Comparison of the intracellular trafficking of two alternatively spliced isoforms of pp120, a substrate of the insulin receptor tyrosine kinase

pp120, a substrate of the insulin receptor tyrosine kinase, is a plasma membrane glycoprotein in the hepatocyte. It is expressed as two spliced isoforms differing by the presence (full length) or absence (truncated) of most of the intracellular domain including all phosphorylation sites. Because the two isoforms differ by their ability to regulate receptor‐mediated insulin endocytosis and degradation, we aimed to investigate the cellular basis for this functional difference by comparing their intracellular trafficking. During its intracellular assembly, pp120 is transported from the trans‐Golgi network to the sinusoidal domain of the plasma membrane before its final transcytosis to the bile canalicular domain. Because both isoforms are expressed in hepatocytes, we examined their intracellular trafficking in NIH 3T3 fibroblasts individually transfected with each isoform. Pulse‐chase experiments demonstrated that most of the newly synthesized full‐length isoform reached complete maturation at about 60 min of chase. By contrast, only about 40% of the newly synthesized truncated isoform underwent complete maturation, even at more prolonged chase. Moreover, a significant portion of the truncated isoform appeared to be targeted to lysosomes. Abolishing basal phosphorylation on Ser⁵⁰³ by cAMP‐dependent serine kinase by mutating this residue to alanine was correlated with incomplete maturation of full length pp120 in NIH 3T3 cells and hepatocytes. This finding suggests that the intracellular domain of pp120 contains information that regulates its vectorial sorting from the trans‐Golgi network to the plasma membrane. J. Cell. Biochem. 76:133–142, 1999. © 1999 Wiley‐Liss, Inc.     

Cell Adhesion Properties and Effects on Receptor-Mediated Insulin Endocytosis Are Independent Properties of pp120, a Substrate of the Insulin Receptor Tyrosine Kinase

pp120 undergoes phosphorylation by the tyrosine kinase of the insulin, not the insulin-like growth factor 1 (IGF-1), receptor. Moreover, pp120 stimulates receptor-mediated insulin, but not IGF-1, endocytosis, suggesting that pp120 phosphorylation underlies its effect on insulin endocytosis. pp120 phosphorylation also underlies its bile acid transport and tumor suppression functions. In addition to depending on the intracellular tail, the cell adhesion property of pp120 depends on Arg⁹⁸ in the N-terminal IgV-like ectoplasmic domain. To investigate whether this domain mediates the effect of pp120 on insulin endocytosis, we mutated Arg⁹⁸ to Ala and examined whether this mutation altered pp120 phosphorylation and its effect on ligand endocytosis in transfected NIH 3T3 cells. This mutation did not modify either pp120 phosphorylation or its effect on receptor-mediated ligand endocytosis. These findings support the hypothesis that stimulation of insulin endocytosis by pp120 is not mediated by Arg⁹⁸ in the N-terminal IgV-like ectoplasmic domain of pp120.     

The localization of FGFR3 mutations causing thanatophoric dysplasia type I differentially affects phosphorylation, processing and ubiquitylation of the receptor

Recurrent missense fibroblast growth factor receptor 3 (FGFR3) mutations have been ascribed to skeletal dysplasias of variable severity including the lethal neonatal thanatophoric dysplasia types I (TDI) and II (TDII). To elucidate the role of activating mutations causing TDI on receptor trafficking and endocytosis, a series of four mutants located in different domains of the receptor were generated and transiently expressed. The putatively elongated X807R receptor was identified as three isoforms. The fully glycosylated mature isoform was constitutively but mildly phosphorylated. Similarly, mutations affecting the extracellular domain (R248C and Y373C) induced moderate constitutive receptor phosphorylation. By contrast, the K650M mutation affecting the tyrosine kinase 2 (TK2) domain produced heavy phosphorylation of the nonglycosylated and mannose-rich isoforms that impaired receptor trafficking through the Golgi network. This resulted in defective expression of the mature isoform at the cell surface. Normal processing was rescued by tyrosine kinase inhibitor treatment. Internalization of the R248C and Y373C mutant receptors, which form stable disulfide-bonded dimers at the cell surface was less efficient than the wild-type, whereas ubiquitylation was markedly increased but apparently independent of the E3 ubiquitin-ligase casitas B-lineage lymphoma (c-Cbl). Constitutive phosphorylation of c-Cbl by the K650M mutant appeared to be related to the intracellular retention of the receptor. Therefore, although mutation K650M affecting the TK2 domain induces defective targeting of the overphosphorylated receptor, a different mechanism characterized by receptor retention at the plasma membrane, excessive ubiquitylation and reduced degradation results from mutations that affect the extracellular domain and the stop codon.     

