Identification of the full-length AE2 (AE2a) isoform as the Golgi-associated anion exchanger in fibroblasts.

Na(+)-independent Cl(-)/HCO(3)(-) exchangers (AE1, AE2, AE3) are generally known as ubiquitous, multispanning plasma membrane proteins that regulate intracellular pH and transepithelial acid-base balance in animal tissues. However, previous immunological evidence has suggested that anion exchanger (AE) proteins may also be present in intracellular membranes, including membranes of the Golgi complex and mitochondria. Here we provide several lines of evidence to show that an AE protein is indeed a resident of the Golgi membranes and that this protein corresponds to the full-length AE2a isoform in fibroblasts. First, both the N- and C-terminal antibodies to AE2 (but not to AE1) detected an AE protein in the Golgi membranes. Golgi localization of this AE2 antigen was evident also in cycloheximide-treated cells, indicating that it is a true Golgi-resident protein. Second, our Northern blotting and RT-PCR analyses demonstrated the presence of only the full-length AE2a mRNA in cells that show prominent Golgi staining with antibodies to AE2. Third, antisense oligonucleotides directed against the translational initiation site of the AE2a mRNA markedly inhibited the expression of the endogenous AE2 protein in the Golgi. Finally, transient expression of the GFP-tagged full-length AE2a protein resulted in predominant accumulation of the fusion protein in the Golgi membranes in COS-7 and CHO-K1 cells. Golgi localization of the AE2a probably involves its oligomerization and/or association with the recently identified Golgi membrane skeleton, because a substantial portion of both the endogenous AE2a and the GFP-tagged fusion protein resisted detergent extraction in cold. (J Histochem Cytochem 49:259-269, 2001)     

Targeting of the AE2 anion exchanger to the Golgi apparatus is cell type-dependent and correlates with the expression of Ank(195), a Golgi membrane skeletal protein

Sodium-independent anion exchangers (AE1-4) show remarkable variability in their tissue-specific expression and subcellular localization. Currently, isoform-specific targeting mechanisms are considered to be responsible for this variable localization. Here, we report that targeting can also be cell type-specific. We show that the full-length AE2 protein and its green fluorescent protein- or DsRed-tagged variants localize predominantly either to the Golgi apparatus in COS-7 cells, or to the plasma membrane in HeLa cells. This alternative targeting did not seem to result from either translational or post-translational differences, but rather from differential expression of at least one of the Golgi membrane skeletal proteins, ankyrin(195) (Ank(195)), between the two cell types. Comparative studies with several different cell lines revealed that the Golgi localization of the AE2 protein correlated strictly with the expression of Ank(195) in the cells. The two Golgi-associated proteins also co-localized well and similarly resisted detergent extraction in the cold, whereas the plasma membrane-localized AE2 in Ank(195)-deficient cells was mostly detergent-soluble. Collectively, our results suggest that Ank(195) expression is a key determinant for the variable and cell type-dependent localization of the AE2 protein in the Golgi apparatus in mammalian cells.     

The membrane transport factor p115 recycles only between homologous compartments in intact heterokaryons

Cytosolic proteins that participate in membrane traffic are assumed to be recruited from the cytosol onto specific membrane sites where they perform their function, and then released into cytosol before rebinding to catalyze another round of transport. To examine whether the ER to Golgi transport factor p115 recycles through release into a cytosolic pool, we formed heterokaryons between rat NRK and simian COS-7 cells and examined the dynamics of rat p115 transfer from the rat to the simian portion of the heterokaryon. The heterokaryons shared a common cytosolic pool, as shown by the efficient relocation of a cytosolic green fluorescent protein (GFP) from the COS-7 to the NRK part of the heterokaryon. Unexpectedly, even 24 h after cell fusion, rat p115 did not redistribute to the COS-7 part of the heterokaryon. This was not due to the inability of the rat p115 to associate with simian membranes since rat p115 expressed in COS-7 cells was efficiently targeted to and associated with simian Golgi complex. Furthermore, rat p115 associated with heterologous simian membranes after the NRK and COS-7 Golgi fused into a single chimeric structure. Our results indicate that p115 is not freely diffusible in intact cells and might remain tethered to membranes throughout its life cycle. These findings suggest that p115, and perhaps other cytosolic proteins involved in membrane traffic, recycle not by being released into cytosol, but in association with recycling membranes.     

