We propose to crystallize the complex of the prokaryotic ribosome or its 50S subunit with the SecYEG translocon. Two SecYEG heterotrimers bind to the ribosome at the end of the nascent polypeptide tunnel to form a transmembrane channel which is part of a translocase complex that allows for secreted and membrane embedded molecules to be transcribed through or into the membrane bilayer respectively.Building on previously established expression and complex formation protocols that allowed the analysis of this complex via electron microscopy we will proceed to crystallize the complex. We will rely on a multi-pronged approach involving ribosomes and SecYEG proteins from various organisms, which are able to form heterologous complexes, as well as engineered/truncated versions of the SecYEG proteins.Our goal is to solve the crystal structure of this complex at atomic resolution to shed light on the exact mechanisms of one of the most basic and important biological processes. Questions we hope the structure will help us address are how the channel is opened and closed, how membrane proteins are embedded into the bilayer from the channel and how the membrane seal is maintained throughout the whole process.The proposed project contributes to the development of x-ray crystallography of macromolecular assemblies and membrane proteins which are both regarded as future focal points of structural biology. As such it is valuable for European research and also provides excellent future perspectives for the researcher.The techniques involved in the project complement the skill-set of the researcher well and will leave him with well-rounded crystallographic expertise focused on two of the most promising areas in his field of research. Furthermore, funding this project would al low the researcher, who is from an objective 1 less-favoured region, to return to Europe from his current position in the United States.