Mutant rab8 Impairs docking and fusion of rhodopsin-bearing post-Golgi membranes and causes cell death of transgenic Xenopus rods

Rab8 is a GTPase involved in membrane trafficking. In photoreceptor cells, rab8 is proposed to participate in the late stages of delivery of rhodopsin-containing post-Golgi membranes to the plasma membrane near the base of the connecting cilium. To test the function of rab8 in vivo, we generated transgenic Xenopus laevis expressing wild-type, constitutively active (Q67L), and dominant negative (T22N) forms of canine rab8 in their rod photoreceptors as green fluorescent protein (GFP) fusion proteins. Wild-type and constitutively active GFP-rab8 proteins were primarily associated with Golgi and post-Golgi membranes, whereas the dominant negative protein was primarily cytoplasmic. Expression of wild-type GFP-rab8 had minimal effects on cell survival and intracellular structures. In contrast, GFP-rab8T22N caused rapid retinal degeneration. In surviving peripheral rods, tubulo-vesicular structures accumulated at the base of the connecting cilium. Expression of GFP-rab8Q67L induced a slower retinal degeneration in some tadpoles. Transgene effects were transmitted to F1 offspring. Expression of the GFP-rab8 fusion proteins appears to decrease the levels of endogenous rab8 protein. Our results demonstrate a role for rab8 in docking of post-Golgi membranes in rods, and constitute the first report of a transgenic X. laevis model of retinal degenerative disease.

FIP-2, a coiled-coil protein, links Huntingtin to Rab8 and modulates cellular morphogenesis

Huntington's disease is characterised by the death of cortical and striatal neurons, and is the result of an expanded polyglutamine tract in the Huntingtin protein [1]. Huntingtin is present on both endocytic and secretory membrane organelles but its function is unclear [2,3]. Rab GTPases regulate both of these transport pathways [4]. We have previously shown that Rab8 controls polarised membrane transport by modulating cell morphogenesis [5]. To understand Rab8-mediated processes, we searched for Rab8-interacting proteins by the yeast two-hybrid system. Here, we report that Huntingtin is linked to the Rab8 protein through FIP-2, a tumour necrosis factor-alpha (TNF-alpha)-inducible coiled-coil protein related to the NEMO protein [6,7]. The activated form of Rab8 interacted with the amino-terminal region of FIP-2, whereas dominant-negative Rab8 did not. Huntingtin bound to the carboxy-terminal region of FIP-2. Coexpressed FIP-2 and Huntingtin enhanced the recruitment of Huntingtin to Rab8-positive vesicular structures, and FIP-2 promoted cell polarisation in a similar way to Rab8. We propose a model in which Huntingtin, together with FIP-2 and Rab8, are part of a protein network that regulates membrane trafficking and cellular morphogenesis.

Mouse A6/twinfilin is an actin monomer-binding protein that localizes to the regions of rapid actin dynamics

In our database searches, we have identified mammalian homologues of yeast actin-binding protein, twinfilin. Previous studies suggested that these mammalian proteins were tyrosine kinases, and therefore they were named A6 protein tyrosine kinase. In contrast to these earlier studies, we did not find any tyrosine kinase activity in our recombinant protein. However, biochemical analysis showed that mouse A6/twinfilin forms a complex with actin monomer and prevents actin filament assembly in vitro. A6/twinfilin mRNA is expressed in most adult tissues but not in skeletal muscle and spleen. In mouse cells, A6/twinfilin protein is concentrated to the areas at the cell cortex which overlap with G-actin-rich actin structures. A6/twinfilin also colocalizes with the activated forms of small GTPases Rac1 and Cdc42 to membrane ruffles and to cell-cell contacts, respectively. Furthermore, expression of the activated Rac1(V12) in NIH 3T3 cells leads to an increased A6/twinfilin localization to nucleus and cell cortex, whereas a dominant negative form of Rac1(V12,N17) induces A6/twinfilin localization to cytoplasm. Taken together, these studies show that mouse A6/twinfilin is an actin monomer-binding protein whose localization to cortical G-actin-rich structures may be regulated by the small GTPase Rac1.

