Biomedical subjects

S L Allison

Publications and source records attributed to S L Allison.

24 records · Page 2Linked to original sources

PubMed1995

Oligomeric rearrangement of tick-borne encephalitis virus envelope proteins induced by an acidic pH.

The flavivirus envelope protein E undergoes irreversible conformational changes at a mildly acidic pH which are believed to be necessary for membrane fusion in endosomes. In this study we used a combination of chemical cross-linking and sedimentation analysis to show that the envelope proteins of the flavivirus tick-borne encephalitis virus also change their oligomeric structure when exposed to a mildly acidic environment. Under neutral or slightly alkaline conditions, protein E on the surface of native virions exists as a homodimer which can be isolated by solubilization with the nonionic detergent Triton X-100. Solubilization with the same detergent after pretreatment at an acidic pH, however, yielded homotrimers rather than homodimers, suggesting that exposure to an acidic pH had induced a simultaneous weakening of dimeric contacts and a strengthening of trimeric ones. The pH threshold for the dimer-to-trimer transition was found to be 6.5. Because the pH dependence of this transition parallels that of previously observed changes in the conformation and hydrophobicity of protein E and that of virus-induced membrane fusion, it appears likely that the mechanism of fusion with endosomal membranes involves a specific rearrangement of the proteins in the viral envelope. Immature virions in which protein E is associated with the uncleaved precursor (prM) of the membrane protein M did not undergo a low-pH-induced rearrangement. This is consistent with a protective role of protein prM for protein E during intracellular transport of immature virions through acidic compartments of the trans-Golgi network.

Animals↗

PubMed1995

Synthesis and secretion of recombinant tick-borne encephalitis virus protein E in soluble and particulate form.

A quantitative study was performed to investigate the requirements for secretion of recombinant soluble and particulate forms of the envelope glycoprotein E of tick-borne encephalitis (TBE) virus. Full-length E and a carboxy terminally truncated anchor-free form were expressed in COS cells in the presence and absence of prM, the precursor of the viral membrane protein M. Formation of a heteromeric complex with prM was found to be necessary for efficient secretion of both forms of E, whereas only low levels of anchor-free E were secreted in the absence of prM. The prM-mediated transport function could also be provided by coexpression of prM and E from separate constructs, but a prM-to-E ratio of greater than 1:1 did not further enhance secretion. Full-length E formed stable intracellular heterodimers with prM and was secreted as a subviral particle, whereas anchor-free E was not associated with particles and formed a less stable complex with prM, suggesting that prM interacts with both the ectodomain and anchor region of E.

Animals↗

PubMed1994

Structural changes and functional control of the tick-borne encephalitis virus glycoprotein E by the heterodimeric association with protein prM.

We have used tick-borne encephalitis virus to study the involvement of acidic compartments during the entry and release phases of flavivirus infection and to elucidate the role of protein prM in immature virions. Elevation of the pH in acidic intracellular compartments by either bafilyomycin A1, a specific inhibitor of the vacuolar type H(+)-ATPase or by NH4Cl had a strong inhibitory effect during virus penetration and also prevented the cleavage of prM when added in the late phase of the viral life cycle. In the latter case the release of virus particles was not impaired. These immature (prM-containing) virions exhibited a 20- to 50-fold lower specific infectivity and HA activity than mature virions and in contrast to these did not undergo low pH-triggered aggregation. The presence of prM also affected the binding of monoclonal antibodies to protein E, especially at sites which have been shown to undergo acid pH-induced conformational changes in mature virions. Crosslinking, solubilization, and sedimentation analyses revealed the existence of prM-E heterooligomeric complexes, suggesting that the function of prM is to protect protein E from undergoing the irreversible conformational changes in acidic compartments of the secretory pathway that are necessary for triggering fusion activity in the endosome during virus entry.

Ammonium Chloride↗

PubMed1994

Expression of cloned envelope protein genes from the flavivirus tick-borne encephalitis virus in mammalian cells and random mutagenesis by PCR.