The regulatory or phosphorylation domain of p120 catenin controls E-cadherin dynamics at the plasma membrane

In contrast to growth factor-stimulated tyrosine phosphorylation of p120, its relatively constitutive serine/threonine phosphorylation is not well understood. Here we examined the role of serine/threonine phosphorylation of p120 in cadherin function. Expression of cadherins in cadherin-null cells converted them to an epithelial phenotype, induced p120 phosphorylation and localized it to sites of cell contact. Detergent solubility and immunofluorescence confirmed that phosphorylated p120 was at the plasma membrane. E-cadherin constructs incapable of traveling to the plasma membrane did not induce serine/threonine phosphorylation of p120, nor did cadherins constructs incapable of binding p120. However, an E-cadherin cytoplasmic domain construct artificially targeted to the plasma membrane did induce serine/threonine phosphorylation of p120, suggesting phosphorylation occurs independently of signals from cadherin dimerization and trafficking through the ER/Golgi. Solubility assays following calcium switch showed that p120 isoform 3A was more effective at stabilizing E-cadherin at the plasma membrane relative to isoform 4A. Since the major phosphorylation domain of p120 is included in isoform 3A but not 4A, we tested p120 mutated in the known phosphorylation sites in this domain and found that it was even less effective at stabilizing E-cadherin. These data suggest that serine/threonine phosphorylation of p120 influences the dynamics of E-cadherin in junctions.     

Shc and CEACAM1 interact to regulate the mitogenic action of insulin.

CEACAM1, a tumor suppressor (previously known as pp120), is a plasma membrane protein that undergoes phosphorylation on Tyr(488) in its cytoplasmic tail by the insulin receptor tyrosine kinase. Co-expression of CEACAM1 with insulin receptors decreased cell growth in response to insulin. Co-immunoprecipitation experiments in intact NIH 3T3 cells and glutathione S-transferase pull-down assays revealed that phosphorylated Tyr(488) in CEACAM1 binds to the SH2 domain of Shc, another substrate of the insulin receptor. Overexpressing Shc SH2 domain relieved endogenous Shc from binding to CEACAM1 and restored MAP kinase activity, growth of cells in response to insulin, and their colonization in soft agar. Thus, by binding to Shc, CEACAM1 sequesters this major coupler of Grb2 to the insulin receptor and down-regulates the Ras/MAP kinase mitogenesis pathway. Additionally, CEACAM1 binding to Shc enhances its ability to compete with IRS-1 for phosphorylation by the insulin receptor. This leads to a decrease in IRS-1 binding to phosphoinositide 3'-kinase and to the down-regulation of the phosphoinositide 3'-kinase/Akt pathway that mediates cell proliferation and survival. Thus, binding to Shc appears to constitute a major mechanism for the down-regulatory effect of CEACAM1 on cell proliferation.     

Differential Phosphorylation of Myelin-Associated Glycoprotein Isoforms in Cell Culture

The alternative splicing of myelin-associated glycoprotein (MAG) mRNA generates two isoforms that harbor distinct potential phosphorylation sites in their cytoplasmic tails. Here we characterize the in vivo phosphorylation of MAG isoforms in NIH 3T3 cells transfected with the cDNAs encoding the two isoforms of MAG. Our results demonstrate that the longer isoform, L-MAG, is phosphorylated constitutively mainly on serine, but also on threonine and tyrosine residues. This phosphorylation is subject to change by 12-O-tetradecanoylphorbol 13-acetate (TPA) and ammonium vanadate, but not by dibutyryl-cyclic AMP. The shorter isoform, S-MAG, is constitutively phosphorylated only on serine residues. While TPA and dibutyryl-cyclic AMP have no detectable effect, ammonium vanadate induces tyrosine and threonine phosphorylation in S-MAG. 32P labeling of v-src-transformed NIH 3T3 cells that express L-MAG also show that L-MAG is likely to be an in vivo substrate for pp60v-src tyrosine kinase activity. These results demonstrate that both MAG isoforms are phosphorylated in a heterologous cell system and that this phosphorylation is subject to pharmacological manipulation.     

