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The cryoprotective effects of glycine betaine on bacteria
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topology and was now sitting the right way around in the membrane. Full function was also restored to this re-folded LacY as it could catalyze full active transport. This result is significant, as it suggests that changes in the lipid composition of a membrane can change the topology, and hence properties, of the membrane proteins within it. Exactly how the presence of PE results in the refolding of LacY is unclear, although
TRENDS in Microbiology Vol.10 No.7 July 2002
Bogdanov et al. favour a mechanism whereby PE destabilizes the partially functioning LacY, resulting in re-entry of the protein into the insertion apparatus where the native structure is formed via a post-assembly proofreading process. These investigators concluded that protein topology can be controlled by determinants in both the protein sequence and the membrane lipids, and that postassembly reorganization of the TM helices
The cryoprotective effects of glycine betaine on bacteria It is well known that in response to a decrease in environmental temperature, some plants and animals accumulate several cryoprotective substances inside the cell. However, the role of glycine betaine as a bacterial cryoprotectant is only beginning to be understood. The growth-enhancing effect of glycine betaine on the food-borne pathogen Listeria monocytogenes at low temperature was first reported by a laboratory at the University of California, USA. This study also revealed that the uptake of glycine betaine into the bacterial cell was stimulated at low temperature. Recently, the same laboratory demonstrated a role for an ATP-dependent transport system in the cold-induced uptake of glycine betaine. Additionally, this transporter was shown to be inhibited by the glycine betaine analogs carnitine, dimethylglycine and γ-butyrobetaine [1]. The growth-enhancing effect of glycine betaine on L. monocytogenes at low temperature was attributed to its ability to prevent cold-induced aggregation of cellular proteins and its role in maintaining membrane fluidity at low temperature. It has been shown that following inductive synthesis of some heat shock proteins (HSPs), which are believed to protect the cell from thermal stress, the frequency of survival of an Escherichia coli strain under frozen storage conditions was remarkably increased; this was thought to result from a chaperoning effect of the HSPs preventing denaturation of the cellular proteins at low temperature. Hence, it seems possible that the cryoprotective properties of glycine betaine result from a similar effect. The fact that this osmoprotectant is already known to act as a http://tim.trends.com
chemical chaperone during thermal stress strengthens this suggestion. An additional role of glycine betaine in regulating membrane fluidity cannot be ruled out, as its ability to increase membrane fluidity was demonstrated earlier using bilayers of small unilamellar vesicles. It has also been observed that when two strains of L. monocytogenes were grown in the presence of glycine betaine at low temperature, the amount of a branched-chain fatty acid that was essential for survival of the strains in cold (anteiso-C 15:0) was slightly enhanced. Subsequently, a cold-sensitive mutant deficient in this branched-chain fatty acid was found to have decreased membrane fluidity [2]. It therefore appears that glycine betaine helps to maintain membrane fluidity at low temperature by promoting the synthesis of specific fatty acids. Recent investigations on isolated thylakoid membrane demonstrate the mutifaceted nature of its cryoprotective effects. Similar studies involving liposomes made of total lipids extracted from some cold-adapted bacteria promise further insights into the mechanism responsible for the cryoprotective effects of glycine betaine. 1 Mendum, M.L. and Smith, L.T. (2002) Characterization of glycine betaine porter I from Listeria monocytogenes and its roles in salt and chill tolerance. Appl. Environ. Microbiol. 68, 813–819 2 Jones, S.L. et al. (2002) Correlation of long-range membrane order with temperaturedependent growth characteristics of parent and a cold-sensitive, branched-chain-fatty-aciddeficient mutant of Listeria monocytogenes. Arch. Microbiol. 177, 217–222
M.K.Chattopadhyay [email protected]
311
can occur in membrane proteins in response to changes in the lipid environment. 1 Bogdanov, M. et al. (2002) A polytopic membrane protein displays a reversible topology dependent on membrane lipid composition. EMBO J. 21, 2107–2116
Gavin H. Thomas [email protected]
Evolution of gene fusions One of the major mechanisms of protein evolution is the formation of multidomain proteins as a result of gene fusion. Linking genes that functionally cooperate can confer significant selective advantages. For example, the fusion of protein domains from enzymes belonging to the same metabolic pathway simplifies transcription regulation, alleviates the problem of protein diffusion in the cytoplasm and increases the efficiency of coupling of different biochemical reactions previously catalyzed by distinct enzymes. Fused genes often show dispersed phylogenetic distribution, which suggests a role for processes other than vertical transmission in their evolution. Yanai et al. [1] have now tried to elucidate the evolutionary history of gene fusions. The authors studied 51 gene fusions identified from 32 sequenced microbial genomes, each belonging to at least two of the three kingdoms, Bacteria, Archaea and Eukarya. The gene fusions were split into the individual protein domains and phylogenetic trees were built for each of the fusion components, as well as for corresponding orthologous domains of stand-alone genes. The resulting trees were compared with each other and to the species phylogenetic trees based on ribosomal proteins. This work indicates that, although independent protein domain fusions in distinct evolutionary lineages are not uncommon, the horizontal transfer of gene fusions plays a major role in the evolution and dissemination of protein-domain architectures, particularly within the bacterial and archaeal kingdoms. 1 Yanai, I. et al. (2002) Evolution of gene fusions: horizontal transfer versus independent events. Genome Biol. 3, 0024.1-0024.13
Ivan Matic [email protected]
0966-842X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
topology and was now sitting the right way around in the membrane. Full function was also restored to this re-folded LacY as it could catalyze full active transport. This result is significant, as it suggests that changes in the lipid composition of a membrane can change the topology, and hence properties, of the membrane proteins within it. Exactly how the presence of PE results in the refolding of LacY is unclear, although
TRENDS in Microbiology Vol.10 No.7 July 2002
Bogdanov et al. favour a mechanism whereby PE destabilizes the partially functioning LacY, resulting in re-entry of the protein into the insertion apparatus where the native structure is formed via a post-assembly proofreading process. These investigators concluded that protein topology can be controlled by determinants in both the protein sequence and the membrane lipids, and that postassembly reorganization of the TM helices
The cryoprotective effects of glycine betaine on bacteria It is well known that in response to a decrease in environmental temperature, some plants and animals accumulate several cryoprotective substances inside the cell. However, the role of glycine betaine as a bacterial cryoprotectant is only beginning to be understood. The growth-enhancing effect of glycine betaine on the food-borne pathogen Listeria monocytogenes at low temperature was first reported by a laboratory at the University of California, USA. This study also revealed that the uptake of glycine betaine into the bacterial cell was stimulated at low temperature. Recently, the same laboratory demonstrated a role for an ATP-dependent transport system in the cold-induced uptake of glycine betaine. Additionally, this transporter was shown to be inhibited by the glycine betaine analogs carnitine, dimethylglycine and γ-butyrobetaine [1]. The growth-enhancing effect of glycine betaine on L. monocytogenes at low temperature was attributed to its ability to prevent cold-induced aggregation of cellular proteins and its role in maintaining membrane fluidity at low temperature. It has been shown that following inductive synthesis of some heat shock proteins (HSPs), which are believed to protect the cell from thermal stress, the frequency of survival of an Escherichia coli strain under frozen storage conditions was remarkably increased; this was thought to result from a chaperoning effect of the HSPs preventing denaturation of the cellular proteins at low temperature. Hence, it seems possible that the cryoprotective properties of glycine betaine result from a similar effect. The fact that this osmoprotectant is already known to act as a http://tim.trends.com
chemical chaperone during thermal stress strengthens this suggestion. An additional role of glycine betaine in regulating membrane fluidity cannot be ruled out, as its ability to increase membrane fluidity was demonstrated earlier using bilayers of small unilamellar vesicles. It has also been observed that when two strains of L. monocytogenes were grown in the presence of glycine betaine at low temperature, the amount of a branched-chain fatty acid that was essential for survival of the strains in cold (anteiso-C 15:0) was slightly enhanced. Subsequently, a cold-sensitive mutant deficient in this branched-chain fatty acid was found to have decreased membrane fluidity [2]. It therefore appears that glycine betaine helps to maintain membrane fluidity at low temperature by promoting the synthesis of specific fatty acids. Recent investigations on isolated thylakoid membrane demonstrate the mutifaceted nature of its cryoprotective effects. Similar studies involving liposomes made of total lipids extracted from some cold-adapted bacteria promise further insights into the mechanism responsible for the cryoprotective effects of glycine betaine. 1 Mendum, M.L. and Smith, L.T. (2002) Characterization of glycine betaine porter I from Listeria monocytogenes and its roles in salt and chill tolerance. Appl. Environ. Microbiol. 68, 813–819 2 Jones, S.L. et al. (2002) Correlation of long-range membrane order with temperaturedependent growth characteristics of parent and a cold-sensitive, branched-chain-fatty-aciddeficient mutant of Listeria monocytogenes. Arch. Microbiol. 177, 217–222
M.K.Chattopadhyay [email protected]
311
can occur in membrane proteins in response to changes in the lipid environment. 1 Bogdanov, M. et al. (2002) A polytopic membrane protein displays a reversible topology dependent on membrane lipid composition. EMBO J. 21, 2107–2116
Gavin H. Thomas [email protected]
Evolution of gene fusions One of the major mechanisms of protein evolution is the formation of multidomain proteins as a result of gene fusion. Linking genes that functionally cooperate can confer significant selective advantages. For example, the fusion of protein domains from enzymes belonging to the same metabolic pathway simplifies transcription regulation, alleviates the problem of protein diffusion in the cytoplasm and increases the efficiency of coupling of different biochemical reactions previously catalyzed by distinct enzymes. Fused genes often show dispersed phylogenetic distribution, which suggests a role for processes other than vertical transmission in their evolution. Yanai et al. [1] have now tried to elucidate the evolutionary history of gene fusions. The authors studied 51 gene fusions identified from 32 sequenced microbial genomes, each belonging to at least two of the three kingdoms, Bacteria, Archaea and Eukarya. The gene fusions were split into the individual protein domains and phylogenetic trees were built for each of the fusion components, as well as for corresponding orthologous domains of stand-alone genes. The resulting trees were compared with each other and to the species phylogenetic trees based on ribosomal proteins. This work indicates that, although independent protein domain fusions in distinct evolutionary lineages are not uncommon, the horizontal transfer of gene fusions plays a major role in the evolution and dissemination of protein-domain architectures, particularly within the bacterial and archaeal kingdoms. 1 Yanai, I. et al. (2002) Evolution of gene fusions: horizontal transfer versus independent events. Genome Biol. 3, 0024.1-0024.13
Ivan Matic [email protected]
0966-842X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.