Insertion of membrane proteins in artificial polymer membranes

Abstract

The last few decades have seen a huge growth in research on “soft materials”. A large part of the research in this field was dedicated to the preparation of new types of artificial membranes, which behave similar to lipid or cell membranes. A particular challenge is the preparation of stabilized, flexible, adaptable and responsive materials. Similar to nature such systems can only be realized using hierarchically self-assembled systems. In this context we have introduced a new way of stabilizing lipid-bilayers using hydrophobic polymer scaffold. In contrast to other approaches, presented by Ringsdorf et al., 1988, the hydrophobic polymer scaffold allowed us to insert membrane proteins into the polymer stabilized membranes. One representative example of the functional insertion of a membrane protein into such stabilized membranes will be described in the present work. In another approach we used the self assembling capacity of amphiphilic block copolymers to prepare stable biomimetic membranes. The last few years have seen considerable progress in the development of block copolymer chemistry. Particularly, a whole series of new amphiphilic block copolymers with low glass-transition temperatures have been introduced. The diversity of block copolymer chemistry allows to vary the chemical constitution, such as the nature and the sequence of the repeat unit (as mentioned in a later section), the length and the different structures of the different blocks and even the molecular architecture of the whole polymer, block, graft, star, etc. This may lead to the formation of new types of self-assembled superstructures that are not accessible to conventional low molar mass amphiphiles. Recently our group introduced a new type of amphiphilic block copolymer composed of two hydrophilic side blocks poly-methyloxazoline (PMOXA) and one hydrophobic middle block polydimethylsiloxane (PDMS), forming an ABA triblock copolymer. The physico-chemical characterization of the ABA block copolymer has been described by Nardin et al. Similar to conventional low molar mass amphiphiles (lipids, surfactants), this polymer selfassembles in aqueous media and forms well defined superstructures. Depending on its molecular composition and the experimental conditions various lyotropic mesophases, vesicles and nanotubes have been identified. Most interestingly it has been shown that membranes formed by such polymers could be used as a matrix for the incorporation of integral membrane proteins. In the present work we developed new procedures for membrane proteins that are adapted and optimized with respect to the artificial polymer membranes. For that purpose we performed a series of experiments with different membrane proteins that have different structural properties and functionality. In all systems investigated we could successfully proof the incorporation and the functionality of the proteins. For a first experiment we used well characterized and stable membrane proteins like bacterial porins. Porins are well characterized integral membrane proteins possessing interesting structural and physical properties, such as hydrophobic β-strands, which can interact and insert into the hydrophobic part of the block copolymer. Additionally, the porins form β-stranded pores, which allow a passive and selective transfer of small molecules across a membrane. Then, more complex membrane proteins were used such as hemagglutinin or NADH:oxydoreductase. Both proteins are composed of a large soluble part which contributes to their structural and functional particularities. The globular part of hemagglutinin is involved in the fusion of two membranes whereas the soluble part of NADH:oxydoreductase is responsible for proton and electron transfer across the membrane. The combination of natural proteins with artificial polymer membranes allows the formation of a new type of hybrid material combining the mechanical, chemical, and biological stability of the amphiphile block copolymer and the functional specificity of membrane proteins.