A fundamental problem in molecular cell biology is to understand how newly synthesized proteins reach their proper intracellular address. In many cases, one or even two or three membranes separate the place of synthesis from the final destination. Thus, precursor proteins have to be translocated across or inserted into membranes, a process that is not trivial considering the relatively large size of protein molecules and the hydrophobic core of biological membranes. The protein translocation process is understood to involve information encoded in the protein sequence itself as well as the cellular machinery that decodes this information and delivers the protein to its correct location. Although this solution is rather general, Mother Nature provides many variations to this common theme.One level of variability is provided by the fact that the cellular protein translocation machineries are actually multi subunit complexes that can adopt themselves to their substrate proteins. Some of these translocases thread precursor proteins across the membrane in their unfolded state while others allow or even require a folded substrate. Even more variability arises from differences in the structure of the actual translocation pore. Structural studies together with biochemical and genetic approaches provided detailed information on the structure-function relationship of some of these protein translocases. In some cases the translocation pore is built from trans-membrane helices while in other systems it is composed mainly from one or several membrane embedded beta-barrels. It seems that we are only starting to understand the complexity of the fascinating process of protein translocation.
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