteins. Transfection of ankyrin promoter/reporter plasmids into tissue culture cell lines yielded expression in erythroid, but not muscle, neural, or HeLa cells. Electrophoretic mobility shift assays, including competition and antibody supershift experiments, demonstrated binding of GATA-1, BKLF, and Sp1 to core ankyrin promoter sequences. In transfection assays, mutation of the Sp1 site had no effect on reporter gene expression, mutation of the CACCC site decreased expression by half, and mutation of the GATA-1 site completely abolished activity. The ankyrin gene erythroid promoter was transactivated in heterologous cells by forced expression of GATA-1 and to a lesser degree BKLF.

The spectrin-based skeleton at the postsynaptic membrane of the neuromuscular junction

Membrane skeletons, in particular the spectrin-based skeleton, are thought to participate in the organization of specialized membrane domains by restricting integral proteins to specific membrane sites. In the neuromuscular junction, discrete isoforms of spectrin and ankyrin, the peripheral protein that links spectrin to the membrane, colocalize with voltage-dependent sodium channels and N-CAM at the troughs of the postsynaptic membrane folds. Moreover, beta-spectrin, N-CAM, and sodium channels become clustered at the endplate during a period of time coincident with postsynaptic fold formation and synapse maturation. These observations suggest a role of the spectrin skeleton in directing and maintaining postsynaptic accumulations of sodium channels and N-CAM. In addition, the coexistence of spectrin and dystrophin at the troughs of the junctional folds raises the question of their respective functions in this membrane domain, where both cytoskeletal proteins have the potential to associate with sodium channels via ankyrin and syntrophin, respectively. Possible scenarios are discussed here with respect to accumulating evidence from studies of assembly of similar membrane domains in neurons.

Ankyrin-B is required for intracellular sorting of structurally diverse Ca2+ homeostasis proteins

This report describes a congenital myopathy and major loss of thymic lymphocytes in ankyrin-B (-/-) mice as well as dramatic alterations in intracellular localization of key components of the Ca(2+) homeostasis machinery in ankyrin-B (-/-) striated muscle and thymus. The sarcoplasmic reticulum (SR) and SR/T-tubule junctions are apparently preserved in a normal distribution in ankyrin-B (-/-) skeletal muscle based on electron microscopy and the presence of a normal pattern of triadin and dihydropyridine receptor. Therefore, the abnormal localization of SR/ER Ca ATPase (SERCA) and ryanodine receptors represents a defect in intracellular sorting of these proteins in skeletal muscle. Extrapolation of these observations suggests defective targeting as the basis for abnormal localization of ryanodine receptors, IP3 receptors and SERCA in heart, and of IP3 receptors in the thymus of ankyrin-B (-/-) mice. Mis-sorting of SERCA 2 and ryanodine receptor 2 in ankyrin-B (-/-) cardiomyocytes is rescued by expression of 220-kD ankyrin-B, demonstrating that lack of the 220-kD ankyrin-B polypeptide is the primary defect in these cells. Ankyrin-B is associated with intracellular vesicles, but is not colocalized with the bulk of SERCA 1 or ryanodine receptor type 1 in skeletal muscle. These data provide the first evidence of a physiological requirement for ankyrin-B in intracellular targeting of the calcium homeostasis machinery of striated muscle and immune system, and moreover, support a catalytic role that does not involve permanent stoichiometric complexes between ankyrin-B and targeted proteins. Ankyrin-B is a member of a family of adapter proteins implicated in restriction of diverse proteins to specialized plasma membrane domains. Similar mechanisms involving ankyrins may be essential for segregation of functionally defined proteins within specialized regions of the plasma membrane and within the Ca(2+) homeostasis compartment of the ER.