The signal for Golgi retention of bovine beta 1,4-galactosyltransferase is in the transmembrane domain.

The expression and localization of bovine beta 1,4-galactosyltransferase (Gal T) has been studied in mammalian cells transfected with Gal T cDNA constructs, and the role of the amino-terminal domains of Gal T in Golgi localization examined. Here we demonstrate that the transmembrane (signal/anchor) domain of bovine Gal T contains a positive Golgi retention signal. Bovine Gal T was characterized in transfected cells with anti-bovine Gal T antibodies, affinity-purified from a rabbit antiserum using a bacterial recombinant fusion protein. These affinity-purified antibodies recognized native bovine Gal T and showed minimum cross-reactivity with Gal T from non-bovine sources. Bovine Gal T cDNA was expressed, as active enzyme, transiently in COS-1 cells and stably in murine L cells, and the product was shown to be localized to the Golgi complex by immunofluorescence using the polypeptide-specific antibodies. A low level of surface bovine Gal T was also detected in the transfected L cells by flow cytometry. The removal of 18 of the 24 amino acids from the cytoplasmic domain of bovine Gal T did not alter the Golgi localization of the product transiently expressed in COS-1 cells or stably expressed in L cells. Both the full-length bovine Gal T and the cytoplasmic domain deletion mutant were N-glycosylated in the transfected L cells, indicating both proteins have the correct N(in)/C(out) membrane orientation. Deletion of both the cytoplasmic and signal/anchor domains of bovine Gal T and incorporation of a cleavable signal sequence resulted in a truncated soluble bovine Gal T that was rapidly secreted (within 1 h) from transfected COS-1 cells. Replacement of the signal/anchor domain of bovine Gal T with the signal/anchor domain of the human transferrin receptor resulted in the transport of the hybrid molecule to the cell surface of transfected COS-1 cells. Furthermore, a hybrid construct containing the signal/anchor domain of Gal T with ovalbumin was efficiently retained in the Golgi complex, whereas ovalbumin anchored to the membrane by the transferrin receptor signal/anchor was expressed at the cell surface of transfected COS-1 cells. Overall, these studies show that the hydrophobic, signal/anchor domain of Gal T is both necessary and sufficient for Golgi localization.     

Association of TAG-1 with Caspr2 is essential for the molecular organization of juxtaparanodal regions of myelinated fibers

Myelination results in a highly segregated distribution of axonal membrane proteins at nodes of Ranvier. Here, we show the role in this process of TAG-1, a glycosyl-phosphatidyl-inositol-anchored cell adhesion molecule. In the absence of TAG-1, axonal Caspr2 did not accumulate at juxtaparanodes, and the normal enrichment of shaker-type K+ channels in these regions was severely disrupted, in the central and peripheral nervous systems. In contrast, the localization of protein 4.1B, an axoplasmic partner of Caspr2, was only moderately altered. TAG-1, which is expressed in both neurons and glia, was able to associate in cis with Caspr2 and in trans with itself. Thus, a tripartite intercellular protein complex, comprised of these two proteins, appears critical for axo-glial contacts at juxtaparanodes. This complex is analogous to that described previously at paranodes, suggesting that similar molecules are crucial for different types of axo-glial interactions.     

Characterization of Cos-7 cells overexpressing the rat secretory pathway Ca2+-ATPase

