Chloride is an allosteric effector of copper assembly for the yeast multicopper oxidase Fet3p: An unexpected role for intracellular chloride channels

GEF1 is a gene in Saccharomyces cerevisiae, which encodes a putative voltage-regulated chloride channel. gef1 mutants have a defect in the high-affinity iron transport system, which relies on the cell surface multicopper oxidase Fet3p. The defect is due to an inability to transfer Cu+ to apoFet3p within the secretory apparatus. We demonstrate that the insertion of Cu into apoFet3p is dependent on the presence of Cl−. Cu-loading of apoFet3p is favored at acidic pH, but in the absence of Cl− there is very little Cu-loading at any pH. Cl− has a positive allosteric effect on Cu-loading of apoFet3p. Kinetic studies suggest that Cl− may also bind to Fet3p and that Cu+ has an allosteric effect on the binding of Cl− to the enzyme. Thus, Cl− may be required for the metal loading of proteins within the secretory apparatus. These results may have implications in mammalian physiology, as mutations in human intracellular chloride channels result in disease.

A method that allows the assembly of kinetochore components onto chromosomes condensed in clarified Xenopus egg extracts

Kinetochores are complex macromolecular structures that link mitotic chromosomes to spindle microtubules. Although a small number of kinetochore components have been identified, including the kinesins CENP-E and XKCM1 as well as cytoplasmic dynein, neither how these and other proteins are organized to produce a kinetochore nor their exact functions within this structure are understood. For this reason, we have developed an assay that allows kinetochore components to assemble onto discrete foci on in vitro-condensed chromosomes. The source of the kinetochore components is a clarified cell extract from Xenopus eggs that can be fractionated or immunodepleted of individual proteins. Kinetochore assembly in these clarified extracts requires preincubating the substrate sperm nuclei in an extract under low ATP conditions. Immunodepletion of XKCM1 from the extracts prevents the localization of kinetochore-associated XKCM1 without affecting the targeting of CENP-E and cytoplasmic dynein or the binding of monomeric tubulin to the kinetochore. Extension of this assay for the analysis of other components should help to dissect the protein–protein interactions involved in kinetochore assembly and function.

Differential Epitope Tagging of Actin in Transformed Drosophila Produces Distinct Effects on Myofibril Assembly and Function of the Indirect Flight Muscle

We have tested the impact of tags on the structure and function of indirect flight muscle (IFM)-specific Act88F actin by transforming mutant Drosophila melanogaster, which do not express endogenous actin in their IFMs, with tagged Act88F constructs. Epitope tagging is often the method of choice to monitor the fate of a protein when a specific antibody is not available. Studies addressing the functional significance of the closely related actin isoforms rely almost exclusively on tagged exogenous actin, because only few antibodies exist that can discriminate between isoforms. Thereby it is widely presumed that the tag does not significantly interfere with protein function. However, in most studies the tagged actin is expressed in a background of endogenous actin and, as a rule, represents only a minor fraction of the total actin. The Act88F gene encodes the only Drosophila actin isoform exclusively expressed in the highly ordered IFM. Null mutations in this gene do not affect viability, but phenotypic effects in transformants can be directly attributed to the transgene. Transgenic flies that express Act88F with either a 6x histidine tag or an 11-residue peptide derived from vesicular stomatitis virus G protein at the C terminus were flightless. Overall, the ultrastructure of the IFM resembled that of the Act88F null mutant, and only low amounts of C-terminally tagged actins were found. In contrast, expression of N-terminally tagged Act88F at amounts comparable with that of wild-type flies yielded fairly normal-looking myofibrils and partially reconstituted flight ability in the transformants. Our findings suggest that the N terminus of actin is less sensitive to modifications than the C terminus, because it can be tagged and still polymerize into functional thin filaments.

