Most cellular processes are intimately connected to the transport and to the folding of proteins. To achieve their remarkable conformational flexibility, proteins are intrinsically labile structures, which fold and unfold constantly. In principle, all the information required for the acquisition of the functional three dimensional structure is encoded in the protein sequence. In the cell, the folding process is however governed and regulated by a conserved multi-membered ATP-driven protein machinery. These molecular chaperones and folding catalysts allow the maintenance of protein structure even under unfavourable conditions. This is of special importance in the transport of proteins across membranes. We plan to investigate these folding problems in collaboration with the groups working on single molecule spectroscopy and time resolved spectroscopy (area A). Protein trafficking and translocation are essential processes in even the simplest living cells. Many proteins have to cross from one up to as many as five membranes to reach their destination. The compartmentalisation of the eukaryotic cell relies entirely on high fidelity trafficking and translocation systems. Multiple pathways have evolved to translocate different proteins across the same membrane according to their specific properties (folded or unfolded, presence or absence of hydrophobic segments, etc). Some transport pathways are universally conserved; others that developed in bacteria are found in present-day organelles that originated by endosymbiosis, while yet others evolved specifically in eukaryotes. In addition intracellular architecture requires continuous trafficking to maintain the endomembrane system.