On the basis of sequence similarities to the yeast PMR1 and hSPCA gene, the rat alternatively spliced mRNA has been suggested to be a Golgi secretory pathway Ca²⁺-ATPase (SPCA). Data in this report lend further support for this hypothesis in that sucrose gradient fractionation of rat liver microsomes resulted in SPCA comigrating with the Golgi calcium binding protein CALNUC, which was well resolved from the endoplasmic reticulum marker calreticulin. Also, in PC-12 cells, antibody to SPCA colocalized with an antibody to the Golgi marker -mannosidase II. To study the biological effects of SPCA expression, we performed stable overexpression of SPCA in COS-7 cells. Seven clones were selected for further comparison with COS-7 cells containing an empty expression vector. Overexpression of SPCA resulted in a significant reduction of plasma membrane Ca²⁺-ATPase, sarco(endo)plasmic reticulum Ca²⁺-ATPase, and calreticulin expression in these clones. In contrast, the expression of the Golgi calcium-binding protein CALNUC increased significantly. The phosphoenzyme intermediate formed using membranes from clone G11/5 was calcium dependent, significantly more intense than in COS-7 cells, and not affected by La³⁺ treatment. Calcium uptake by G11/5 microsomes was ATP dependent and significantly greater than in microsomes from parent COS-7 cells. The overexpression of SPCA significantly increased the growth rate of these cells compared with COS-7 cells containing only the empty vector. These data demonstrate that overexpression of the rat SPCA results in significant changes in the expression of calcium transport and storage proteins in COS-7 cells. calcium transport     

Molecular cloning of the 3'' half of the Clostridium perfringens enterotoxin gene and demonstration that this region encodes receptor-binding activity.

Clostridium perfringens type A enterotoxin (CPE) causes the symptoms associated with C. perfringens food poisoning. To determine whether the C-terminal half of CPE contains receptor-binding activity, the 3' half of the cpe structural gene was cloned with an Escherichia coli expression vector system. E. coli lysates containing the expressed C-terminal CPE fragment (CPEfrag) were then assayed for CPE-like serologic, receptor-binding, and cytotoxic activities. CPEfrag was shown to contain an epitope located at or near the receptor-binding domain of the CPE molecule. Competitive-binding studies showed specific competition for CPE receptors between CPE and CPEfrag lysates. CPEfrag lysates did not cause cytotoxicity in Vero (African green monkey kidney) cells. However, preincubation with CPEfrag lysates specifically protected Vero cells from subsequent CPE challenge. This indicates that CPEfrag recognizes the physiologic receptor which mediates CPE cytotoxicity. Collectively, these studies indicate that the C-terminal half of CPE contains a receptor-binding domain but additional amino acid sequences appear to be required for CPE cytotoxicity.     

Ctr2 is partially localized to the plasma membrane and stimulates copper uptake in COS-7 cells

Ctr1 (copper transporter 1) mediates high-affinity copper uptake. Ctr2 (copper transporter 2) shares sequence similarity with Ctr1, yet its function in mammalian cells is poorly understood. In African green monkey kidney COS-7 cells and rat tissues, Ctr2 migrated as a predominant band of approximately 70 kDa and was most abundantly expressed in placenta and heart. A transiently expressed hCtr2-GFP (human Ctr2-green fluorescent protein) fusion protein and the endogenous Ctr2 in COS-7 cells were mainly localized to the outer membrane of cytoplasmic vesicles, but were also detected at the plasma membrane. Biotinylation of Ctr2 with the membrane-impermeant reagent sulfo-NHS-SS-biotin [sulfosuccinimidyl-2-(biotinamido)ethyl-1,3-dithiopropionate] confirmed localization at the cell surface. Cells expressing hCtr2-GFP hyperaccumulated copper when incubated in medium supplemented with 10 microM CuSO(4), whereas cells depleted of endogenous Ctr2 by siRNAs (small interfering RNAs) accumulated lower levels of copper. hCtr2-GFP expression did not affect copper efflux, suggesting that hCtr2-GFP increased cellular copper concentrations by promoting uptake at the cell surface. Kinetic analyses showed that hCtr2-GFP stimulated saturable copper uptake with a K(m) of 11.0+/-2.5 microM and a K(0.5) of 6.9+/-0.7 microM when data were fitted to a rectangular hyperbola or Hill equation respectively. Competition experiments revealed that silver completely inhibited hCtr2-GFP-dependent copper uptake, whereas zinc, iron and manganese had no effect on uptake. Furthermore, increased copper concentrations in hCtr2-GFP-expressing cells were inversely correlated with copper chaperone for Cu/Zn superoxide dismutase protein expression. Collectively, these results suggest that Ctr2 promotes copper uptake at the plasma membrane and plays a role in regulating copper levels in COS-7 cells.     

