CAS Researchers Reveal the Structure and Mechanism of a Group-I cobalt Energy Coupling Factor Transporter

Powered by ATP hydrolysis, ATP-binding cassette (ABC) transporters comprise a large superfamily of protein complexes that transport a variety of substrates across cell membranes. In recent years, a new family of ABC transporters, known as energy-coupling factor (ECF) transporters, has been identified. ECF transporters are modular transporters composed of a membrane substrate-binding component EcfS and an ECF module of an integral membrane scaffold component EcfT and two cytoplasmic ATP binding/hydrolysis components EcfA/A’. To date 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 located within one operon. Group-II ECF transporters, whose coding genes are distributed throughout the genome, include several EcfS proteins and can bind to a common ECF module. Responsible for micronutrients, like vitamins and metals, uptake from the environment, ECF transporters mainly exist in prokaryote and plants. In the previous works, Professor ZHANG Peng’s lab had solved several structures of the group-II ECF transporters and interpreted the mechanisms of substrate binding, ECF module sharing and transport process. However, the molecular understanding of group-I ECF transporters is very limited, partly due to a lack of transporter complex structural information.

In this study, Professor ZHANG Peng and his colleagues at CAS Center for Excellence in Molecular Plant Sciences, Shanghai Institute of Plant Physiology and Ecology (SIPPE), Chinese Academy of Sciences (CAS), and in collaboration with other groups, explore the structure and mechanism of group-I ECF transporters using a cobalt ECF transporter-CbiMNQO. CbiMNQO constitutes transmembrane subunits CbiM/CbiN, CbiQ, and cytoplasm subunits CbiO (Fig.1a). Researchers set up a transport assay using ICP-MS (inductively coupled plasma mass spectrometry) and find that CbiN is essential to the transport activity of CbiMNQO (Fig.1b). However, CbiN has little effect on the ATPase activity of the complex. They also find that CbiM is either required for or greatly stimulates the ATP hydrolysis by the CbiMNQO transporter and this stimulation is independent of the presence or binding of Co2+. (Fig.1c). Then the researchers solve the crystal structure of CbiMQO at a resolution of 2.8? (Fig.1d). By comparing the structures of CbiM without substrate and NikM bound with substrate, they find that L1 loop, connecting transmembrane helix 2 and 3, shows major conformational changes and can gate substrate binding and releasing (Fig.1e). They further solve the structure of CbiO in a closed conformation, compare it with the structure of CbiO in CbiMQO complex which adopts an open conformation (Fig.1f), and reveal the conformational changes of CbiO upon ATP binding and product release. Based on the results that the CbiN subunit is essential for cobalt transport but not required for the complex ATPase activity, they postulate that CbiN might interact with CbiM and CbiQ, and is required for coupling TMs movements of CbiQ to CbiM (Fig.1g). This is the first structure of group-I ECF transporters and represents a milestone in the field of ECF-type ABC transporters.

The study entitled “Structure and mechanism of a group-I cobalt energy coupling factor transporter” has been published online in Cell Research on March 21, 2017.

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).

This study was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences.

Article website:：http://www.nature.com/cr/journal/vaop/ncurrent/full/cr201738a.html

Author Contacts:
Peng Zhang, Ph.D.
Professor of Structural Biology
National Key Laboratory of Plant Molecular Genetics
CAS Center for Excellence in Molecular Plant Sciences
Institute of Plant Physiology and Ecology
Chinese Academy of Sciences.
300 Fenglin Road,Shanghai, 200032, China.
Tel: 86-21-54924219
Email: pengzhang01@sibs.ac.cn
Lab page: http://pzhangxtal.sippe.ac.cn