Immunoprecipitation of steroidogenic enzyme autoantigens with autoimmune polyglandular syndrome type I (APS I) sera; further evidence for independent humoral immunity to P450c17 and P450c21

Three steroidogenic P450 cytochromes, steroid 17alpha-hydroxylase (P450c17), steroid 21-hydroxylase (P450c21) and side-chain cleavage enzyme (P450scc), have been described as autoantigens in APS I. In this study we report an immunoprecipitation assay for the detection of autoantibodies to these three enzymes using in vitro 35S-labelled antigens. Overall, 33 out of 46 (72%) patients with APS I had autoantibodies to at least one of the three proteins and each protein was recognized by patient sera with equal frequency. A higher rate of autoantibody positivity was observed in APS I patients with Addison's disease compared with patients without Addison's disease (85% versus 39%). All 11 patients with ovarian failure had anti-P450c17 or anti-P450scc antibodies. The immunoprecipitation results with P450c17, P450c21 and P450scc correlated well with the results obtained by immunoblotting assays. In addition, the steroidogenic enzymes 11beta-hydroxylase (P450c11beta), aromatase (P450arom), 3beta-hydroxysteroid dehydrogenase (3betaHSD) and adrenodoxin were studied by immunoprecipitation assay, but no reaction was found either with 46 APS I or with 26 healthy control sera. To study the suggested immunological cross-reactivity between P450c17 and P450c21 enzymes, nine APS I patient sera were preabsorbed with bacterially expressed P450c17 or P450c21 and subsequently used in immunoprecipitation assay. The absorption experiments clearly indicated that the preincubation inhibited only the reactivity of corresponding antigen, suggesting independent autoantibody response to the two enzymes. Our results suggest that the immune response to some but not to all steroidogenic enzymes is a specific feature of APS I that may be pathogenically significant.

Expression and intracellular localization of catechol O-methyltransferase in transfected mammalian cells

The intracellular localization of soluble and membrane-bound isoforms of rat and human catechol O-methyltransferase (COMT) was studied by expressing the recombinant COMT proteins either separately or together in mammalian cell lines (HeLa and COS-7 cells) and in rat primary neurons. The distribution of soluble and membrane-bound COMT enzyme was visualized by immunocytochemistry. For comparison, the localization of native COMT was studied in rat C6 glioma cells by immunoelectron microscopy. Staining of cells expressing membrane-bound COMT with a COMT-specific antiserum revealed an immunofluorescence signal in intracellular reticular structures and in the nuclear membrane. Double-staining of the cells with antisera against proteins specific for the rough endoplasmic reticulum indicated that they colocalized with membrane-bound COMT, suggesting that it resided in the endoplasmic reticulum. Notably, no COMT-specific fluorescence of plasma membranes was detected. The signal in the endoplasmic reticulum was also evident in the cells expressing both recombinant COMT forms. Intracellular native COMT reaction was detected by immunoelectron microscopy in rat C6 glioma cells and an intense cytoplasmic signal was seen in the primary neurons infected with the recombinant Semliki Forest virus. The cells expressing recombinant soluble COMT revealed intense nuclear staining together with diffuse cytoplasmic immunoreactivity, suggesting that a part of soluble COMT is transported to nuclei. Western blotting from rat liver and brain revealed soluble COMT in the nuclei. Enzyme activity measurements from liver cytoplasmic and nuclear fractions suggested that about 5% of the soluble COMT resided in nuclei. The intracellular localization of both COMT forms implies that COMT acts in the cytoplasm and possibly also in the nuclear compartment, and that the physiological substrates of COMT enzymes may have to be internalized before their methylation by COMT.

Rab8 promotes polarized membrane transport through reorganization of actin and microtubules in fibroblasts

Rab8 is a small Ras-like GTPase that regulates polarized membrane transport to the basolateral membrane in epithelial cells and to the dendrites in neurons. It has recently been demonstrated that fibroblasts sort newly synthesized proteins into two different pathways for delivery to the cell surface that are equivalent to the apical and the basolateral post-Golgi routes in epithelial cells (Yoshimori, T., P. Keller, M.G. Roth, and K. Simons. 1996. J. Cell Biol. 133:247-256). To determine the role of Rab8 in fibroblasts, we used both transient expression systems and stable cell lines expressing mutant or wild-type (wt) Rab8. A dramatic change in cell morphology occurred in BHK cells expressing both the wt Rab8 and the activated form of the GTPase, the Rab8Q67L mutant. These cells formed processes as a result of a reorganization of both their actin filaments and microtubules. Newly synthesized vesicular stomatitis virus G glycoprotein, a basolateral marker protein in MDCK cells, was preferentially delivered into these cell outgrowths. Based on these findings, we propose that Rab8 provides a link between the machinery responsible for the formation of cell protrusions and polarized biosynthetic membrane traffic.