Juxtaparanodal clustering of Shaker-like K+ channels in myelinated axons depends on Caspr2 and TAG-1

In myelinated axons, K+ channels are concealed under the myelin sheath in the juxtaparanodal region, where they are associated with Caspr2, a member of the neurexin superfamily. Deletion of Caspr2 in mice by gene targeting revealed that it is required to maintain K+ channels at this location. Furthermore, we show that the localization of Caspr2 and clustering of K+ channels at the juxtaparanodal region depends on the presence of TAG-1, an immunoglobulin-like cell adhesion molecule that binds Caspr2. These results demonstrate that Caspr2 and TAG-1 form a scaffold that is necessary to maintain K+ channels at the juxtaparanodal region, suggesting that axon-glia interactions mediated by these proteins allow myelinating glial cells to organize ion channels in the underlying axonal membrane.     

Internodal specializations of myelinated axons in the central nervous system

We have examined the localization of contactin-associated protein (Caspr), the Shaker-type potassium channels, Kv1.1 and Kv1.2, their associated beta subunit, Kvbeta2, and Caspr2 in the myelinated fibers of the CNS. Caspr is localized to the paranodal axonal membrane, and Kv1.1, Kv1.2, Kvbeta2 and Caspr2 to the juxtaparanodal membrane. In addition to the paranodal staining, an internodal strand of Caspr staining apposes the inner mesaxon of the myelin sheath. Unlike myelinated axons in the peripheral nervous system, there was no internodal strand of Kv1.1, Kv1.2, Kvbeta2, or Caspr2. Thus, the organization of the nodal, paranodal, and juxtaparanodal axonal membrane is similar in the central and peripheral nervous systems, but the lack of Kv1.1/Kv1.2/Kvbeta2/Caspr2 internodal strands indicates that the oligodendrocyte myelin sheaths lack a trans molecular interaction with axons, an interaction that is present in Schwann cell myelin sheaths.     

Molecular and cellular properties of PECAM-1 (endoCAM/CD31): a novel vascular cell-cell adhesion molecule

PECAM-1 is a 130-120-kD integral membrane glycoprotein found on the surface of platelets, at endothelial intercellular junctions in culture, and on cells of myeloid lineage. Previous studies have shown that it is a member of the immunoglobulin gene superfamily and that antibodies against the bovine form of this protein (endoCAM) can inhibit endothelial cell-cell interactions. These data suggest that PECAM-1 may function as a vascular cell adhesion molecule. The function of this molecule has been further evaluated by transfecting cells with a full-length PECAM-1 cDNA. Transfected COS-7, mouse 3T3 and L cells expressed a 130-120-kD glycoprotein on their cell surface that reacted with anti-PECAM-1 polyclonal and monoclonal antibodies. COS-7 and 3T3 cell transfectants formed cell-cell junctions that were highly enriched in PECAM-1, reminiscent of its distribution at endothelial cell-cell borders. In contrast, this protein remained diffusely distributed within the plasma membrane of PECAM-1 transfected cells that were in contact with mock transfectants. Mouse L cells stably transfected with PECAM-1 demonstrated calcium-dependent aggregation that was inhibited by anti-PECAM antibodies. These results demonstrate that PECAM-1 mediates cell-cell adhesion and support the idea that it may be involved in some of the interactive events taking place during thrombosis, wound healing, and angiogenesis.     

