Molecular Mechanism of Secretory Protein Translocation by the SecA ATPase

Abstract

In bacteria, most secretory proteins are translocated across the plasma membrane by the interplay of the SecA ATPase and the SecY channel. How SecA moves a broad range of polypeptide substrates is only poorly understood. Here, we use a combination of bulk biochemistry and single molecule Foerster Resonance Energy Transfer (smFRET) assays to show that SecA moves polypeptides through the SecY-channel by a “push and slide” mechanism. In its ATP-bound state, SecA interacts through a two- helix finger with a subset of amino acids in a substrate, pushing them into the channel. A polypeptide can also passively slide back and forth when SecA is in the predominant ADP-bound state, or when SecA encounters a poorly interacting amino acid in its ATP- bound state. SecA performs multiple rounds of ATP hydrolysis before dissociating from SecY. The proposed “push and slide” mechanism is supported by a mathematical model and explains how SecA moves polypeptides independently of their sequence. This mechanism may also apply to hexameric polypeptide-translocating ATPases.