Abstract
It is difficult to maintain a target membrane protein in a soluble and functional form in aqueous solution without biological membranes. Use of surfactants can improve solubility, but it remains challenging to identify adequate surfactants that can improve solubility without damaging their native structures and biological functions. Here we report the use of a new class of lipopeptides to solubilize photosystem I (PS-I), a well known membrane protein complex. Changes in the molecular structure of these surfactants affected their amphiphilicity and the goal of this work was to exploit a delicate balance between detergency and biomimetic performance in PS-I solubilization via their binding capacity. Meanwhile, the effects of these surfactants on the thermal and structural stability and functionality of PS-I in aqueous solution were investigated by circular dichroism, fluorescence spectroscopy, SDS-PAGE analysis and O2 uptake measurements, respectively. Our studies showed that the solubility of PS-I depended on both the polarity and charge in the hydrophilic head of the lipopeptides and the length of its hydrophobic tail. The best performing lipopeptides in favour of PS-I solubility turned out to be C14DK and C16DK, which were comparable to the optimal amphiphilicity of the conventional chemical surfactants tested. Lipopeptides showed obvious advantages in enhancing PS-I thermostability over sugar surfactant DDM and some full peptide amphiphiles reported previously. Fluorescence spectroscopy along with SDS-PAGE analysis demonstrated that lipopeptides did not undermine the polypeptide composition and conformation of PS-I after solubilization; instead they showed better performance in improving the structural stability and integrity of this multi-subunit membrane protein than conventional detergents. Furthermore, O2 uptake measurements indicated that PS-I solubilized with lipopeptides maintained its functionality. The underlying mechanism for the favorable actions of lipopeptide in PS-I solubilization and stabilization is discussed.
Highlights
Surfactants play important roles in membrane protein research, from isolation, purification, crystallization, structural determination to functional studies [1,2,3,4,5]
We describe the design and synthesis of a series of ultrashort lipopeptides consisting of only two amino acid residues
Because of the combined characteristics of lipids and peptides, lipopeptides are promising in many biological applications
Summary
Surfactants play important roles in membrane protein research, from isolation, purification, crystallization, structural determination to functional studies [1,2,3,4,5]. The use of surfactants is to disrupt biological membranes in which membrane proteins are embedded and maintain proteins in a solubilized form away from the complex native environment. Labor-intensive screening is typically required to obtain an appropriate one from a multitude of surfactants/detergents for a membrane protein of interest and a specific purpose [6,7,8]. Despite these disadvantages, mounting experience and some general trends observed in this field can be useful as guides for surfactant selection. Surfactants with small and charged heads like sodium dodecyl sulfate (SDS) are highly effective in the solubilization of membrane proteins but tend to be denaturing, associated with unfolding of native protein structures; surfactants with large and nonionic heads like ndodecyl-β-D-maltopyranoside (DDM) are mild and nondenaturing but the resulting large protein-detergent complexes disfavor the structural analyses [5,9,10,11]
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