Genetic analysis of the subunit organization and function of the conserved oligomeric golgi (COG) complex: studies of COG5- and COG7-deficient mammalian cells

The conserved oligomeric Golgi (COG) complex is an eight-subunit (Cog1-8) peripheral Golgi protein involved in Golgi-associated membrane trafficking and glycoconjugate synthesis. We have analyzed the structure and function of COG using Cog1 or Cog2 null Chinese hamster ovary cell mutants, fibroblasts from a patient with Cog7-deficient congenital disorders of glycosylation, and stable Cog5-deficient HeLa cells generated by RNA interference. Although the dilation of some Golgi cisternae in Cog5-deficient cells resembled that observed in Cog1- or Cog2-deficient cells, their global glycosylation defects (less severe) and intracellular processing and function of low density lipoprotein receptors (essentially normal) differed from Cog1- and Cog2-deficient cells. Immunoblotting, gel filtration, and immunofluorescence microscopy analyses of the COG-deficient cells and cell extracts indicated that 1) Cog2-4 and Cog5-7 form stable subcomplexes, 2) Cog1 mediates Golgi association of a Cog2-4 plus Cog8 subcomplex, 3) Cog8 associates stably with both Cog5-7 and Cog1-4 subcomplexes, and thus 4) Cog8 helps assemble the Cog1-4 and Cog5-7 subcomplexes into the complete COG complex. This model of the subunit organization of COG is in excellent agreement with in vitro data presented in an accompanying paper (Ungar, D., Oka, T., Vasile, E., Krieger, M., and Hughson, F. M. (2005) J. Biol. Chem. 280, 32729-32735). Only one or two of the seven Cog1- or Cog2-dependent Golgi membrane proteins called GEARs are also sensitive to Cog5 or Cog7 deficiency, indicating that the COG subunits play distinctive roles in controlling Golgi structure and function.     

The AE2 anion exchanger is necessary for the structural integrity of the Golgi apparatus in mammalian cells

The structural integrity of the Golgi apparatus is known to be dependent on multiple factors, including the organizational status of microtubules, actin and the ankyrin/spectrin-based Golgi membrane skeleton, as well as vesicular trafficking and pH homeostasis. In this respect, our recently identified Golgi-associated anion exchanger, AE2, may also be of importance, since it potentially acts as a Golgi pH regulator and as a novel membrane anchor for the spectrin-based Golgi membrane skeleton. Here, we show that inhibition (>75%) of AE2 expression by antisense oligonucleotides in COS-7 cells results in the fragmentation of the juxtanuclear Golgi apparatus and in structural disorganization of the Golgi stacks, the cisternae becoming generally shorter, distorted, vesiculated and/or swollen. These structural changes occurred without apparent dissociation of the Golgi membrane skeletal protein Ankyrin(195), but were accompanied by the disappearance of the well-focused microtubule-organizing center (MTOC), suggesting the involvement of microtubule reorganization. Similar changes in Golgi structure and assembly of the MTOC were also observed upon transient overexpression of the EGFP-AE2 fusion protein. These data implicate a clear structural role for the AE2 protein in the Golgi and in its cytological positioning around the MTOC.     

B2-1, a Sec7- and pleckstrin homology domain-containing protein, localizes to the Golgi complex

B2-1 is a human protein that contains both a Sec7 and a pleckstrin homology domain. The yeast Sec7 protein was previously shown to be involved in vesicle formation in the Golgi and endoplasmic reticulum. Recently, several groups have shown that B2-1 and highly similar proteins (e.g., ARNO, ARNO3) have varied cellular functions and subcellular locations. One of these is an association of the B2-1 Sec7 domain with the plasma membrane, binding to the cytoplasmic portion of the integrin beta2 chain (CD18) and is postulated to be involved in inside-out signaling. Other groups have shown that B2-1 and these related proteins are guanine nucleotide-exchange factors that act upon ADP ribosylation factors (ARFs) and are localized to the Golgi or plasma membrane. Here we report the subcellular localization of B2-1 protein. Interestingly, B2-1 does not localize to the plasma membrane but rather associates with a distinct Golgi complex compartment. B2-1's distribution can be disrupted by brefeldin A, a drug that rapidly disrupts the Golgi apparatus by inhibiting ARF activity. Furthermore, transient transfection of GFP-tagged B2-1 shows Golgi complex targeting. Excessive overexpression of transfected B2-1 causes partial Golgi dispersion.     