The differential effects of pp120 (Ceacam 1) on the mitogenic action of insulin and insulin-like growth factor 1 are regulated by the nonconserved tyrosine 1316 in the insulin receptor

pp120 (Ceacam 1) undergoes ligand-stimulated phosphorylation by the insulin receptor, but not by the insulin-like growth factor 1 receptor (IGF-1R). This differential phosphorylation is regulated by the C terminus of the beta-subunit of the insulin receptor, the least conserved domain of the two receptors. In the present studies, deletion and site-directed mutagenesis in stably transfected hepatocytes derived from insulin receptor knockout mice (IR(-/-)) revealed that Tyr(1316), which is replaced by the nonphosphorylatable phenylalanine in IGF-1R, regulated the differential phosphorylation of pp120 by the insulin receptor. Similarly, the nonconserved Tyr(1316) residue also regulated the differential effect of pp120 on IGF-1 and insulin mitogenesis, with pp120 downregulating the growth-promoting action of insulin, but not that of IGF-1. Thus, it appears that pp120 phosphorylation by the insulin receptor is required and sufficient to mediate its downregulatory effect on the mitogenic action of insulin. Furthermore, the current studies revealed that the C terminus of the beta-subunit of the insulin receptor contains elements that suppress the mitogenic action of insulin. Because IR(-/-) hepatocytes are derived from liver, an insulin-targeted tissue, our observations have finally resolved the controversy about the role of the least-conserved domain of insulin and IGF-1Rs in mediating the difference in the mitogenic action of their ligands, with IGF-1 being more mitogenic than insulin.     

Phosphatidylinositol 3-kinase activation is required for insulin stimulation of pp70 S6 kinase, DNA synthesis, and glucose transporter translocation.

Phosphatidylinositol 3-kinase (PI 3-kinase) is stimulated by insulin and a variety of growth factors, but its exact role in signal transduction remains unclear. We have used a novel, highly specific inhibitor of PT 3-kinase to dissect the role of this enzyme in insulin action. Treatment of intact 3T3-L1 adipocytes with LY294002 produced a dose-dependent inhibition of insulin-stimulated PI 3-kinase (50% inhibitory concentration, 6 microM) with > 95% reduction in the levels of phosphatidylinositol-3,4,5-trisphosphate without changes in the levels of phosphatidylinositol-4-monophosphate or its derivatives. In parallel, there was a complete inhibition of insulin-stimulated phosphorylation and activation of pp70 S6 kinase. Inhibition of PI 3-kinase also effectively blocked insulin- and serum-stimulated DNA synthesis and insulin-stimulated glucose uptake by inhibiting translocation of GLUT 4 glucose transporters to the plasma membrane. By contrast, LY294002 had no effect on insulin stimulation of mitogen-activated protein kinase or pp90 S6 kinase. Thus, activation of PI 3-kinase plays a critical role in mammalian cells and is required for activation of pp70 S6 kinase and DNA synthesis and certain forms of intracellular vesicular trafficking but not mitogen-activated protein kinase or pp90 S6 kinase activation. These data suggest that PI 3-kinase is not only an important component but also a point of divergence in the insulin signaling network.     

An endogenous substrate for the insulin receptor-associated tyrosine kinase

Insulin binding to its receptor stimulates a tyrosine-specific protein kinase. This enzyme phosphorylates the insulin receptor, as well as a variety of exogenous substrates in vitro. In the present studies, we have identified an endogenous substrate for the insulin receptor-associated kinase. We studied insulin-stimulated protein phosphorylation in partially purified insulin receptor preparations from the livers of dexamethasone-treated rats. In this cell-free system, insulin stimulated the phosphorylation of its own receptor as well as of a phosphoprotein of apparent Mr = 120,000 (termed pp120). pp120 was not immunoprecipitated by three anti-receptor antisera, nor was the receptor immunoprecipitated by antisera raised against pp120, suggesting that pp120 is not antigenically related or tightly bound to the insulin receptor. Dose-response curves for receptor and pp120 phosphorylation stimulated by pork insulin were essentially identical, and showed the appropriate specificity (insulin much greater than proinsulin) for a receptor-mediated event. Phosphoamino acid analysis revealed that insulin stimulated the incorporation of 32P predominantly into tyrosine residues of pp120. Casein, an artificial substrate for the insulin receptor kinase, competed with pp120 for insulin-stimulated phosphorylation. Phosphorylation of pp120 was rapid (half-maximal effect within 2 min at 24 degrees C) and, like receptor phosphorylation, was supported with Mn2+ or Mg2+ as divalent cation and ATP as the phosphate donor. While receptor autophosphorylation and artificial substrate phosphorylation were not altered by prior treatment of the rats with dexamethasone, insulin-stimulated pp120 phosphorylation was enhanced in preparations derived from dexamethasone-treated rats, suggesting an alteration of pp120, not the receptor, as a result of dexamethasone-treatment. Further studies of this newly identified endogenous substrate may help clarify the physiologic role of the insulin receptor-associated kinase.     

