Streptavidin forms two-dimensional crystals when specifically bound to layers of biotinylated lipids at the air/water interface. The three-dimensional structure of streptavidin determined from the crystals by electron crystallography corresponds well with the structure determined by x-ray crystallography. Comparison of the electron and x-ray crystallographic structures reveals the occurrence of free biotin-binding sites on the surface of the two-dimensional crystals facing the aqueous solution. The free biotin-binding sites could be specifically labeled with biotinylated ferritin. The streptavidin/biotinylated lipid system may provide a general approach for the formation of two-dimensional crystals of biotinylated macromolecules.
Highly specific ligand-receptor interactions generally characterize surface recognition reactions. Such processes can be simulated by streptavidin-biotin-specific binding. Biotin lipids have thus been synthesized, and their interaction with streptavidin (or avidin) at the air-water interface was directly shown by measurement of surface pressure isotherms and fluorescence microscopy. These proteins interact with the biotin lipid monolayer via specific binding or nonspecific adsorption. Both phenomena were clearly distinguished by use of the inactivated form of streptavidin. The binding of fluorescein-labeled streptavidin to monolayers was also directly observed by fluorescence microscopy. The fluorescence of the protein domains is directly related to the state of polarization of the exciting light. This anisotropy can only be explained by the formation of oriented two-dimensional biotin lipid-streptavidin domains.
Interactions of proteins with membranes are of importance in many fields of biochemistry and In particular, studying membrane-bound receptors in vitro requires working models for biological membranes. As a model for lipid membranes, monolayers of amphiphiles on an aqueous subphase, or mono-and multilayers of amphiphiles on solid substrates (Langmuir-Blodgett films) are often used. An evaluation of specific and unspecific interactions of polypeptides with several types of films is discussed in this paper.Proteins can exhibit several modes of interaction with the hydrophilic or the hydrophobic sites of lipid-covered surfaces: these can be, for example, ionic interactions with the charged hydrophilic head groups of lipids, that may eventually lead to an intercalation of the proteins into the lipid layer between their hydrophobic tails. Proteins are also well known to adsorb on hydrophobic surfaces, and so it is to be expected that they do on Langmuir-Blodgett films.Furthermore, a specific interaction of proteins with lipid monolayers can be demonstrated when functionalized lipids are used. A very intelligent sensor mimicking a biological function could exist in a lipid film containing molecules with
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