VIP17/MAL, a proteolipid in apical transport vesicles

VIP17 is a proteolipid enriched in the CHAPS-insoluble complexes from MDCK cells, and a candidate component of the molecular machinery responsible for the sorting and targeting of proteins to the apical surface. Cloning and sequencing of the cDNA encoding the protein revealed that it is the canine homolog of the human and rat MAL proteins. Analysis by immunofluorescence microscopy of epitope-tagged VIP17/MAL expressed transiently in BHK cells and stably in MDCK cells revealed a perinuclear, vesicular, and plasmalemmal staining. In MDCK cells the distribution was mainly in vesicular structures in the apical cytoplasm. These and other results suggest that VIP17/MAL is an important component in vesicular trafficking cycling between the Golgi complex and the apical plasma membrane.

N-glycans as apical sorting signals in epithelial cells

In epithelial Madin-Darby canine kidney (MDCK) cells newly synthesized molecules are sorted in the trans-Golgi network and directly delivered to their apical and basolateral surface destinations. Sorting is mediated by signals in the cytoplasmic domains of basolateral transmembrane proteins whereas glycosylphosphatidylinositol-linked proteins have apical sorting information in their glycolipid tails. Signals for apical transmembrane proteins are thought to reside in their ectodomains, because truncated forms lacking the cytoplasmic tail and the membrane anchor are secreted apically. Here we demonstrate that carbohydrates act as an apical targeting signal for secretory proteins. Growth hormone, which is non-glycosylated and secreted from both sides of MDCK cell layers, is secreted from the apical side when glycosylated. Thus glycans not only play a general role in protein folding but also appear to function in protein sorting in biosynthetic traffic.

VIP21/caveolin is a cholesterol-binding protein

VIP21/caveolin is localized to both caveolae and apical transport vesicles and presumably cycles between the cell surface and the Golgi complex. We have studied the lipid interactions of this protein by reconstituting Escherichia coli-expressed VIP21/caveolin into liposomes. Surprisingly, the protein reconstituted only with cholesterol-containing lipid mixtures. We demonstrated that the protein binds at least 1 mol of cholesterol per mole of protein and that this binding promotes formation of protein oligomers. These findings suggest that VIP21/caveolin, through its cholesterol-binding properties, serves a specific function in microdomain formation during membrane trafficking.

The alphavirus replicase protein nsP1 is membrane-associated and has affinity to endocytic organelles

In alphavirus-infected cells the four virus-specific nonstructural proteins (nsP1-nsP4) are located on modified endosomes and lysosomes known as type I cytopathic vacuoles. In this paper, we show that when nsP1 was expressed alone in HeLa cells with the aid of the recombinant T7 RNA polymerase vaccinia (vTF7-3) virus system, it was tightly associated with intracellular smooth membranes. The membrane association may be due to acylation, since nsP1 could be labeled with [3H]palmitic acid in both Semliki Forest virus-infected and nsP1-transfected HeLa cells. Release of the 3H-label by alkaline methanolysis suggests that the palmitate was associated with nsP1 via an ester bond. Pulse-chase experiments done on nsP1-transfected cells revealed that this protein was rapidly associated with the membranes. After synchronizing the synthesis of the nsP1 gene product in transfected cells, nsP1 appeared first at the plasma membrane and thereafter on vesicles, many of which contained the endosomal transferrin receptor marker. Later, nsP1 appeared on large vacuoles, which contained the lysosome specific h-lamp-1 protein. Membrane association of nsP1, and its affinity to endosomes and lysosomes, suggest a role of this protein in the biogenesis of the alphavirus-specific RNA replication complex.