The Multiple Roles of Cyk1p in the Assembly and Function of the Actomyosin Ring in Budding Yeast

The budding yeast IQGAP-like protein Cyk1p/Iqg1p localizes to the mother-bud junction during anaphase and has been shown to be required for the completion of cytokinesis. In this study, video microscopy analysis of cells expressing green fluorescent protein-tagged Cyk1p/Iqg1p demonstrates that Cyk1p/Iqg1p is a dynamic component of the contractile ring during cytokinesis. Furthermore, in the absence of Cyk1p/Iqg1p, myosin II fails to undergo the contraction-like size change at the end of mitosis. To understand the mechanistic role of Cyk1p/Iqg1p in actomyosin ring assembly and dynamics, we have investigated the role of the structural domains that Cyk1p/Iqg1p shares with IQGAPs. An amino terminal portion containing the calponin homology domain binds to actin filaments and is required for the assembly of actin filaments to the ring. This result supports the hypothesis that Cyk1p/Iqg1p plays a direct role in F-actin recruitment. Deletion of the domain harboring the eight IQ motifs abolishes the localization of Cyk1p/Iqg1p to the bud neck, suggesting that Cyk1p/Iqg1p may be localized through interactions with a calmodulin-like protein. Interestingly, deletion of the COOH-terminal GTPase-activating protein-related domain does not affect Cyk1p/Iqg1p localization or actin recruitment to the ring but prevents actomyosin ring contraction. In vitro binding experiments show that Cyk1p/Iqg1p binds to calmodulin, Cmd1p, in a calcium-dependent manner, and to Tem1p, a small GTP-binding protein previously found to be required for the completion of anaphase. These results demonstrate the critical function of Cyk1p/Iqg1p in regulating various steps of actomyosin ring assembly and cytokinesis.

Cytoplasmic Dynein Heavy Chain 1b Is Required for Flagellar Assembly in Chlamydomonas

A second cytoplasmic dynein heavy chain (cDhc) has recently been identified in several organisms, and its expression pattern is consistent with a possible role in axoneme assembly. We have used a genetic approach to ask whether cDhc1b is involved in flagellar assembly in Chlamydomonas. Using a modified PCR protocol, we recovered two cDhc sequences distinct from the axonemal Dhc sequences identified previously. cDhc1a is closely related to the major cytoplasmic Dhc, whereas cDhc1b is closely related to the minor cDhc isoform identified in sea urchins, Caenorhabditis elegans, and Tetrahymena. The Chlamydomonas cDhc1b transcript is a low-abundance mRNA whose expression is enhanced by deflagellation. To determine its role in flagellar assembly, we screened a collection of stumpy flagellar (stf) mutants generated by insertional mutagenesis and identified two strains in which portions of the cDhc1b gene have been deleted. The two mutants assemble short flagellar stumps (<1–2 μm) filled with aberrant microtubules, raft-like particles, and other amorphous material. The results indicate that cDhc1b is involved in the transport of components required for flagellar assembly in Chlamydomonas.

Assembly of Lampbrush Chromosomes from Sperm Chromatin

We have examined the behavior of demembranated sperm heads when injected into the germinal vesicle (GV) of amphibian oocytes. Xenopus sperm heads injected into Xenopus GVs swelled immediately and within hours began to stain with an antibody against RNA polymerase II (Pol II). Over time each sperm head became a loose mass of chromosome-like threads, which by 24–48 h resolved into individually recognizable lampbrush chromosomes (LBCs). Although LBCs derived from sperm are unreplicated single chromatids, their morphology and immunofluorescent staining properties were strikingly similar to those of the endogenous lampbrush bivalents. They displayed typical transcriptionally active loops extending from an axis of condensed chromomeres, as well as locus-specific “landmarks.” Experiments with [3H]GTP and actinomycin D demonstrated that transcription was not necessary for the initial swelling of the sperm heads and acquisition of Pol II but was required for maintenance of the lampbrush loops. Splicing was not required at any stage during formation of sperm LBCs. When Xenopus sperm heads were injected into GVs of the newt Notophthalmus, the resulting sperm LBCs displayed very long loops with pronounced Pol II axes, like those of the endogenous newt LBCs; as expected, they stained with antibodies against newt-specific proteins. Other heterologous injections, including sperm heads of the frog Rana pipiens and the zebrafish Danio rerio in Xenopus GVs, confirm that LBCs can be derived from taxonomically distant organisms. The GV system should help identify both cis- and trans-acting factors needed to convert condensed chromatin into transcriptionally active LBCs. It may also be useful in producing cytologically analyzable chromosomes from organisms whose oocytes do not go through a typical lampbrush phase or cannot be manipulated by current techniques.

