We have characterized a new ankyrin gene, expressed in brain and other tissues, that is subject to extensive tissue-specific alternative mRNA processing. The full-length polypeptide has a molecular mass of 480 kDa and includes a predicted globular head domain, with membrane- and spectrin-binding activities, as well as an extended "tail" domain. We term this gene ankyrinG based on its giant size and general expression. Two brain-specific isoforms of 480 kDa and 270 kDa were identified that contain a unique stretch of sequence highly enriched in serine and threonine residues immediately following the globular head domain. Antibodies against the serine-rich domain and spectrin-binding domain revealed labeling of nodes of Ranvier and axonal initial segments. Ankyrin-binding proteins also known to be localized in these specialized membrane domains include the voltage-dependent sodium channel, the sodium/potassium ATPase, sodium/calcium exchanger, and members of the neurofascin/L1 family of cell adhesion molecules. The neural-specific ankyrinG polypeptides are candidates to participate in maintenance/targeting of ion channels and cell adhesion molecules to nodes of Ranvier and axonal initial segments.

Abstract. Two variants of ankyrin have been distinguished in rat brain tissue using antibodies: a broadly distributed isoform (ankyrinB) that represents the major form of ankyrin in brain and another isoform with a restricted distribution (ankyrinR) that shares epitopes with erythrocyte ankyrin. The ankyrinR isoform was localized by immunofluorescence in cryosections of rat spinal cord gray matter and myelinated central and peripheral nerves to: (a) perikarya and initial axonal segments of neuron cells, (b) nodes of Ranvier of myelinated nerve with no detectable labeling in other areas of the myelinated axons, and (c) the axolemma of unmyelinated axons. Immunogold EM on ultrathin cryosections of myelinated nerve showed that ankyrinR was localized on the cytoplasmic face of the axolemma and was restricted to the nodal and, in some cases, paranodal area. The major isoform of ankyrin in brain (ankyrinB) displayed a broad distribution on glial and neuronal cells of the gray matter and a mainly glial distribution in central myelinated axons with no significant labeling on the axolemma. These results show that (a) ankyrin isoforms display a differential distribution on glial and neuronal cells of the nervous tissue; (b) an isoform of ankyrin codistributes with the voltage-dependent sodium channel in both myelinated and unmyelinated nerve fibers. Ankyrin interacts in vitro with the voltage-dependent sodium channel (Srinivasan, Y., L. Elmer, J. Davis, V. Bennett, and K. Angelides. 1988. Nature (Lond.). 333:177-180). A specific interaction of an isoform of ankyrin with the sodium channel thus may play an important role in the morphogenesis and/or maintenance of the node of Ranvier.

Abstract. 440-kD ankyrinB is an alternatively spliced variant of 220-kD ankyrins, with a predicted 220-kD sequence inserted between the membrane/spectrin binding domains and COOH-terminal domain (Kunimoto, M., E. Otto, and V. . J. Cell Biol. 236:1372-1379. This paper presents the sequence of 2085 amino acids comprising the alternatively spliced portion of 440-kD ankyrine, and provides evidence that much of the inserted sequence has the configuration of an extended random coil. Notable features of the inserted sequence include a hydrophilicity profile that contains few hydrophobic regions, and 220 predicted sites for phosphorylation by protein kinases (casein kinase 2, protein kinase C, and prolinedirected protein kinase). Secondary structure and folding of the inserted amino acid residues were deduced from properties of recombinant polypeptides. Frictional ratios of 1.9-2.4 were calculated from Stokes radii and sedimentation coefficients, for polypeptides comprising 70% of the inserted sequence, indicating a highly asymmetric shape. Circular dichroism spectra of these polypeptides indicate a nonglobular structure with negligible a-helix or/3 sheet folding. These results suggest a ball-and-chain model for 440-kD ankyrine with a membrane-associated globular head domain and an extended filamentous tail domain encoded by the inserted sequence. Immunofluorescence and immunoblot studies of developing neonatal rat optic nerve indicate that 440-kD ankyrins is selectively targeted to premyelinated axons, and that 440-kD ankyrine disappears from these axons coincident with myelination. Hypomyelinated nerve tracts of the myelin-deficient Shiverer mice exhibit elevated levels of 440-kD ankyrins. 440-kD ankyrins thus is a specific component of unmyelinated axons and expression of 440-kD ankyrins may be downregulated as a consequence of myelination.KYRINS are a family of spectrin-binding proteins that link the spectrin/actin network to cytoplasmic domains of integral proteins that include ion channels and cell adhesion molecules (Bennett, 1992; Bennett and Giliigan, 1993;Davis et al., 1993). Ankyrins contain three structural domains: (a) an NH2-terminal 89-95-kD membrane-binding domain (Davis and Bennett, 1990a); (b) a 62-kD domain that binds to spectrin (Bennett, 1978); and (c) a COOH-terminai domain that is the target of alternative splicing and represents the most variable domain among different ankyrins. A striking feature of the membranebinding domain is the presence of 24 tandem repeats of 33 amino acids. The 33-residue repeats are necessary and sufliciem for association of ankyrin with the anion exchanger (Davis and Bennett, 1990b;Davis et al., 1991), the voltagedependent sodium channel (Srinivasan et al., 1992), and nervous system cell adhesion molecules related to L1 and neurofascin (Davis et al., 1993).