Expression of Semliki Forest virus nsP1-specific methyltransferase in insect cells and in Escherichia coli

We have expressed the Semliki Forest virus (SFV)-specific nonstructural protein nsP1 both in insect cells and in Escherichia coli in the absence of other viral proteins. A substantial amount of nsP1 was synthesized in Sf9 cells infected with the recombinant Autographa californica nuclear polyhedrosis virus (AcNPV) AcNPV-nsP1. These cells had a high level of guanine-7-methyltransferase activity compared with that of wild-type AcNPV-infected cells. The methyltransferase activity and nsP1 were mostly in the mitochondrial pellet fraction (P15). The enzymatic activity was increased by treatment with deoxycholate (DOC), as in the case of SFV-infected BHK cells. The material released by DOC treatment from P15 of the AcNPV-nsP1-infected cells was analyzed by gel filtration and sucrose gradient centrifugation. Both the methyltransferase activity and nsP1 were in aggregates. nsP1 expressed in E. coli at 37 degrees C sedimented at 15,000 x g, whereas after expression at 15 degrees C, both nsP1 and methyltransferase activity were in the supernatant fraction. Paradoxically, the activity from E. coli was completely inhibited by Triton X-100 and DOC. Sucrose gradient analysis showed that even the "soluble" nsP1-methyltransferase was in aggregates. The methyltransferase activities in the P15 fractions of SFV-infected BHK cells and AcNPV-nsP1-infected Sf9 cells and in E. coli catalyzed linear incorporation of the [3H]methyl group from S-adenosylmethionine to GTP for a 60-min period. The enzymes from the three sources had similar substrate specificities and Km values for S-adenosylmethionine. In addition to GTP, they all methylated dGTP and GpppG, but not m7GTP or GpppA, or in vitro-transcribed RNAs with GpppA and GpppG caps. The unique properties of SFV-specific nsP1 methyltransferase are discussed.

ATPase and GTPase activities associated with Semliki Forest virus nonstructural protein nsP2

The replication of Semliki Forest virus requires four nonstructural proteins (nsP1 to nsP4), all derived from the same polyprotein. One of these, nsP2, is a multifunctional protein needed in RNA replication and in the processing of the nonstructural polyprotein. On the basis of amino acid sequence homologies, nsP2 was predicted to possess nucleoside triphosphatase and RNA helicase activities. Here, we report the engineered expression in Escherichia coli of nsP2 and of an amino-terminal fragment of it by use of the highly efficient T7 expression system. Both polypeptides were produced as fusion proteins with a histidine tag at the amino terminus and purified by immobilized-metal affinity chromatography. The two recombinant proteins exhibited ATPase and GTPase activities, which were further stimulated by the presence of single-stranded RNA. The activities were not found in similarly prepared fractions from uninduced control cells or cells expressing an unrelated polypeptide. Radiolabeled ribonucleoside triphosphates could be cross-linked to both the full-length and the carboxy-terminally truncated nsP2 protein, and both polypeptides had RNA-binding capacity. We also expressed and purified an nsP2 variant which had a single amino acid substitution in the nucleotide-binding motif (Lys-192-->Asn). No nucleoside triphosphatase activity was associated with this mutant protein.

Nuclear targeting of Semliki Forest virus nsP2

The Semliki Forest virus-specific nonstructural protein nsP2 is transported into the nuclei of both infected and transfected BHK cells. The pentapeptide sequence P 648R RRV is an essential part of the nuclear localization signal (NLS) of nsP2, the middle arginine being the most critical residue for nuclear targeting. Host DNA and RNA syntheses are rapidly inhibited in virus-infected cells, and nsP2 could be involved in these processes. It has been postulated that the inhibition of cellular replication could be due to viral NTPase activity. We have expressed and purified nsP2 in E. coli using the highly efficient T7 based expression system. Purified nsP2 was shown to have ATPase and GTPase activities, and these specific activities were increased in the presence of single-stranded RNA, a typical feature of RNA helicases. The role of nsP2 in the nucleus was studied by creating a mutant virus SFV-RDR, which contained an altered NLS (PRDRV). The mutation affected neither the processing nor the stability of nsP2, but it did render nsP2 completely cytoplasmic. SFV-RDR was shown to be fully infectious, and no difference could be seen in the expression of viral proteins. In addition, the inhibition of host DNA synthesis was almost equally efficient in both wild-type and mutant-infected cells. The pathogenic properties of the mutant will be further studied.

