Physiological Characterization of a Plant Mitochondrial Calcium Uniporter in Vitro and in Vivo

Over the recent years, several proteins that make up the mitochondrial calcium uniporter complex (MCUC) mediating Ca²⁺uptake into the mitochondrial matrix have been identified in mammals, including the channel-forming protein MCU. Although six MCU gene homologs are conserved in the model plant Arabidopsis (Arabidopsis thaliana) in which mitochondria can accumulate Ca²⁺, a functional characterization of plant MCU homologs has been lacking. Using electrophysiology, we show that one isoform, AtMCU1, gives rise to a Ca²⁺-permeable channel activity that can be observed even in the absence of accessory proteins implicated in the formation of the active mammalian channel. Furthermore, we provide direct evidence that AtMCU1 activity is sensitive to the mitochondrial calcium uniporter inhibitors Ruthenium Red and Gd³⁺, as well as to the Arabidopsis protein MICU, a regulatory MCUC component. AtMCU1 is prevalently expressed in roots, localizes to mitochondria, and its absence causes mild changes in Ca²⁺ dynamics as assessed by in vivo measurements in Arabidopsis root tips. Plants either lacking or overexpressing AtMCU1 display root mitochondria with altered ultrastructure and show shorter primary roots under restrictive growth conditions. In summary, our work adds evolutionary depth to the investigation of mitochondrial Ca²⁺ transport, indicates that AtMCU1, together with MICU as a regulator, represents a functional configuration of the plant mitochondrial Ca²⁺ uptake complex with differences to the mammalian MCUC, and identifies a new player of the intracellular Ca²⁺ regulation network in plants.