Dissecting the conformational complexity and flipping mechanism of a prokaryotic heme transporter

Iron-bound cyclic tetrapyrroles (hemes) are key redox-active cofactors in membrane-integrated oxygen reductases and other bioenergetic enzymes. However, the mechanisms of heme transport and insertion into respiratory chain complexes remain unclear. Here, we used a combination of cellular, biochemical, structural and computational methods to resolve ongoing controversies around the function of the heterodimeric bacterial ABC transporter CydDC. We provide multi-level evidence that CydDC is a heme transporter required for assembly and functional maturation of cytochrome bd, a pharmaceutically relevant drug target. Our systematic single-particle cryo-EM approach combined with atomistic molecular dynamics simulations provides detailed insight into the conformational landscape of CydDC during substrate binding and occlusion. Our simulations reveal that heme binds laterally from the membrane space to the transmembrane region of CydDC, enabled by a highly asymmetrical inward-facing CydDC conformation. During the binding process, heme propionates interact with positively charged residues on the surface and later in the substrate-binding pocket of the transporter, causing the heme orientation to flip 180 degrees. The membrane-accessible heme entry site of CydDC is primarily controlled by the conformational plasticity of CydD transmembrane helix 4, the extended cytoplasmic segment of which also couples heme confinement to a rotational movement of the CydC nucleotide-binding domain. Our cryo-EM data highlight that this signal transduction mechanism is necessary to drive conformational transitions toward occluded and outward-facing states. One Sentence SummaryThe heterodimeric bacterial ABC transporter CydDC is a heme flippase essential for the functional maturation of cytochrome bd.