A method for exposing hidden antigenic sites in paraformaldehyde-fixed cultured cells, applied to initially unreactive antibodies

We describe a simple protocol that allows the retrieval of masked or hidden intracellular antigens in cultured cells. The protocol is based on the exposure of paraformaldehyde-fixed and Triton X-100-permeabilized cells to guanidine hydrochloride (GdnHCl). We used it for localization of different antigens in BHK-21 cells by immunofluorescence microscopy. Our results showed that five out of six antibodies, initially unreactive, became excellent localization tools when used in conjunction with GdnHCl. Denaturation of fixed cells with GdnHCl did not affect the overall cell architecture, when monitored with different organelle-specific antibodies. In two cases out of 17 the antigenic sites were lost after denaturation. However, this problem could be partially overcome by including low amounts of glutaraldehyde in the fixative. We think that this method could be generally useful in immunofluorescence localization studies, particularly in cases where the antibodies are known to react only with denatured antigens.

Rapid affinity-purification and biotinylation of antibodies

A simple and rapid method to affinity-purify and biotinylate antibodies was developed. The method utilizes separation of antigens by sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by transfer to nitrocellulose and binding of the antibodies to the specific antigen. The antibodies are biotinylated, while still bound to the antigen, thus avoiding the conjugation of the active antigen-binding sites of the antibodies. These antibodies have been successfully used in double-label immunofluorescence studies, but they should be likewise applicable in other immunological protocols.

Nuclear and nucleolar targeting signals of Semliki Forest virus nonstructural protein nsP2

Semliki Forest virus nonstructural protein nsP2, when expressed in the absence of other viral proteins, is transported into the nucleus, with a specific enrichment in nucleoli. This implies that information for its nuclear targeting must be in nsP2 itself. To define the nuclear and nucleolar targeting signals of nsP2, a series of in-frame deletions were made in its coding sequence. We have identified one amino-terminal (residues 25-110) and another carboxy-terminal (residues 634-661) deletion, both of which rendered nsP2 cytoplasmic. The carboxy-terminal region had a pentapeptide sequence P 648R RRV, which resembles some of the known nuclear localization signals. When the three arginines were substituted with aspartic acids, the mutant nsP2 was localized in the cytoplasm. Especially arginines 648-649 were shown to be critical. The carboxy-terminal 232 amino acid residues of nsP2 were able to translocate beta-galactosidase to the nucleus as a fusion protein, whereas the amino-terminal 110 residues failed to do so. Studies with different beta-galactosidase-nsP2 fusions showed that the pentapeptide is necessary for nuclear transport but that its activity is strongly affected by the context. The deletion analysis indicated also that the nuclear localization signal was not sufficient for nucleolar targeting of nsP2. The nucleolar targeting sequence of nsP2 was tentatively mapped between residues 470 and 539, and at least residues 470-489 are an essential part of it.

Expression of enzymatically active rat liver and human placental catechol-O-methyltransferase in Escherichia coli; purification and partial characterization of the enzyme

To produce sufficient amounts of recombinant catechol-O-methyltransferase (COMT) for structural and functional studies the coding regions of the rat liver and human placental COMT genes have been introduced into a bacterial expression vector pKEX14. Recombinant COMT was produced in Escherichia coli up to 10% of total bacterial protein after the induction of the T7 RNA polymerase gene with isopropyl-beta-D-thiogalactopyranoside. Both the rat and human enzymes were enzymatically active, soluble and reacted with anti-COMT antiserum in Western blotting. Both enzymes were purified from E. coli cells and partially characterized by determining their specific activity, apparent molecular weight and pI.

Proteolytic processing of Semliki Forest virus-specific non-structural polyprotein

