Gram-negative bacteria such as Escherichia coli are surrounded by two lipid bilayer membranes, which are separated by the murein-containing periplasmic space. About 50% of the mass of the outer membrane consists of protein. The most abundant proteins include OmpA, BraunŐs lipoprotein, and the porins. Transcription of the ompA gene leads to a very stable messenger RNA (mRNA), whose degradation pathway is beginning to be understood. The OmpA protein is synthesized as a precursor pre-protein that is secreted into the periplasmic space. After proteolytic processing of the signal peptide, the protein is assembled into the outer membrane. As one of the first proteins characterized with respect to secretion and outer membrane localization, OmpA has become a paradigm for the study of the underlying processes. The one-domain membrane moiety of this relatively small and monomeric protein represents a very useful model for the investigation of membrane assembly of beta-structured membrane proteins in vivo as well as in vitro.
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