Dissection of De Novo Membrane Insertion Activities of Internal Transmembrane Segments of ATP-Binding-Cassette Transporters: Toward Understanding Topological Rules for Membrane Assembly of Polytopic Membrane Proteins

The membrane assembly of polytopic membrane proteins is a complicated process. Using Chinese hamster P-glycoprotein (Pgp) as a model protein, we investigated this process previously and found that Pgp expresses more than one topology. One of the variations occurs at the transmembrane (TM) domain including TM3 and TM4: TM4 inserts into membranes in an Nin-Cout rather than the predicted Nout-Cin orientation, and TM3 is in cytoplasm rather than the predicted Nin-Cout orientation in the membrane. It is possible that TM4 has a strong activity to initiate the Nin-Cout membrane insertion, leaving TM3 out of the membrane. Here, we tested this hypothesis by expressing TM3 and TM4 in isolated conditions. Our results show that TM3 of Pgp does not have de novo Nin-Cout membrane insertion activity whereas TM4 initiates the Nin-Cout membrane insertion regardless of the presence of TM3. In contrast, TM3 and TM4 of another polytopic membrane protein, cystic fibrosis transmembrane conductance regulator (CFTR), have a similar level of de novo Nin-Cout membrane insertion activity and TM4 of CFTR functions only as a stop-transfer sequence in the presence of TM3. Based on these findings, we propose that 1) the membrane insertion of TM3 and TM4 of Pgp does not follow the sequential model, which predicts that TM3 initiates Nin-Cout membrane insertion whereas TM4 stops the insertion event; and 2) “leaving one TM segment out of the membrane” may be an important folding mechanism for polytopic membrane proteins, and it is regulated by the Nin-Cout membrane insertion activities of the TM segments.

Fluid-Phase Markers in the Basolateral Endocytic Pathway Accumulate in Response to the Actin Assembly-promoting Drug Jasplakinolide

To investigate the role of filamentous actin in the endocytic pathway, we used the cell-permeant drug Jasplakinolide (JAS) to polymerize actin in intact polarized Madin–Darby canine kidney (MDCK) cells. The uptake and accumulation of the fluid-phase markers fluorescein isothiocyanate (FITC)-dextran and horseradish peroxidase (HRP) were followed in JAS-treated or untreated cells with confocal fluorescence microscopy, biochemical assays, and electron microscopy. Pretreatment with JAS increased the uptake and accumulation of fluid-phase markers in MDCK cells. JAS increased endocytosis in a polarized manner, with a marked effect on fluid-phase uptake from the basolateral surface but not from the apical surface of polarized MDCK cells. The early uptake of FITC-dextran and HRP was increased more than twofold in JAS-treated cells. At later times, FITC-dextran and HRP accumulated in clustered endosomes in the basal and middle regions of JAS-treated cells. The large accumulated endosomes were similar to late endosomes but they were not colabeled for other late endosome markers, such as rab7 or mannose-6-phosphate receptor. JAS altered transport in the endocytic pathway at a later stage than the microtubule-dependent step affected by nocodazole. JAS also had a notable effect on cell morphology, inducing membrane bunching at the apical pole of MDCK cells. Although other studies have implicated actin in endocytosis at the apical cell surface, our results provide novel evidence that filamentous actin is also involved in the endocytosis of fluid-phase markers from the basolateral membrane of polarized cells.

Fibronectin Regulates Assembly of Actin Filaments and Focal Contacts in Cultured Cells via the Heparin-binding Site in Repeat III13

Fibroblasts, when plated on the extracellular matrix protein fibronectin (FN), rapidly spread and form an organized actin cytoskeleton. This process is known to involve both the central α5β1 integrin-binding and the C-terminal heparin-binding regions of FN. We found that within the heparin-binding region, the information necessary for inducing organization of stress fibers and focal contacts was located in a 29–amino acid segment of FN type III module 13 (III13). We did not find a cytoskeleton-organizing role for repeat III14, which had previously been implicated in this process. Within III13, the same five basic amino acids known to be most important for heparin binding were also necessary for actin organization. A substrate of III13 alone was only weakly adhesive but strongly induced formation of filopodia and lamellipodia. Stress fiber formation required a combination of III13 and III7–11 (which contains the integrin α5β1 recognition site), either as a single fusion protein or as separate polypeptides, and the relative amounts of the two binding sites appeared to determine whether stress fibers or filopodia and lamellipodia were the predominant actin structures formed. We propose that a balance of signals from III13 and from integrins regulates the type of actin structures assembled by the cell.