Retention of a cell adhesion complex at the paranodal junction requires the cytoplasmic region of Caspr

An axonal complex of cell adhesion molecules consisting of Caspr and contactin has been found to be essential for the generation of the paranodal axo-glial junctions flanking the nodes of Ranvier. Here we report that although the extracellular region of Caspr was sufficient for directing it to the paranodes in transgenic mice, retention of the Caspr-contactin complex at the junction depended on the presence of an intact cytoplasmic domain of Caspr. Using immunoelectron microscopy, we found that a Caspr mutant lacking its intracellular domain was often found within the axon instead of the junctional axolemma. We further show that a short sequence in the cytoplasmic domain of Caspr mediated its binding to the cytoskeleton-associated protein 4.1B. Clustering of contactin on the cell surface induced coclustering of Caspr and immobilized protein 4.1B at the plasma membrane. Furthermore, deletion of the protein 4.1B binding site accelerated the internalization of a Caspr-contactin chimera from the cell surface. These results suggest that Caspr serves as a "transmembrane scaffold" that stabilizes the Caspr/contactin adhesion complex at the paranodal junction by connecting it to cytoskeletal components within the axon.     

Increasing the expression of calcium-permeable TRPC3 and TRPC7 channels enhances constitutive secretion

The hTRPC [human TRPC (canonical transient receptor potential)] family of non-selective cation channels is proposed to mediate calcium influx across the plasma membrane via PLC (phospholipase C)-coupled receptors. Heterologously expressed hTRPC3 and hTRPC7 have been localized at the cell surface; however, a large intracellular component has also been noted but not characterized. In the present study, we have investigated the intracellular pool in COS-7 cells and have shown co-localization with markers for both the TGN (trans-Golgi network) and the cis-Golgi cisternae by immunofluorescence microscopy. Addition of BFA (Brefeldin A) to cells expressing hTRPC3 or hTRPC7 resulted in the redistribution of the Golgi component to the endoplasmic reticulum, indicating that this pool is present in both the Golgi stack and the TGN. Expression of either TRPC3 or TRPC7, but not TRPC1 or the cell surface marker CD8, resulted in a 2-4-fold increase in secreted alkaline phosphatase in the extracellular medium. Based on these results, we propose that an additional function of these members of the hTRPC family may be to enhance secretion either by affecting transport through the Golgi stack or by increasing fusion at the plasma membrane.     

The transmembrane segment of ryanodine receptor contains an intracellular membrane retention signal for Ca(2+) release channel.

The ryanodine receptor (RyR) is a large homotetrameric protein with a hydrophobic domain at the C-terminal end that resides in the endoplasmic reticulum (ER) or sarcoplasmic reticulum membrane and forms the conduction pore of a Ca(2+) release channel. Our previous studies showed that RyR expressed in heterologous cells localized to the ER membrane. Confocal microscopic imaging indicated that the ER retention signal is likely present within the C-terminal portion of RyR, a region that contains four putative transmembrane segments. To identify the amino acid sequence responsible for ER retention of RyR, we expressed fusion proteins containing intercellular adhesion molecule (ICAM), various fragments of RyR, and green fluorescent protein (GFP) in Chinese hamster ovary and COS-7 cells. ICAM is a plasma membrane-resident glycoprotein and serves as a reporter for protein trafficking to the cell surface membrane. Imaging analyses indicated that ICAM-GFP fusion proteins with RyR sequence preceding the four transmembrane segments, ICAM-RyR-(3661-3993)-GFP, and with RyR sequence corresponding to transmembrane segments 1, 2, and 3, ICAM-RyR-(4558-4671)-GFP and ICAM-RyR-(4830-4919)-GFP, were localized to the plasma membrane; fusion proteins containing the fourth transmembrane segment of RyR, ICAM-RyR-(4913-4943)-GFP, were retained in the ER. Biochemical assay showed that ICAM-RyR-GFP fusion proteins that target to the plasma membrane are fully glycosylated, and those retained in the intracellular membrane are core-glycosylated. Together our data indicate that amino acids 4918-4943 of RyR contain the signal sequence for ER retention of the Ca(2+) release channel.     

