Abstract
S-layers commonly cover archaeal cell envelopes and are composed of proteins that self-assemble into a paracrystalline surface structure. Despite their detection in almost all archaea, there are few reports investigating the structural properties of these proteins, with no reports exploring this topic for halophilic S-layers. The objective of the present study was to investigate the secondary and tertiary organization of the Haloferax volcanii S-layer protein. Such investigations were performed using circular dichroism, fluorescence spectroscopy, dynamic light scattering and transmission electron microscopy. The protein secondary structure is centered on β-sheets and is affected by environmental pH, with higher disorder in more alkaline conditions. The pH can also affect the protein’s tertiary structure, with higher tryptophan side-chain exposure to the medium under the same conditions. The concentrations of Na, Mg and Ca ions in the environment also affect the protein structures, with small changes in α-helix and β-sheet content, as well as changes in tryptophan side chain exposure. These changes in turn influence the protein’s functional properties, with cell envelope preparations revealing striking differences when in different salt conditions. Thermal denaturation assays revealed that the protein is stable. It has been reported that the S-layer protein N-glycosylation process is affected by external factors and the present study indicates for the first time changes in the protein structure.
Highlights
Haloarchaea are able to thrive in hyper saline environments such as salt lakes, the Dead Sea, natural brines and marine solar salterns [1]
The H. volcanii S-layer protein theoretical molecular weight is of approximately 85.2 kDa and its primary structure consists of 827 amino acids, with seven potential N-glycosylation sites
H. volcanii cells were grown to late exponential phase and cell envelope preparations conducted according to protocols described by Kessel et al, 1988 [21], with minor modifications
Summary
Haloarchaea are able to thrive in hyper saline environments such as salt lakes, the Dead Sea, natural brines and marine solar salterns [1]. Some of these microbes exhibit optimal growth at salt concentrations approaching the saturation point [2]. In order to survive in such conditions, they present several adaptations to maintain their cellular proteins stable and active [3]. Proteins tend to be rich in surfaceexposed negatively charged amino acids [6,7,8], improving solubility at high salt concentrations.
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