Translocation and secretion of proteins across microbial membrane.

Microorganisms secrete proteins into medium or translocates them in specific locations in the cell. The mechanisms of translocation and secretion of proteins in microorganisms and in mammalian organella are found to be similar in many respects. Most proteins to be localized outside of cytoplasm have an amino-terminal extension called signal sequence. Genetic studies in Escherichia coli were effectively done in place of biochemical studies in mammalian cells. Analyses of mutations in signal sequence proved this peptide is essential for translocation or secretion. Machinery essential for protein translocation or secretion has been found in mammalian cells such as Signal Recognition Particle or its receptor on endoplasmic reticulum. Though such machinery is yet to be found in microorganisms, existence of their counterparts is supported by analysis of temperature-sensitive mutations such as secA or secY defective in protein translocation and secretion. In vitro translocation and secretion systems using inverted bacterial membrane vesicles showed many proteins can translocate or secrete not only co-translationally but also post-translationally, i. e. after the protein synthesis has been completed. In vitro systems were also effectively studied to show that ATP is a major energy source of protein translocation or secretion.