membrane via the presequence translocase, which consists of membrane-integrated
channel proteins and the matrix Hsp70 import motor. It has not been known how preproteins
are directed to the import channel. We have identified the essential protein Tim50, which
exposes its major domain to the intermembrane space. Tim50 interacts with preproteins in
transit and directs them to the channel protein Tim23. Inactivation of Tim50 strongly inhibits …