The effect of SecB on protein folding : a single molecule study

In biology, "life" is the condition which distinguishes active organisms from inorganic matter. Life of organisms is characterized by the capacity for growth, functional activity and continual changes preceding death. For all three kingdoms of life, i.e. Eukarya, Bacteria or Archaea, the process of growing involves transcription (DNA to mRNA) and translation (mRNA to protein) of genetic information (DAN) into proteins. Proteins or plypeptides fulfill diverse functions in cells. Many catalyze chemical reactions (enzymes), others are used for structural and mechanical functions (cytoskeleton) and some are used for communication between cells. Proteins are built from twenty different amino acids in various compositions and lengths. Each protein is synthesized as a linear chain of amino acids when translated from a mRNA sequence into protein and initially has no structure. In order to work properly, proteins have to fold from a random coil into a functional three-dimensional structure, the so called native state. The final three-dimensional structure is determined by the amino acid sequence of the protein. Nevertheless, many proteins cannot fold out of themselves and need help to obtain their native state, especially under stress conditions, and within the cell context with its highly crowded protein-containing cytosol. This help is provided by other proteins, so called chaperones. Many chaperones have the ability to alternate the folding pathway in favor towards the native state. However, enhancing folding and remodeling misfolded proteins are not the only functions of molecular chaperones. They also play a role in other cellular aspects, including translocation through membranes or remodelling of protein complexes. In the chapters of this thesis, I have studied the function of the chaperone SecB that plays a highly specialized role in the translocation of proteins across the cytoplasmic membrane in Escherichia coli.