Energy-coupling factor (ECF) transporters has been identified recently as a new family of ATP-binding cassette (ABC) transporters which comprise a large superfamily of protein complexes that transport a variety of substrates across cell membranes. They are composed of a membrane substrate-binding component EcfS, an ECF module of an integral membrane scaffold component EcfT and two cytoplasmic ATP binding/hydrolysis components EcfA/A’.

Two groups of ECF transporters have been identified with group-I ECF transporters being an EcfS protein associated with a dedicated ECF module and the encoding genes, and group-II ECF transporters which include several EcfS proteins and can bind to a common ECF module. These transporters mainly exist in prokaryote and plants and are responsible for micronutrients, like vitamins and metals, uptake from the environment, ECF

Professor ZHANG Peng’s lab at Shanghai Institute of Plant Physiology and Ecology of Chinese Academy of Sciences in the previous studies had revealed several structures of the group-II ECF transporters and interpreted the mechanisms of substrate binding, ECF module sharing and transport process. However, the molecular mechanism of group-I ECF transporters remains pretty much unknown, partly due to a lack of transporter complex structural information.

Using a cobalt ECF transporter-CbiMNQO, researhcers set out to explore the structure and mechanism of group-I ECF transporters. New findings indicated that CbiMNQO constitutes transmembrane subunits CbiM/CbiN, CbiQ, and cytoplasm subunits CbiO. The study has been published online in Cell Research.

Researchers set up a transport assay using ICP-MS (inductively coupled plasma mass spectrometry) and found that CbiN is essential to the transport activity of CbiMNQO. Yet CbiN has little effect on the ATPase activity of the complex, and CbiM is either required for or greatly stimulates the ATP hydrolysis by the CbiMNQO transporter. This stimulation is independent of the presence or binding of Co²⁺.

This study analyzed the crystal structure of CbiMQO at a resolution of 2.8Å. By comparing the structures of CbiM without substrate and NikM bound with substrate, L1 loop, connecting transmembrane helix 2 and 3, was found showing major conformational changes and can gate substrate binding and releasing.

The structure of CbiO in a closed conformation was further indicated, and was compared with the structure of CbiO in CbiMQO complex which adopts an open conformation, the conformational changes of CbiO upon ATP binding and product release were revealed.

Based on the result that the CbiN subunit is essential for cobalt transport but not required for the complex ATPase activity, one favored postulate from the researchers is that CbiN might interact with CbiM and CbiQ, and is required for coupling TMs movements of CbiQ to CbiM. This is the first structure of group-I ECF transporters, which represents a milestone in the field of ECF-type ABC transporters.

Data collection was technically assisted by the staff members at BL19U/18U of National Center for Protein Science Shanghai (NCPSS) and BL17U of Shanghai Synchrotron Radiation Facility (SSRF). The study was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences.