Abstract
Cold-adaptation strategies have been studied in multiple psychrophilic organisms, especially for psychrophilic enzymes. Decreased enzyme activity caused by low temperatures as well as a higher viscosity of the aqueous environment require certain adaptations to the metabolic machinery of the cell. In addition to this, low temperature has deleterious effects on the lipid bilayer of bacterial membranes and therefore might also affect the embedded membrane proteins. Little is known about the adaptation of membrane proteins to stresses of the cold. In this study we investigate a set of 66 membrane proteins from the core genome of the bacterial family Vibrionaceae to identify general characteristics that discern psychrophilic and mesophilic membrane proteins. Bioinformatical and statistical methods were used to analyze the alignments of the three temperature groups mesophilic, intermediate and psychrophilic. Surprisingly, our results show little or no adaptation to low temperature for those parts of the proteins that are predicted to be inside the membrane. However, changes in amino acid composition and hydrophobicity are found for complete sequences and sequence parts outside the lipid bilayer. Among others, the results presented here indicate a preference for helix-breaking and destabilizing amino acids Ile, Asp and Thr and an avoidance of the helix-forming amino acid Ala in the amino acid composition of psychrophilic membrane proteins. Furthermore, we identified a lower overall hydrophobicity of psychrophilic membrane proteins in comparison to their mesophilic homologs. These results support the stability-flexibility hypothesis and link the cold-adaptation strategies of membrane proteins to those of loop regions of psychrophilic enzymes.
Highlights
Cold adapted bacteria colonize habitats that are hostile to most organisms, e.g. the arctic ocean or the deep-sea, with temperature minima close to or even below the freezing point of water
In total the dataset used in the presented study includes four psychrophilic bacteria, six isolates assigned to the intermediate temperature group and 54 mesophilic bacteria
We identified 66 membrane proteins, 52 transmembrane proteins (TMP), 11 signal peptides and 3 b-barrel containing membrane proteins, present in all genomes in our dataset
Summary
Cold adapted bacteria colonize habitats that are hostile to most organisms, e.g. the arctic ocean or the deep-sea, with temperature minima close to or even below the freezing point of water. To maintain growth and survival at these low temperatures coldadapted (psychrophilic) bacteria require an array of specific adaptations in the cellular components, protein synthesis machinery and enzymes [1,2]. The main challenges psychrophiles have to overcome are a decrease in enzyme activity and increased viscosity of the aqueous environment due to the low temperature. The results show that psychrophilic enzymes tend to be more flexible and less stable to maintain their catalytic activity. Studies on psychrophilic enzymes produced differing results, indicating that psychrophilic organisms developed more than one strategy to adapt to the cold [7,8]
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