The diversity of FtsY-lipid interactions.

The bacterial signal recognition particle (SRP) receptor FtsY forms a complex with the SRP Ffh to target nascent polypeptide chains to the bacterial inner membrane. How FtsY interacts with lipids and associates to the membrane is unclear. Here, we show that vesicle binding leads to partial protection against proteolytic degradation and a change in secondary structure, which differs depending on whether the lipids are simple mixtures of zwitterionic and anionic lipids, mimics of Escherichia coli lipids, or lysolipids. Lipid binding alters the stability of FtsY. Thermal unfolding of FtsY in buffer shows two transitions, one occurring at approximately 60 degrees C and the other at approximately 90 degrees C. The thermal intermediate accumulating between 60 and 90 degrees C has structural features in common with the state induced by binding to E. coli lipids. E. coli lipid extract induces a single transition around 70 degrees C, anionic lipids have no effect while cooperative unfolding is completely removed in lysolipids. Thus, the lipid environment profoundly influences the dynamic properties of FtsY, leading to three different kinds of FtsY-lipid interactions with different effects on structure, proteolytic protection, and stability, and is driven both by hydrophobic and electrostatic interactions. Trypsin digestion experiments highlight the central role of the N-domain in lipid contacts, whereas the A- and G-domains appear to play a more minor part.