Sec-dependent membrane protein insertion: sequential interaction of nascent FtsQ with SecY and YidC

Recent studies identified YidC as a novel membrane factor that may play a k ey role in membrane insertion of inner membrane proteins (IMPs), both in co njunction with the Sec-translocase and as a separate entity. Here, we show that the type II IMP FtsQ requires both the translocase and, to a lesser ex tent, YidC in vivo. Using photo-crosslinking we demonstrate that the transm embrane (TM) domain of the nascent IMP FtsQ inserts into the membrane close to SecY and lipids, and moves to a combined YidC/lipid environment upon el ongation. These data are consistent with a crucial role for YidC in the lat eral transfer of TM domains from the Sec translocase into the lipid bilayer .