Dimerization mechanism of an inverted-topology ion channel in membranes

Mechanism of MCUB-Dependent Inhibition of Mitochondrial Calcium Uptake

Mitochondrial Ca2+ levels are regulated to balance stimulating respiration against the harm of Ca2+ overload. Contributing to this balance, the main channel transporting Ca2+ into the matrix, the mitochondrial Ca2+ uniporter, can incorporate a dominant-negative subunit (MCUB). MCUB is homologous to the pore-forming subunit MCU, but when present in the pore-lining tetramer, inhibits Ca2+ transport. Here, using cell lines deleted of both MCU and MCUB, we identify three factors that contribute to MCUB-dependent inhibition. First, MCUB protein requires MCU to express. The effect is mediated via the N-terminal domain (NTD) of MCUB. Replacement of the MCUB NTD with the MCU NTD recovers autonomous expression but fails to rescue Ca2+ uptake. Surprisingly, mutations to MCUB that affect interactions with accessory subunits or the conduction pore all failed to rescue Ca2+ uptake, suggesting the mechanism of inhibition may involve more global domain rearrangements. Second, using concatemeric tetramers with varying MCU:MCUB ratios, we find that MCUB incorporation does not abolish conduction, but rather inhibits Ca2+ influx proportional to the amount of MCUB present in the channel. Reducing rather than abolishing Ca2+ transport is consistent with MCUB retaining the highly-conserved selectivity filter DIME sequence. Finally, we apply live-cell Förster resonance energy transfer to establish that the endogenous stoichiometry is 2:2 MCU:MCUB. Taken together, our results suggest MCUB preferentially incorporates into nascent uniporters, and the amount of MCUB protein present linearly correlates with the degree of inhibition of Ca2+ transport, creating a precise, tunable mechanism for cells to regulate mitochondrial Ca2+ uptake.

Mechanism of MCUB-dependent inhibition of mitochondrial calcium uptake

Mitochondrial Ca 2+ levels are regulated to balance stimulating respiration against the harm of Ca 2+ overload. Contributing to this balance, the main channel transporting Ca 2+ into the matrix, the mitochondrial Ca 2+ uniporter, can incorporate a dominant-negative subunit (MCUB). MCUB is homologous to the pore-forming subunit MCU, but when present in the pore-lining tetramer, inhibits Ca 2+ transport. Here, using cell lines deleted of both MCU and MCUB, we identify three factors that contribute to MCUB-dependent inhibition. First, MCUB protein requires MCU to express. The effect is mediated via the N-terminal domain (NTD) of MCUB. Replacement of the MCUB NTD with the MCU NTD recovers autonomous expression but fails to rescue Ca 2+ uptake. Surprisingly, mutations to MCUB that affect interactions with accessory subunits or the conduction pore all failed to rescue Ca 2+ uptake, suggesting the mechanism of inhibition may involve global rearrangements. Second, using concatemeric tetramers with varying MCU:MCUB ratios, we find that MCUB incorporation does not abolish conduction, but rather inhibits Ca 2+ influx proportional to the amount of MCUB present in the channel. Reducing rather than abolishing Ca 2+ transport is consistent with MCUB retaining the highly-conserved selectivity filter DIME sequence. Finally, we apply live-cell Förster resonance energy transfer to establish that the endogenous stoichiometry is 2:2 MCU:MCUB. Taken together, our results suggest MCUB preferentially incorporates into nascent uniporters, and the amount of MCUB protein present linearly correlates with the degree of inhibition of Ca 2+ transport, creating a precise, tunable mechanism for cells to regulate mitochondrial Ca 2+ uptake.

Molecular mechanisms of Na+-driven bile acid transport in human NTCP

Human Na+ taurocholate co-transporting protein (hNTCP) is a key bile salt transporter to maintain enterohepatic circulation and is responsible for the recognition of hepatitis B and D viruses. Despite landmark cryoelectron microscopy studies revealing open-pore and inward-facing states of hNTCP stabilized by antibodies, the transport mechanism remains largely unknown. To address this knowledge gap, we used molecular dynamics and enhanced sampling metadynamics simulations to elucidate the intrinsic mechanism of hNTCP-mediated taurocholate acid (TCA) transport driven by Na+ binding. We uncovered three TCA-binding modes, including one that closely matched the limited cryoelectron microscopy density observed in the open-pore hNTCP. We also captured several key hNTCP conformations in the substrate transport cycle, particularly including an outward-facing, substrate-bound state. Furthermore, we provided thermodynamic evidence supporting that changes in the Na+-binding state drive the TCA transport by exploiting the amphiphilic nature of the substrate and modulating the protein environment, thereby enabling the TCA molecule to flip through. Understanding these mechanistic details of Na+-driven bile acid transport may aid in the development of hNTCP-targeted therapies for liver diseases.

Lanthanum-fluoride electrode-based methods to monitor fluoride transport in cells and reconstituted lipid vesicles

Fluoride (F-) export proteins, including F- channels and F- transporters, are widespread in biology. They contribute to cellular resistance against fluoride ion, which has relevance as an ancient xenobiotic, and in more modern contexts like organofluorine biosynthesis and degradation or dental medicine. This chapter summarizes quantitative methods to measure fluoride transport across membranes using fluoride-specific lanthanum-fluoride electrodes. Electrode-based measurements can be used to measure unitary fluoride transport rates by membrane proteins that have been purified and reconstituted into lipid vesicles, or to monitor fluoride efflux into living microbial cells. Thus, fluoride electrode-based measurements yield quantitative mechanistic insight into one of the major determinants of fluoride resistance in microorganisms, fungi, yeasts, and plants.