image: (A) Negative stain EM map obtained by single particle analysis method (estimated resolution ~18 Å). (B) The pore surface representation showing a defined polar cavity for calcium transport. (C) Cartoon representation of the NMR structure showing the formation of the uniporter core, which consists of the transmembrane pore (yellow) and the coiled-coil domain (blue). (D) Two subunits of the pentamer showing subunit folding and definition of the helical segments. view more
Credit: Prof. James Chou's lab
Mitochondria are key integrators of cellular calcium (Ca2+) signaling and energy metabolism. Early studies demonstrated that isolated mitochondria could buffer huge amounts of Ca2+ via a highly selective channel called the "uniporter". Uptake of Ca2+ via the uniporter is known to activate the citric acid cycle, while its overload leads to cell death. Although the uniporter has been studied extensively for over 50 years and its molecular identity remained elusive until in 2011, computational genomics studies discovered its molecular components. The centerpiece of the uniporter, the calcium-conducting subunit, is MCU (mitochondrial calcium uniporter). MCU exhibits the unique property of both high selectivity and high conductance for calcium, which makes it an intriguing structural target being pursued by many structural biology labs worldwide.
Prof. James J. Chou's lab, together with their collaborators, Prof. CONG Yao's group and Prof. OUYANG Bo's group in National Center for Protein Science ·Shanghai, Institute of Biochemistry and Cell Biology (SIBCB), Institutions of Shanghai Institutes for Biological Sciences, CAS, and Prof. Vamsi Mootha's group in Department of Molecular Biology and Howard Hughes Medical Institute, Massachusetts General Hospital, Boston, USA, revealed the MCU architecture using nuclear magnetic resonance (NMR) and electron microscopy (EM).
The MCU structure is a homopentamer with the second transmembrane helix forming a hydrophilic pore across the membrane. The critical DxxE amino acid sequence motif forms the pore entrance featuring two carboxylate rings, which appear to be the ion selectivity filter. The structure represents a novel ion channel architecture and suggests a passage for calcium transport. This is one of the largest structures characterized by NMR, providing a structural blueprint for understanding the function of this channel.
In addition to MCU, the uniporter complex also includes other components, including the channel gate keeper EMRE (essential MCU regulator), and regulatory subunits MICU1 (mitochondrial calcium uptake 1), MICU2 (mitochondrial calcium uptake 2), and MCUb (MCU isoform b). The complexity implies intricate regulation of MCU, which appears to be an ancient calcium channel that is a part of the earliest eukaryotes. The reported MCU pore architecture represents the first step towards understanding how this complex calcium uniporter works.
The high quality NMR and EM data were collected in National Center for Protein Science ·Shanghai, owing to technical supports by LIU Zhijun on NMR and by KONG Liangliang on EM. The Center is equipped with the state-of-the-art facilities and enabling technologies for life science study and provides superb service for domestic and international users.
###
This work entitled "Architecture of the Mitochondrial Calcium Uniporter", was published in Nature on May 2, 2016.
This work was supported by grants from the Strategic Priority Research Program of the Chinese Academy of Sciences, National Institutes of Health (NIH), and Howard Hughes Medical Institute.
Journal
Nature