The proper function of every living cell depends on the delivery of gene products, the proteins, to their various locations in the cell. Some cells also produce proteins for secretion. All cells and compartments within them are surrounded by a membrane consisting of a thin lipid film. Special transport machines are required to move newly produced protein cargo through these membranes, or out of the cell. This process needs to be error-free and strictly regulated.
Cells of higher organisms have several different compartments. Bacteria are relatively simple cells with only one or two different compartments. The most important protein translocation system in bacteria is similar to one also found in higher organisms, including humans. Escherichia coli and other bacteria have a ubiquitous transport system known as the Sec pathway, and the core component SecYEG consists of three different membrane proteins SecY, SecE, and SecG. Most Escherichia coli proteins that are not destined for the main, cytoplasmic compartment, are transported by the SecYEG complex, including those destined for insertion into the membrane itself. They typically contain a signal sequence, which acts like a post-code and is recognized by the transport machinery. Proteins are then transported through a channel formed by the SecYEG complex.
Researchers at the Max Planck Institute of Biophysics in Frankfurt have now determined the first structure of the SecYEG protein translocation machinery from the bacterium Escherichia coli. They were able to grow small, two-dimensional crystals of the membrane-inserted complex, and determined its structure by electron cryo-microscopy and image processing.
The structure for the first time provides a detailed view of a protein-transporting machine (Figure 1 & 2). The images show a so called "dimer"(chunk or unit) of the SecYEG complex in its native environment, the lipid membrane. The dimer is thought to be the active form of the complex, which binds and transports secretory and membrane proteins. The structure shows elements of a secondary structure, which are 15 transmembrane segments in each monomer. At the dimer interface there is a cavity, which may be a doorway through which proteins are pushed by associated partner proteins. This structure is the first step on the way towards an in-depth understanding of this fundamental biological process.
Breyton C, Haase W, Rapoport TA, Kuhlbrandt W, Collinson I, 'Three-dimensional structure of the bacterial protein-translocation complex SecYEG.' Nature 2002 August 8;418(6898):662-5