The processing and stability of the non-structural (ns) proteins of Semliki Forest virus were studied in vivo. Virus-specific proteins from infected cells were identified by immunoprecipitation with monospecific antisera. The complete ns precursor, P1234, translated within 7 to 9 min of the start of translation, was processed before the completion of translation into P123 and nsP4. Pulse-chase experiments showed that the mature ns proteins were relatively stable for at least 2 h. Interestingly, the decrease in the amount of the P34 precursor during chase is accompanied by an increase only in the amount of nsP3, which could explain the observed lower amount of nsP4 in infected cells. In cells infected with SFV RNA- mutants ts4 and ts6 maintained at the restrictive temperature, nsP4 but no nsP1, nsP2 or nsP3 accumulated in addition to the ns precursor proteins P1234, P123, P12 and P34. Translation in vitro of mRNAs from a cDNA clone, encoding P1234 with a deletion in the carboxy-terminal proteinase domain of nsP2, did not yield nsP1, nsP2 or nsP3 but only nsP4, indicating that cleavage at the nsP3/4 sites is independent. Evidently, nsP4 is produced by a nascent cleavage of the growing P1234, catalysed by its own proteinase activity; the proteolytic cleavages at the nsP1/2 and nsP2/3 sites are catalysed by the proteinase moiety of nsP2.

Biogenesis of type I cytopathic vacuoles in Semliki Forest virus-infected BHK cells

We investigated the biogenesis of type I cytopathic vacuoles (CPVIs) in Semliki Forest virus (SFV)-infected cells by immunofluorescence and electron microscopy. By using the ts1 mutant of SFV at the restrictive temperature to avoid superinfection, we showed that the multiplicity of infection affects the time of appearance but not the number of CPVIs in a cell. Formation of CPVIs did not require incoming virus particles, because they were found in BHK cells transfected with infectious RNA from the SFV prototype strain or ts1 mutant. When the SFV gene for nsP3 was expressed alone in BHK cells, the nsP3 protein was localized to numerous vesiclelike structures and large vacuoles. The nsP3 protein may function as an anchoring protein for the RNA replication complex of SFV.

Localization and phosphorylation of Semliki Forest virus non-structural protein nsP3 expressed in COS cells from a cloned cDNA

The nsP3 protein of Semliki Forest virus is a phosphoprotein, which is processed from a large nonstructural polyprotein. The nsP3 gene was isolated from the large coding region by the polymerase chain reaction technique and cloned into a eukaryotic expression vector. Using the constructed pSVNS3 expression vector it was shown that nsP3 could be phosphorylated in the absence of other virus-specific proteins. This suggests that the formation of a complex with the other non-structural proteins is not required for the phosphorylation of nsP3. About half of the synthesized nsP3, in pSVNS3-transfected COS cells, could be fractionated into the mitochondrial pellet fraction indicating that nsP3 is associated with large intracellular structures. Immunofluorescence microscopy of pSVNS3-transfected COS cells showed that nsP3 was found in the cytoplasm localized to vesicle-like structures. These results suggest that nsP3 contains information for specific interaction with large intracellular vesicular structures.

A single amino acid change in E3 of ts1 mutant inhibits the intracellular transport of SFV envelope protein complex

At 39 degrees the envelope protein complex (E1-p62) of Semliki Forest virus mutant ts1 is arrested in the rough endoplasmic reticulum (RER). When the infected cultures are shifted to 28 degrees, the complex is transported to the cell surface. During the transport p62 is cleaved into E2 under conditions in which no virus budding takes place. We have sequenced the cDNA, which encodes the envelope proteins of ts1. Comparison with the respective wild-type nucleotide sequence shows only one nucleotide change, G----A, causing a replacement of cysteine-58 (TGC) with tyrosine (TAC) in the E3 protein of ts1. A cDNA fragment from the ts1 genome encoding the mutation in E3 was used to replace the respective fragment of prototype SFV in an eukaryotic expression vector. Intracellular arrest of envelope proteins at 39 degrees was seen in transfected BHK21 cells. A shift of the transfected cells to 28 degrees resulted in the appearance of the envelope proteins at the cell surface. We conclude that the single point mutation is solely responsible for the temperature-sensitive transport defect of ts1 envelope glycoproteins.