Identification of tyrosines 154 and 307 in the extracellular domain and 653 and 766 in the intracellular domain as phosphorylation sites in the heparin-binding fibroblast growth factor receptor tyrosine kinase (flg).

Four tyrosine residues have been identified as phosphorylation sites in the tyrosine kinase isoform of the heparin-binding fibroblast growth factor receptor flg (FGF-R1). Baculoviral-insect cell-derived recombinant FGF-R1 was phosphorylated and fragmented with trypsin while immobilized on heparin-agarose beads. Phosphotyrosine peptides were purified by chromatography on immobilized anti-phosphotyrosine antibody and analyzed by Edman degradation and electrospray tandem mass spectrometry. Tyrosine residue 653, which is in a homologous spatial position to major autophosphorylation sites in the catalytic domain of the src and insulin receptor kinases, is the major intracellular FGF-R1 phosphorylation site. Residue 766 in the COOH-terminus outside the kinase domain is a secondary site. Tyrosine residues 154 and 307, which are in the extracellular domain of transmembrane receptor isoforms and are in an unusual sequence context for tyrosine phosphorylation, were also phosphorylated.     

Role of the diacylglycerol kinase alpha-conserved domains in membrane targeting in intact T cells

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to phosphatidic acid, modifying the cellular levels of these two lipid mediators. Ten DGK isoforms, grouped into five subtypes, are found in higher organisms. All contain a conserved C-terminal domain and at least two cysteine-rich motifs of unknown function. DGKalpha is a type I enzyme that acts as a negative modulator of diacylglycerol-based signals during T cell activation. Here we studied the functional role of the DGKalpha domains using mutational analysis to investigate membrane binding in intact cells. We show that the two atypical C1 domains are essential for plasma membrane targeting of the protein in intact cells but unnecessary for catalytic activity. We also identify the C-terminal sequence of the protein as essential for membrane binding in a phosphatidic acid-dependent manner. Finally we demonstrate that, in the absence of the calcium binding domain, receptor-dependent translocation of the truncated protein is regulated by phosphorylation of Tyr(335). This functional study provides new insight into the role of the so-called conserved domains of this lipid kinase family and demonstrates the existence of additional domains that confer specific plasma membrane localization to this particular isoform.     

Phosphoinositide-dependent phosphorylation of PDK1 regulates nuclear translocation

3-phosphoinositide-dependent kinase 1 (PDK1) phosphorylates the activation loop of a number of protein serine/threonine kinases of the AGC kinase superfamily, including protein kinase B (PKB; also called Akt), serum and glucocorticoid-induced kinase, protein kinase C isoforms, and the p70 ribosomal S6 kinase. PDK1 contains a carboxyl-terminal pleckstrin homology domain, which targets phosphoinositide lipids at the plasma membrane and is central to the activation of PKB. However, PDK1 subcellular trafficking to other compartments is not well understood. We monitored the posttranslational modifications of PDK1 following insulin-like growth factor 1 stimulation. PDK1 underwent rapid and transient phosphorylation on S396, which was dependent upon plasma membrane localization. Phosphorylation of S396 was necessary for nuclear shuttling of PDK1, possibly through its influence on an adjacent nuclear export sequence. Thus, mitogen-stimulated phosphorylation of PDK1 provides a means for directed PDK1 subcellular trafficking, with potential implications for PDK1 signaling.     

Insulin-stimulated phosphorylation of a Rab GTPase-activating protein regulates GLUT4 translocation

Insulin stimulates the rapid translocation of intracellular glucose transporters of the GLUT4 isotype to the plasma membrane in fat and muscle cells. The connections between known insulin signaling pathways and the protein machinery of this membrane-trafficking process have not been fully defined. Recently, we identified a 160-kDa protein in adipocytes, designated AS160, that is phosphorylated by the insulin-activated kinase Akt. This protein contains a GTPase-activating domain (GAP) for Rabs, which are small G proteins required for membrane trafficking. In the present study we have identified six sites of in vivo phosphorylation on AS160. These sites lie in the motif characteristic of Akt phosphorylation, and insulin treatment increased phosphorylation at five of the sites. Expression of AS160 with two or more of these sites mutated to alanine markedly inhibited insulin-stimulated GLUT4 translocation in 3T3-L1 adipocytes. Moreover, this inhibition did not occur when the GAP function in the phosphorylation site mutant was inactivated by a point mutation. These findings strongly indicate that insulin-stimulated phosphorylation of AS160 is required for GLUT4 translocation and that this phosphorylation signals translocation through inactivation of the Rab GAP function.     