The structural membrane proteins prM and E of the flavivirus tick-borne encephalitis (TBE) virus were expressed in mammalian cells for the purpose of probing the structure and molecular interactions of these proteins. Advantage was taken of the natural error frequency of the Taq polymerase used in the PCR amplification to generate a randomly mutated population of genes that were then cloned directly into plasmid expression vectors under the control of an SV40 promoter. Analysis of the mutation frequency by direct sequencing of 22 separate clones showed that the PCR produced mutations at a rate yielding an average of one to two amino acid changes per clone in the 496 amino acid long protein E. This is an ideal rate for assessing the importance of individual amino acid residues within protein domains, thus demonstrating the potential value of the PCR as a random mutagenesis method. Clones encoding wild-type prM and E proteins, and a truncated form of E, were also constructed by recombining portions of selected PCR clones. Transfection of COS-1 cells with these constructs resulted in expression of the prM and E proteins, which was demonstrated by indirect immunofluorescence using monoclonal antibodies (Mabs). The intracellular level of TBE virus antigen, measured in lysates of transfected cells by ELISA, reached approximately 25% of that found in virus-infected COS cells. Furthermore, it was shown by immunofluorescence using a panel of 19 anti-E Mabs that the antigenic structure of the expressed E proteins was nearly identical to that of E protein in infected cells, thus confirming the suitability of this model system as a tool for studying flavivirus protein structure.

Animals↗

PubMed1990

Nucleotide sequence of the gene encoding the repressor for the histidine utilization genes of Pseudomonas putida.

The hutC gene of Pseudomonas putida encodes a repressor which, in combination with the inducer urocanate, regulates expression of the five structural genes necessary for conversion of histidine to glutamate, ammonia, and formate. The nucleotide sequence of the hutC region was determined and found to contain two open reading frames which overlapped by one nucleotide. The first open reading frame (ORF1) appeared to encode a 27,648-dalton protein of 248 amino acids whose sequence strongly resembled that of the hut repressor of Klebsiella aerogenes (A. Schwacha and R. A. Bender, J. Bacteriol. 172:5477-5481, 1990) and contained a helix-turn-helix motif that could be involved in operator binding. The gene was preceded by a sequence which was nearly identical to that of the operator site located upstream of hutU which controls transcription of the hutUHIG genes. The operator near hutC would presumably allow the hut repressor to regulate its own synthesis as well as the expression of the divergent hutF gene. A second open reading frame (ORF2) would encode a 21,155-dalton protein, but because this region could be deleted with only a slight effect on repressor activity, it is not likely to be involved in repressor function or structure.

Amino Acid Sequence↗

PubMed1989

Identification of multiple repressor recognition sites in the hut system of Pseudomonas putida.

The hutC gene in Pseudomonas putida encodes a repressor protein that negatively regulates the expression of all hut genes. We have overexpressed this cloned hutC gene in Escherichia coli to identify P. putida hut regions that could specifically bind the repressor. Ten restriction fragments, some of which were partially overlapping and spanned the coding portions of the P. putida hut region, were labeled and tested for their ability to recognize repressor in a filter binding assay. This procedure identified three binding sites, thus supporting previous indications that there were multiple operons. A 1.0-kilobase-pair SalI restriction fragment contained the operator region for the hutUHIG operon, whereas a 1.9-kilobase-pair SmaI fragment contained the hutF operator. A 2.9-kilobase-pair XhoI segment appeared to contain the third operator, corresponding to a separate and perhaps little used control region for hutG expression only. The addition of urocanate, the normal inducer, caused dissociation of all operator-repressor complexes, whereas N-formylglutamate, capable of specifically inducing expression of the hutG gene, inhibited binding only of repressor to fragments containing that gene. Formylglutamate did not affect the action of urocanate on the repressor-hutUHIG operator complex, indicating that it binds to a site separate from urocanate on the repressor. DNA footprinting and gel retardation analyses were used to locate more precisely the operator for the hutUHIG operon. A roughly 40-base-pair portion was identified which contained a 16-base-pair region of dyad symmetry located near the transcription initiation site for this operon.

Base Sequence↗