Histone H1 Reduces the Frequency of Initiation in Xenopus Egg Extract by Limiting the Assembly of Prereplication Complexes on Sperm Chromatin

Somatic histone H1 reduces both the rate and extent of DNA replication in Xenopus egg extract. We show here that H1 inhibits replication directly by reducing the number of replication forks, but not the rate of fork progression, in Xenopus sperm nuclei. Density substitution experiments demonstrate that those forks that are active in H1 nuclei elongate to form large tracts of fully replicated DNA, indicating that inhibition is due to a reduction in the frequency of initiation and not the rate or extent of elongation. The observation that H1 dramatically reduces the number of replication foci in sperm nuclei supports this view. The establishment of replication competent DNA in egg extract requires the assembly of prereplication complexes (pre-RCs) on sperm chromatin. H1 reduces binding of the pre-RC proteins, XOrc2, XCdc6, and XMcm3, to chromatin. Replication competence can be restored in these nuclei, however, only under conditions that promote the loss of H1 from chromatin and licensing of the DNA. Thus, H1 inhibits replication in egg extract by preventing the assembly of pre-RCs on sperm chromatin, thereby reducing the frequency of initiation. These data raise the interesting possibility that H1 plays a role in regulating replication origin use during Xenopus development.

The UNI3 Gene Is Required for Assembly of Basal Bodies of Chlamydomonas and Encodes δ-Tubulin, a New Member of the Tubulin Superfamily

We have cloned the UNI3 gene in Chlamydomonas and find that it encodes a new member of the tubulin superfamily. Although Uni3p shares significant sequence identity with α-, β-, and γ-tubulins, there is a region of Uni3p that has no similarity to tubulins or other known proteins. Mutant uni3–1 cells assemble zero, one, or two flagella. Pedigree analysis suggests that flagellar number in uni3–1 cells is a function of the age of the cell. The uniflagellate uni3–1 cells show a positional phenotype; the basal body opposite the eyespot templates the single flagellum. A percentage of uni3–1 cells also fail to orient the cleavage furrow properly, and basal bodies have been implicated in the placement of cleavage furrows in Chlamydomonas. Finally when uni3–1 cells are observed by electron microscopy, doublet rather than triplet microtubules are observed at the proximal end of the basal bodies. We propose that the Uni3 tubulin is involved in both the function and cell cycle-dependent maturation of basal bodies/centrioles.

Trafficking, Assembly, and Function of a Connexin43-Green Fluorescent Protein Chimera in Live Mammalian CellsV⃞

To examine the trafficking, assembly, and turnover of connexin43 (Cx43) in living cells, we used an enhanced red-shifted mutant of green fluorescent protein (GFP) to construct a Cx43-GFP chimera. When cDNA encoding Cx43-GFP was transfected into communication-competent normal rat kidney cells, Cx43-negative Madin–Darby canine kidney (MDCK) cells, or communication-deficient Neuro2A or HeLa cells, the fusion protein of predicted length was expressed, transported, and assembled into gap junctions that exhibited the classical pentalaminar profile. Dye transfer studies showed that Cx43-GFP formed functional gap junction channels when transfected into otherwise communication-deficient HeLa or Neuro2A cells. Live imaging of Cx43-GFP in MDCK cells revealed that many gap junction plaques remained relatively immobile, whereas others coalesced laterally within the plasma membrane. Time-lapse imaging of live MDCK cells also revealed that Cx43-GFP was transported via highly mobile transport intermediates that could be divided into two size classes of <0.5 μm and 0.5–1.5 μm. In some cases, the larger intracellular Cx43-GFP transport intermediates were observed to form from the internalization of gap junctions, whereas the smaller transport intermediates may represent other routes of trafficking to or from the plasma membrane. The localization of Cx43-GFP in two transport compartments suggests that the dynamic formation and turnover of connexins may involve at least two distinct pathways.