Intracellular targeting of the insulin-regulatable glucose transporter (GLUT4) is isoform specific and independent of cell type

Insulin stimulates glucose transport in adipocytes via the rapid redistribution of the GLUT1 and GLUT4 glucose transporters from intracellular membrane compartments to the cell surface. Insulin sensitivity is dependent on the proper intracellular trafficking of the glucose transporters in the basal state. The bulk of insulin-sensitive transport in adipocytes appears to be due to the translocation of GLUT4, which is more efficiently sequestered inside the cell and is present in much greater abundance than GLUT1. The cell type and isoform specificity of GLUT4 intracellular targeting were investigated by examining the subcellular distribution of GLUT1 and GLUT4 in cell types that are refractory to the effect of insulin on glucose transport. Rat GLUT4 was expressed in 3T3-L1 fibroblasts and HepG2 hepatoma cells by DNA-mediated transfection. Transfected 3T3-L1 fibroblasts over-expressing human GLUT1 exhibited increased glucose transport, and laser confocal immunofluorescent imaging of GLUT1 in these cells indicated that the protein was concentrated in the plasma membrane. In contrast, 3T3-L1 fibroblasts expressing GLUT4 exhibited no increase in transport activity, and confocal imaging demonstrated that this protein was targeted almost exclusively to cytoplasmic compartments. 3T3-L1 fibroblasts expressing GLUT4 were unresponsive to insulin with respect to transport activity, and no change was observed in the subcellular distribution of the protein after insulin administration. Immunogold labeling of frozen ultrathin sections revealed that GLUT4 was concentrated in tubulo-vesicular elements of the trans-Golgi reticulum in these cells. Sucrose density gradient analysis of 3T3-L1 homogenates was consistent with the presence of GLUT1 and GLUT4 in discrete cytoplasmic compartments. Immunogold labeling of frozen thin sections of HepG2 cells indicated that endogenous GLUT1 was heavily concentrated in the plasma membrane. Sucrose density gradient analysis of homogenates of HepG2 cells expressing rat GLUT4 suggested that GLUT4 is targeted to an intracellular location in these cells. The density of the putative GLUT4-containing cytoplasmic membrane vesicles was very similar in HepG2 cells, 3T3-L1 fibroblasts, 3T3-L1 adipocytes, and rat adipocytes. These data indicate that the intracellular trafficking of GLUT4 is isoform specific. Additionally, these observations support the notion that GLUT4 is targeted to its proper intracellular locale even in cell types that do not exhibit insulin-responsive glucose transport, and suggest that the machinery that regulates the intracellular targeting of GLUT4 is distinct from the factors that regulate insulin-dependent recruitment to the cell surface.     

Substrates and signalling complexes: the tortured path to insulin action.

In the last few years several potential substrates of the insulin receptor tyrosine kinase have been identified, purified, and their cDNAs isolated. These putative substrates include: 1) pp15, a fatty acid-binding protein; 2) pp120, a plasma membrane ecto-ATPase; 3) pp42, a MAP serine/threonine kinase; 4) pp85, a subunit of the Type 1 phosphatidylinositol kinase; and 5) pp185, a phosphatidylinositol kinase binding protein. Although the tyrosine phosphorylation of several of these substrates correlates with the signalling capabilities of various mutant receptors, the role of these substrates in mediating any one of insulin's many biological responses is still unknown. In addition, recent data indicate that the tyrosine phosphorylation of pp42 may in fact be due to autophosphorylation, thereby removing it from the list of putative substrates of the insulin receptor kinase. Finally, the present review discusses the question of whether signalling occurs as a result of the tyrosine phosphorylation of substrates or via the formation of signalling complexes.     