Characterization of Chlamydomonas reinhardtii zygote-specific cDNAs that encode novel proteins containing ankyrin repeats and WW domains

Genes that are expressed only in the young zygote are considered to be of great importance in the development of an isogamous green alga, Chlamydomonas reinhardtii. Clones representing the Zys3 gene were isolated from a cDNA library prepared using zygotes at 10 min after fertilization. Sequencing of Zys3 cDNA clones resulted in the isolation of two related molecular species. One of them encoded a protein that contained two kinds of protein-to-protein interaction motifs known as ankyrin repeats and WW domains. The other clone lacked the ankyrin repeats but was otherwise identical. These mRNA species began to accumulate simultaneously in cells beginning 10 min after fertilization, and reached maximum levels at about 4 h, after which time levels decreased markedly. Genomic DNA gel-blot analysis indicated that Zys3 was a single-copy gene. The Zys3 proteins exhibited parallel expression to the Zys3 mRNAs at first, appearing 2 h after mating, and reached maximum levels at more than 6 h, but persisted to at least 1 d. Immunocytochemical analysis revealed their localization in the endoplasmic reticulum, which suggests a role in the morphological changes of the endoplasmic reticulum or in the synthesis and transport of proteins to the Golgi apparatus or related vesicles.

Protein-protein interactions in intracellular Ca2+-release channel function

Release of Ca2+ ions from intracellular stores can occur via two classes of Ca2+-release channel (CRC) protein, the inositol 1,4, 5-trisphosphate receptors (InsP3Rs) and the ryanodine receptors (RyRs). Multiple isoforms and subtypes of each CRC class display distinct but overlapping distributions within mammalian tissues. InsP3Rs and RyRs interact with a plethora of accessory proteins which modulate the activity of their intrinsic channels. Although many aspects of CRC structure and function have been reviewed in recent years, the properties of proteins with which they interact has not been comprehensively surveyed, despite extensive current research on the roles of these modulators. The aim of this article is to review the regulation of CRC activity by accessory proteins and, wherever possible, to outline the structural details of such interactions. The CRCs are large transmembrane proteins, with the bulk of their structure located cytoplasmically. Intra- and inter-complex protein-protein interactions between these cytoplasmic domains also regulate CRC function. Some accessory proteins modulate channel activity of all CRC subtypes characterized, whereas other have class- or even isoform-specific effects. Certain accessory proteins exert both direct and indirect forms of regulation on CRCs, occasionally with opposing effects. Others are themselves modulated by changes in Ca2+ concentration, thereby participating in feedback mechanisms acting on InsP3R and RyR activity. CRCs are therefore capable of integrating numerous signalling events within a cell by virtue of such protein-protein interactions. Consequently, the functional properties of InsP3Rs and RyRs within particular cells and subcellular domains are 'customized' by the accessory proteins present.

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.

AnkyrinG is associated with the postsynaptic membrane and the sarcoplasmic reticulum in the skeletal muscle fiber

Ankyrins are a multi-gene family of peripheral proteins that link ion channels and cell adhesion molecules to the spectrin-based skeleton in specialized membrane domains. In the mammalian skeletal myofiber, ankyrins were immunolocalized in several membrane domains, namely the costameres, the postsynaptic membrane and the triads. Ank1 and Ank3 transcripts were previously detected in skeletal muscle by northern blot analysis. However, the ankyrin isoforms associated with these domains were not identified, with the exception of an unconventional Ank1 gene product that was recently localized at discrete sites of the sarcoplasmic reticulum. Here we study the expression and subcellular distribution of the Ank3 gene products, the ankyrinsG, in the rat skeletal muscle fiber. Northern blot analysis of rat skeletal muscle mRNAs using domain-specific Ank3 cDNA probes revealed two transcripts of 8.0 kb and 5.6 kb containing the spectrin-binding and C-terminal, but not the serine-rich, domains. Reverse transcriptase PCR analysis of rat skeletal muscle total RNA confirmed the presence of Ank3 transcripts that lacked the serine-rich and tail domains, a major insert of 7813 bp at the junction of the spectrin-binding and C-terminal domains that was previously identified in brain Ank3 transcripts. Immunoblot analysis of total skeletal muscle homogenates using ankyrinG-specific antibodies revealed one major 100 kDa ankyrinG polypeptide. Immunofluorescence labeling of rat diaphragm cryosections showed that ankyrin(s)G are selectively associated with (1) the depths of the postsynaptic membrane folds, where the voltage-dependent sodium channel and N-CAM accumulate, and (2) the sarcoplasmic reticulum, as confirmed by codistribution with the sarcoplasmic reticulum Ca2+-ATPase (SERCA 1). At variance with ankyrin(s)G, ankyrin(s)R (ank1 gene products) accumulate at the sarcolemma and at sarcoplasmic structures, in register with A-bands. Both ankyrin isoforms codistributed over Z-lines and at the postsynaptic membrane. These data extend the notion that ankyrins are differentially localized within myofibers, and point to a role of the ankyrinG family in the organization of the sarcoplasmic reticulum and the postsynaptic membrane.

