Molecular basis for substrate transport of <i>Mycobacterium tuberculosis</i> ABC importer DppABCD.

Abstract

The type I adenosine 5'-triphosphate (ATP)-binding cassette (ABC) transporter DppABCD is believed to be responsible for the import of exogenous heme as an iron source into the cytoplasm of the human pathogen <i>Mycobacterium tuberculosis</i> (<i>Mtb</i>). Additionally, this system is also known to be involved in the acquisition of tri- or tetra-peptides. Here, we report the cryo-electron microscopy structures of the dual-function <i>Mtb</i> DppABCD transporter in three forms, namely, the <i>apo</i>, substrate-bound, and ATP-bound states. The <i>apo</i> structure reveals an unexpected and previously uncharacterized assembly mode for ABC importers, where the lipoprotein DppA, a cluster C substrate-binding protein (SBP), stands upright on the translocator DppBCD primarily through its hinge region and N-lobe. These structural data, along with biochemical studies, reveal the assembly of DppABCD complex and the detailed mechanism of DppABCD-mediated transport. Together, these findings provide a molecular roadmap for understanding the transport mechanism of a cluster C SBP and its translocator.