The AAA+ ATPase Msp1 is a processive protein translocase with robust unfoldase activity

Msp1 is a conserved eukaryotic AAA+ ATPase localized to the outer mitochondrial membrane, where it is thought to extract mislocalized tail-anchored proteins. Despite recent in vivo and in vitro studies supporting this function, a mechanistic understanding of how Msp1 extracts its substrates is still lacking. Msp1's ATPase activity depends on its hexameric state, and previous characterizations of the cytosolic AAA+ domain in vitro had proved challenging due to its monomeric nature in the absence of the transmembrane domain. Here, we used a hexamerization scaffold to study the substrate-processing mechanism of the soluble Msp1 motor, the functional homo-hexameric state of which was confirmed by negative-stain electron microscopy. We demonstrate that Msp1 is a robust bidirectional protein translocase that is able to unfold diverse substrates by processive threading through its central pore. This unfoldase activity is inhibited by Pex3, a membrane protein proposed to regulate Msp1 at the peroxisome.