Abstract. Isoforms of ankyrin (ankyrinSR) immunologically related to erythrocyte ankyrin (ankyrrnno) are associated with distinct neuronal plasma membrane domains of functional importance, such as cell bodies and dendrites, axonal hillock and initial segments, and nodes of Ranvier. Ankyrrnno is expressed in brain, and accounts for at least one of the ankyrinR isoforms . Another ankyrin isoform of brain, ankyrinB, is encoded by a distinct gene and is immunologically distinct from ankyrins R. Mutant mice with normoblastosis (nb/nb) constitute the first described genetic model of ankyrin deficiency : they display a severe hemolytic anemia due to a significantly reduced expression of the ankyrrnno gene in reticulocytes as well as brain

Abstract. The subcellular distribution of the 43,000-D protein (43 kD or u~) and of some major cytoskeletal proteins was investigated in Torpedo marmorata electrocytes by immunocytochemical methods (immunofluorescence and immunogold at the electron microscope level) on frozen-fixed sections and homogenates of electric tissue. A monoclonal antibody directed against the 43-kD protein (Nghirm, H. O., J. Cartaud, C. Dubreuil, C. Kordeli, G. Buttin, and J. P. Changeux, 1983, Proc. Natl. Acad. Sci. USA, 80:6403-6407), selectively labeled the postsynaptic membrane on its cytoplasmic face. Staining by anti-actin and anti-desmin antibodies appeared evenly distributed within the cytoplasm: anti-desmin antibodies being associated with the network of intermediate-sized filaments that spans the electrocyte, and anti-actin antibodies making scattered clusters throughout the cytoplasm without preferential labeling of the postsynaptic membrane. On the other hand, a dense coating by anti-actin antibodies became apparent on the postsynaptic membrane in homogenates of electric tissue pointing to the possible artifactual redistribution of a soluble cytoplasmic actin pool.Anti-fodrin and anti-ankyrin antibodies selectively labeled the non-innervated membrane of the cell. F actin was also detected in this membrane. Filamin and vinculin, two actin-binding proteins recently localized at the rat neuromuscular junction (Bloch, R.J., and Z. W. Hall, 1983, J. CellBiol., 97:217-223), were detected in the electrocyte by the immunoblot technique but not by immunocytochemistry.The data are interpreted in terms of the functional polarity of the electrocyte and of the selective interaction of the cytoskeleton with the innervated and noninnervated domains of the plasma membrane.T HE postsynaptic membrane of the neuromuscular junction and of the electromotor synapse corresponds to a local differentiation of the plasma membrane characterized by an accumulation of the nicotinic acetylcholine receptor (Ach-R).t This membrane specialization persists for a long period after denervation (reviewed in references 10 and 18), indicating that physical constraints maintain the Ach-R molecules in place and, in particular, prevent against their lateral diffusion. Interactions of the Ach-R with extrinsic components from the extracellular matrix (13, 55) and/or with the cytoskeleton (17, 48, 67) have been postulated to contribute to this differentiation process. cell--the non-innervated face--is specialized in the regeneration of the electrochemical gradient (40). The Ach-R is present exclusively on the innervated membrane, and Na ÷ K ÷ ATPase accumulates on the non-innervated one. These membranes thus constitute fully differentiated and stable domains of the plasma membrane. From this standpoint the diskshaped electrocyte shows a striking functional and structural polarity ( 18, 21).Postsynaptic membrane fractions purified from Torpedo comprise essentially the intrinsic Ach-R polypeptides plus few extrinsic components of apparent molecular mass 43 kD (63, 64) na...

Subsynaptic membrane fragments prepared from Torpedo marmorata electric organ contain, in addition to the acetylcholine receptor polypeptides, a major protein band of apparent molecular mass 43,000 daltons. On two-dimensional gels, this band yields three spots referred to as v1, v2, and V3. Monoclonal antibodies against the 43,000-dalton proteins were developed in CBA mice. One of them reacted exclusively with the vP polypeptide but not with p2 and v3. Staining by the "immunogold" reaction followed by observation by electron microscopy showed that this antibody exclusively labeled the innervated membrane of T. marmorata electroplaque on its cytoplasmic face. Electroblots of onedimensional gels of membrane preparations from 80-mm embryo electric organ were prepared. After reaction with the anti-vP monoclonal antibody, a strongly stained 43,000-dalton band was revealed.The acetylcholine receptor (AcChoR) protein (1) makes densely packed and stable aggregates in the subsynaptic membrane of the adult neuromuscular junction and of the electromotor synapse. This supramolecular organization develops during embryogenesis from a diffuse and labile distribution of AcChoR molecules present in the membrane of the noninnervated myotube or electrocyte. The mechanisms postulated to account for this important morphogenetic step (for refs., see ref.

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 crossreacted 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.

Voltage-dependant sodium channels at the axon initial segment and nodes of Ranvier colocalize with the nodal isoforms of ankyrin G (Ank G node). Using fusion proteins derived from the intracellular regions of the Nav1.2a subunit and the Ank repeat domain of Ank G node, we mapped a major interaction site in the intracellular loop separating ␣ subunit domains I-II. This 57-amino acid region binds the Ank repeat region with a K D value of 69 nM. We identified another site in intracellular loop III-IV, and we mapped both Nav1.2a binding sites on the ankyrin repeat domain to the region encompassing repeats 12-22. The ankyrin repeat domain did not bind the ␤ 1 and ␤ 2 subunit cytoplasmic regions. We showed that in cultured embryonic motoneurons, expression of the ␤ 2 subunit is not necessary for the colocalization of Ank G node with functional sodium channels at the axon initial segment. Antibodies directed against the ␤ 1 subunit intracellular region, ␣ subunit loop III-IV, and Ank G node could not co-immunoprecipitate Ank G node and sodium channels from Triton X-100 solubilisates of rat brain synaptosomes. Co-immunoprecipitation of sodium channel ␣ subunit and of the 270-and 480-kDa AnkG node isoforms was obtained when solubilization conditions that maximize membrane protein extraction were used. However, we could not find conditions that allowed for co-immunoprecipitation of ankyrin with the sodium channel ␤ 1 subunit.