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iversit sed fo line 1 ropla twin imila otein o dat need to transport proteins across the plasma membrane into This article is concerned with an alternative means of Biochimica et Biophysica Acta 169Gram-positive bacteria. The importance of this process can be judged from the fact that Escherichia coli translocates over 250 proteins, and 7 % of the Bacillus subtilis proteome is believed to be exported across the plasma membrane [1]. A range of mechanisms have evolved for the export of proteins in bacteria, but many of these are used for spe-targeting signal in which a twin-arginine motif plays a prominent role (hence the name), and the pathway was first characterised in plant chloroplasts in the early 1990s, when biochemical studies pointed to the existence of two very different mechanisms for the targeting of lumenal proteins across the thylakoid membrane. Now, it is clear that a very similar system operates in prokaryotes and aria) or into the cell wall or growth medium of archaea andthe periplasm or outer membrane (in Gram-negative bacte- protein export, termed the twin-arginine translocation (Tat) pathway. The Tat system recognises substrates bearing aProtein transport is also a key process in prokaryotes, whichKeywords: Tat; Protein targeting; Prokaryote 1