Phosphatidylserine is involved in the ferrichrome-induced plasma membrane trafficking of Arn1 in Saccharomyces cerevisiae

Arn1 is an integral membrane protein that mediates the uptake of ferrichrome, an important nutritional source of iron, in Saccharomyces cerevisiae. In the absence of ferrichrome, Arn1p is sorted directly from the trans-Golgi network to the vacuolar lumen for degradation. In the presence of low levels of ferrichrome, the siderophore binds to a receptor domain on Arn1, triggering the redistribution of Arn1 to the plasma membrane. When extracellular ferrichrome levels are high, Arn1 cycles between the plasma membrane and intracellular vesicles. To further understand the mechanisms of trafficking of Arn1p, we screened 4580 viable yeast deletion mutants for mislocalization of Arn1-GFP using synthetic genetic array technology. We identified over 100 genes required for trans-Golgi network-to-vacuole trafficking of Arn1-GFP and only two genes, SER1 and SER2, required for the ferrichrome-induced plasma membrane trafficking of Arn1-GFP. SER1 and SER2 encode two enzymes of the major serine biosynthetic pathway, and the Arn1 trafficking defect in the ser1Δ strain was corrected with supplemental serine or glycine. Plasma membrane trafficking of Hxt3, a structurally related glucose transporter, was unaffected by SER1 deletion. Serine is required for the synthesis of multiple cellular components, including purines, sphingolipids, and phospholipids, but of these only phosphatidylserine corrected the Arn1 trafficking defects of the ser1Δ strain. Strains with defects in phospholipid synthesis also exhibited alterations in Arn1p trafficking, indicating that the intracellular trafficking of some transporters is dependent on the phospholipid composition of the cellular membranes.     

Casein Kinase Iγ2 Down-Regulates Trafficking of Ceramide in the Synthesis of Sphingomyelin

Intracellullar trafficking of lipids is fundamental to membrane biogenesis. For the synthesis of sphingomyelin, ceramide is transported from the endoplasmic reticulum to the Golgi apparatus by the ceramide transfer protein CERT. CERT is phosphorylated by protein kinase D at S132 and subsequently multiple times in a serine-repeat motif, resulting in its inactivation. However, the kinase involved in the multiple phosphorylation remains unclear. Here, we identify the γ2 isoform of casein kinase I (CKIγ2) as a kinase whose overexpression confers sphingomyelin-directed toxin-resistance to Chinese hamster ovary cells. In a transformant stably expressing CKIγ2, CERT was hyperphosphorylated, and the intracellular trafficking of ceramide was retarded, thereby reducing de novo sphingomyelin synthesis. The reduction in the synthesis of sphingomyelin caused by CKIγ2 was reversed by the expression of CERT mutants that are not hyperphosphorylated. Furthermore, CKIγ2 directly phosphorylated CERT in vitro. Among three γ isoforms, only knockdown of γ2 isoform caused drastic changes in the ratio of hypo- to hyperphosphorylated form of CERT in HeLa cells. These results indicate that CKIγ2 hyperphosphorylates the serine-repeat motif of CERT, thereby inactivating CERT and down-regulating the synthesis of sphingomyelin.     

Differential effects of over-expressed neural cell adhesion molecule isoforms on myoblast fusion

We have used a transfection based approach to analyze the role of neural cell adhesion molecule (NCAM) in myogenesis at the stage of myoblast fusion to form multinucleate myotubes. Stable cell lines of myogenic C2 cells were isolated that express the transmembrane 140- or 180-kD NCAM isoforms or the glycosylphosphatidylinositol (GPI) linked isoforms of 120 or 125 kD. We found that expression of the 140-kD transmembrane isoform led to a potent enhancement of myoblast fusion. The 125-kD GPI-linked NCAM also enhanced the rate of fusion but less so when a direct comparison of cell surface levels of the 140-kD transmembrane form was carried out. While the 180-kD transmembrane NCAM isoform was effective in promoting C2 cell fusion similar to the 140-kD isoform, the 120-kD isoform did not have an effect on fusion parameters. It is possible that these alterations in cell fusion are associated with cis NCAM interactions in the plane of the membrane. While all of the transfected human NCAMs (the transmembrane 140- and 180-kD isoforms and the 125- and 120-kD GPI isoforms) could be clustered in the plane of the plasma membrane by species-specific antibodies there was a concomitant clustering of the endogenous mouse NCAM protein in all cases except with the 120-kD human isoform. These studies show that different isoforms of NCAM can undergo specific interactions in the plasma membrane which are likely to be important in fusion. While the transmembrane and the 125-kD GPI-anchored NCAMs are capable of enhancing fusion the 120-kD GPI NCAM is not. Thus it is likely that interactions associated with NCAM intracellular domains and also the muscle specific domain (MSD) region in the extracellular domain of the GPI-linked 125-kD NCAM are important. In particular this is the first role ascribed to the O-linked carbohydrate containing MSD region which is specifically expressed in skeletal muscle.