Identification of a candidate human spectrin Src homology 3 domain-binding protein suggests a general mechanism of association of tyrosine kinases with the spectrin-based membrane skeleton

Spectrin is a widely expressed protein with specific isoforms found in erythroid and nonerythroid cells. Spectrin contains an Src homology 3 (SH3) domain of unknown function. A cDNA encoding a candidate spectrin SH3 domain-binding protein was identified by interaction screening of a human brain expression library using the human erythroid spectrin (alphaI) SH3 domain as a bait. Five isoforms of the alphaI SH3 domain-binding protein mRNA were identified in human brain. Mapping of SH3 binding regions revealed the presence of two alphaI SH3 domain binding regions and one Abl-SH3 domain binding region. The gene encoding the candidate spectrin SH3 domain-binding protein has been located to human chromosome 10p11.2 --> p12. The gene belongs to a recently identified family of tyrosine kinase-binding proteins, and one of its isoforms is identical to e3B1, an eps8-binding protein (Biesova, Z., Piccoli, C., and Wong, W. T. (1997)Oncogene 14, 233-241). Overexpression of the green fluorescent protein fusion of the SH3 domain-binding protein in NIH3T3 cells resulted in cytoplasmic punctate fluorescence characteristic of the reticulovesicular system. This fluorescence pattern was similar to that obtained with the anti-human erythroid spectrin alphaI SigmaI/betaI SigmaI antibody in untransfected NIH3T3 cells; in addition, the anti-alphaI SigmaI/betaI SigmaI antibody also stained Golgi apparatus. Immunofluorescence obtained using antibodies against alphaI SigmaI/++betaI SigmaI spectrin and Abl tyrosine kinase but not against alphaII/betaII spectrin colocalized with the overexpressed green fluorescent protein-SH3-binding protein. Based on the conservation of the spectrin SH3 binding site within members of this protein family and published interactions, a general mechanism of interactions of tyrosine kinases with the spectrin-based membrane skeleton is proposed.

An alternative first exon in the distal end of the erythroid ankyrin gene leads to production of a small isoform containing an NH2-terminal membrane anchor

Mouse erythroid ankyrin is encoded by the Ank1 gene on Chromosome 8. The best studied isoform is 210 kDa and contains three large functional domains. We have recently reported a small Ank1 isoform (relative mobility 25 kDa) that localizes to the M and Z lines in skeletal muscle. Analyses of cDNA and genomic clones show that three transcripts of 3.5, 2.0, and 1.6 kb code for this protein. The different transcript sizes are due to their 3'-untranslated regions. They are encoded by a new first exon located in intron 39 of the Ank1 gene and three previously described Ank1 exons (40, 41, and 42). The 5'-flanking region contains a putative muscle-specific promoter. The sequence of the first 72 amino acids is novel and is predicted to form a transmembrane helix at the NH2-terminus. Functional testing of the putative transmembrane segment indicates that it acts as a membrane anchor, suggesting that the new Ank1 isoform may play an important role in organizing the contractile apparatus within the cell.

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.