Cloning and characterization of a 72-kDa inositol-polyphosphate 5-phosphatase localized to the Golgi network

The inositol-polyphosphate 5-phosphatase enzyme family removes the 5-position phosphate from both inositol phosphate and phosphoinositide signaling molecules. We have cloned and characterized a novel 5-phosphatase, which demonstrates a restricted substrate specificity and tissue expression. The 3.9-kb cDNA predicts for a 72-kDa protein with an N-terminal proline rich domain, a central 5-phosphatase domain, and a C-terminal CAAX motif. The 3. 9-kilobase mRNA showed a restricted expression but was abundant in testis and brain. Antibodies against the sequence detected a 72-kDa protein in the testis in the detergent-insoluble fraction. Indirect immunofluorescence of the Tera-1 cell line using anti-peptide antibodies to the 72-kDa 5-phosphatase demonstrated that the enzyme is predominantly located to the Golgi. Expression of green fluorescent protein-tagged 72-kDa 5-phosphatase in COS-7 cells revealed that the enzyme localized predominantly to the Golgi, mediated by the N-terminal proline-rich domain, but not the C-terminal CAAX motif. In vitro, the protein inserted into microsomal membranes on the cytoplasmic face of the membrane. Immunoprecipitated recombinant 72-kDa 5-phosphatase hydrolyzed phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3, 5-bisphosphate, forming phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3-phosphate, respectively. We propose that the novel 5-phosphatase hydrolyzes phosphatidylinositol 3,4, 5-trisphosphate and phosphatidylinositol 3,5-bisphosphate on the cytoplasmic Golgi membrane and thereby may regulate Golgi-vesicular trafficking.     

Caspr2, a new member of the neurexin superfamily, is localized at the juxtaparanodes of myelinated axons and associates with K+ channels

Rapid conduction in myelinated axons depends on the generation of specialized subcellular domains to which different sets of ion channels are localized. Here, we describe the identification of Caspr2, a mammalian homolog of Drosophila Neurexin IV (Nrx-IV), and show that this neurexin-like protein and the closely related molecule Caspr/Paranodin demarcate distinct subdomains in myelinated axons. While contactin-associated protein (Caspr) is present at the paranodal junctions, Caspr2 is precisely colocalized with Shaker-like K+ channels in the juxtaparanodal region. We further show that Caspr2 specifically associates with Kv1.1, Kv1.2, and their Kvbeta2 subunit. This association involves the C-terminal sequence of Caspr2, which contains a putative PDZ binding site. These results suggest a role for Caspr family members in the local differentiation of the axon into distinct functional subdomains.     

Comparative features of copper ATPases ATP7A and ATP7B heterologously expressed in COS-1 cells

ATP7A and ATP7B are P-type ATPases required for copper homeostasis and involved in the etiology of Menkes and Wilson diseases. We used heterologous expression of ATP7A or ATP7B in COS-1 cells infected with adenovirus vectors to characterize differential features pertinent to each protein expressed in the same mammalian cell type, rather than to extrinsic factors related to different cells sustaining expression. Electrophoretic analysis of the expressed protein, before and after purification, prior or subsequent to treatment with endoglycosidase, and evidenced by protein or glycoprotein staining as well as Western blotting, indicates that the ATP7A protein is glycosylated while ATP7B is not. This is consistent with the prevalence of glycosylation motifs in the ATP7A sequence, and not in ATP7B. ATP7A and ATP7B undergo copper-dependent phosphorylation by utilization of ATP, forming equal levels of an "alkali labile" phosphoenzyme intermediate that undergoes similar catalytic (P-type ATPase) turnover in both enzymes. In addition, incubation with ATP yields an "alkali stable" phosphoprotein fraction, attributed to phosphorylation of serines. Alkali stable phosphorylation occurs at lower levels in ATP7A, consistent with a different distribution of serines in the amino acid sequence. Immunostaining of COS-1 cells sustaining heterologous expression shows initial association of both ATP7A and ATP7B with Golgi and the trans-Golgi network. However, in the presence of added copper, ATP7A undergoes prevalent association with the plasma membrane while ATP7B exhibits intense trafficking with cytosolic vesicles. Glycosylation of ATP7A and phosphorylation of ATP7B apparently contribute to their different trafficking and membrane association when expressed in the same cell type.