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Abstract

INTRODUCTION
Proteins fulfill many functions in the cell, based on a large variety of specific architectures. Therefore, for the understanding of the molecular mechanism of a protein, the respective high-resolution structure provides crucial information. In addition, membrane proteins are the target for a majority of drugs, and structures can be used for rational design and development of the compounds. The most widely used method for determination of protein structures at high, near atomic resolution is x-ray crystallography that requires well-ordered three-dimensional crystals grown from protein solution. Whereas several thousand structures of soluble proteins have already been determined by x-ray crystallography, fewer than 50 integral membrane protein structures became known since 1985, when the first membrane protein structure of the bacterial photosynthetic reaction center was presented (Deisenhofer et al., 1985). This is in contrast to the high proportion of integral membrane proteins predicted from a genome analysis of eubacterial, archaean, and eukaryotic organisms, in which 20 to 30% of the open reading frames encode integral membrane proteins (Wallin and von Heijne, 1998).