Nuclear localization of Semliki Forest virus-specific nonstructural protein nsP2

About 50% of Semliki Forest virus-specific nonstructural protein nsP2 is associated with the nuclear fraction in virus-infected BHK cells. Transport into the nucleus must be specific, since only trace amounts of nsP3 and nsP4 and about 13% of nsP1, all derived from the same polyprotein, were found in the nucleus. Subfractionation of [35S]methionine-labeled Semliki Forest virus-infected cells showed that 80 to 90% of the nuclear nsP2 was associated with the nuclear matrix. Indirect immunofluorescence, with anti-nsP2 antiserum, showed the most intensive staining of structures which by Nomarski optics appeared to be nucleoli. In the presence of 1 to 5 micrograms of dactinomycin per ml the nuclei were stained evenly and no nucleoli could be found. Transport of nsP2 into the nucleus occurred early in infection and was fairly rapid. A cDNA encoding the complete nsP2 was isolated by the polymerase chain reaction technique and ligated into a simian virus 40 expression vector derivative. When BHK cells were transfected with this pSV-NS2 vector by the lipofection procedure, nsP2 was expressed in about 1 to 5% of the cells, as shown by indirect immunofluorescence. In positively transfected cells the immunofluorescence stain was most intensive in the nucleoli. Thus, Semliki Forest virus-specific nsP2 must have information which directs it into the nuclear matrix and, more specifically, into the nucleoli.

The Semliki-Forest-virus-specific nonstructural protein nsP4 is an autoproteinase

The Semliki-Forest-virus-specific nonstructural proteins are translated as a large polyprotein (2431 amino acid residues), from which the mature polymerase components nsP1, nsP2, and nsP4 are released by proteolytic cleavages. The complete ns polyprotein (P1234) can be cleaved in two alternative ways yielding either P123 (with sequences of nsP1, nsP2 and nsP3) and nsP4 or P12 (nsP1 plus nsP2) and P34 (nsP3 plus nsP4). We studied the possible autoproteolytic role of nsP4 involved in the cleavage between nsP3 and nsP4 in an in vitro transcription-translation system. cDNAs encoding P34 precursor and shorter precursor protein segments covering the nsP3-nsP4 cleavage region, were cloned under the T7 RNA polymerase promoter. The mRNAs synthesized in vitro were capped and translated in rabbit reticulocyte lysates. The translational products were analyzed by SDS/PAGE. The precursor proteins containing nsP4 sequences were cleaved yielding the products with expected sizes, indicating that the cleavage took place at the nsP3-nsP4 junction. By deleting and truncating the cDNA coding for nsP4, the proteolytic activity was mapped within the 102 amino-terminal amino acids of nsP4. The cleavage between nsP3 and nsP4 can be inhibited by pepstatin A and probably takes place in cis, since exogenously added nsP4 was unable to mediate it.

Semliki Forest virus-specific non-structural protein nsP3 is a phosphoprotein

Antisera were raised in rabbits against fusion proteins consisting of beta-galactosidase and partial amino acid sequences of Semliki Forest virus (SFV)-specific non-structural proteins nsP1, nsP2, nsP3 and nsP4. The antisera were specific since each of them precipitated only one labelled protein of a size expected for nsP1, nsP2, nsP3 or nsP4 from lysates of [35S]methionine-labelled SFV-infected BHK-21 cells. The specific antisera also precipitated p220 (with sequences of nsP1, nsP2 and nsP3), p155 (nsP1 and nsP2) and p135 (nsP3 and nsP4) which have been previously shown to be cleavage products of the polyprotein precursor of the non-structural proteins. nsP1, nsP4 and most of nsP3, together with the virus-specific RNA polymerase activity, were in the mitochondrial pellet (P15) fraction of infected BHK-21 cells whereas nsP2 was evenly distributed between P15 and the supernatant fraction (S15). Only antisera directed against nsP3 sequences precipitated a labelled protein from cells incubated with [32P]orthophosphate during SFV infection. Treatment of the immunoprecipitate with calf alkaline intestinal phosphatase reduced the amount of labelled nsP3 considerably. Immunoprecipitated 32P-labelled nsP3, isolated by SDS-PAGE, was subjected to acid hydrolysis. Both phosphoserine and phosphothreonine but not phosphotyrosine could be identified in the hydrolysate. Approximately twice as much [32P]serine as [32P]threonine was detected in nsP3. P15 and S15 fractions were prepared from [35S]methionine- and 32P-labelled SFV-infected cells and the 35S/32P ratio of nsP3 was determined after immunoprecipitation and SDS-PAGE. The nsP3 in S15 was less heavily phosphorylated (about 50%) than P15-associated nsP3. Anti-nsP3 serum revealed large cytoplasmic vesicles in SFV-infected cells in indirect immunofluorescence microscopy.