The twin arginine transport (Tat) system translocates folded proteins across the bacterial inner membrane. Transport substrates are recognized by means of evolutionarily well-conserved N-terminal signal peptides. The precise role of signal peptides in the actual transport process is not yet fully understood. Potentially, much insight into the molecular details of the transport process could be gained from step-by-step in vitro experiments under controlled conditions. Here, we employ purified preproteins to study their interaction with the phospholipid membrane by using surface plasmon resonance spectroscopy. It turns out that preproteins interact tightly with a model membrane consisting of only phospholipids. This interaction, which is stabilized by both electrostatic and hydrophobic contributions, appears to constitute an early step in protein translocation by the Tat system. © 2006 American Chemical Society.