Assembly of a Novel Cartilage Matrix Protein Filamentous Network: Molecular Basis of Differential Requirement of von Willebrand Factor A Domains

Cartilage matrix protein (CMP) is the prototype of the newly discovered matrilin family, all of which contain von Willebrand factor A domains. Although the function of matrilins remain unclear, we have shown that, in primary chondrocyte cultures, CMP (matrilin-1) forms a filamentous network, which is made up of two types of filaments, a collagen-dependent one and a collagen-independent one. In this study, we demonstrate that the collagen-independent CMP filaments are enriched in pericellular compartments, extending directly from chondrocyte membranes. Their morphology can be distinguished from that of collagen filaments by immunogold electron microscopy, and mimicked by that of self-assembled purified CMP. The assembly of CMP filaments can occur from transfection of a wild-type CMP transgene alone in skin fibroblasts, which do not produce endogenous CMP. Conversely, assembly of endogenous CMP filaments by chondrocytes can be inhibited specifically by dominant negative CMP transgenes. The two A domains within CMP serve essential but different functions during network formation. Deletion of the A2 domain converts the trimeric CMP into a mixture of monomers, dimers, and trimers, whereas deletion of the A1 domain does not affect the trimeric configuration. This suggests that the A2 domain modulates multimerization of CMP. Absence of either A domain from CMP abolishes its ability to form collagen-independent filaments. In particular, Asp22 in A1 and Asp255 in A2 are essential; double point mutation of these residues disrupts CMP network formation. These residues are part of the metal ion–dependent adhesion sites, thus a metal ion–dependent adhesion site–mediated adhesion mechanism may be applicable to matrilin assembly. Taken together, our data suggest that CMP is a bridging molecule that connects matrix components in cartilage to form an integrated matrix network.

BiP and Immunoglobulin Light Chain Cooperate to Control the Folding of Heavy Chain and Ensure the Fidelity of Immunoglobulin Assembly

The immunoglobulin (Ig) molecule is composed of two identical heavy chains and two identical light chains (H2L2). Transport of this heteromeric complex is dependent on the correct assembly of the component parts, which is controlled, in part, by the association of incompletely assembled Ig heavy chains with the endoplasmic reticulum (ER) chaperone, BiP. Although other heavy chain-constant domains interact transiently with BiP, in the absence of light chain synthesis, BiP binds stably to the first constant domain (CH1) of the heavy chain, causing it to be retained in the ER. Using a simplified two-domain Ig heavy chain (VH-CH1), we have determined why BiP remains bound to free heavy chains and how light chains facilitate their transport. We found that in the absence of light chain expression, the CH1 domain neither folds nor forms its intradomain disulfide bond and therefore remains a substrate for BiP. In vivo, light chains are required to facilitate both the folding of the CH1 domain and the release of BiP. In contrast, the addition of ATP to isolated BiP–heavy chain complexes in vitro causes the release of BiP and allows the CH1 domain to fold in the absence of light chains. Therefore, light chains are not intrinsically essential for CH1 domain folding, but play a critical role in removing BiP from the CH1 domain, thereby allowing it to fold and Ig assembly to proceed. These data suggest that the assembly of multimeric protein complexes in the ER is not strictly dependent on the proper folding of individual subunits; rather, assembly can drive the complete folding of protein subunits.

Polymerizing Microtubules Activate Site-directed F-Actin Assembly in Nerve Growth Cones

We identify an actin-based protrusive structure in growth cones termed “intrapodium.” Unlike filopodia, intrapodia are initiated exclusively within lamellipodia and elongate in a continuous (nonsaltatory) manner parallel to the plane of the dorsal plasma membrane causing a ridge-like protrusion. Intrapodia resemble the actin-rich structures induced by intracellular pathogens (e.g., Listeria) or by extracellular beads. Cytochalasin B inhibits intrapodial elongation and removal of cytochalasin B produced a burst of intrapodial activity. Electron microscopic studies revealed that lamellipodial intrapodia contain both short and long actin filaments oriented with their barbed ends toward the membrane surface or advancing end. Our data suggest an interaction between microtubule endings and intrapodia formation. Disruption of microtubules by acute nocodazole treatment decreased intrapodia frequency, and washout of nocodazole or addition of the microtubule-stabilizing drug Taxol caused a burst of intrapodia formation. Furthermore, individual microtubule ends were found near intrapodia initiation sites. Thus, microtubule ends or associated structures may regulate these actin-dependent structures. We propose that intrapodia are the consequence of an early step in a cascade of events that leads to the development of F-actin-associated plasma membrane specializations.