CCDC90A (MCUR1) Is a Cytochrome c Oxidase Assembly Factor and Not a Regulator of the Mitochondrial Calcium Uniporter

Abstract

Mitochondrial calcium is an important modulator of cellular metabolism. CCDC90A was reported to be a regulator of the mitochondrial calcium uniporter (MCU) complex, a selective channel that controls mitochondrial calcium uptake, and hence was renamed MCUR1. Here we show that suppression of CCDC90A in human fibroblasts produces a specific cytochrome c oxidase (COX) assembly defect, resulting in decreased mitochondrial membrane potential and reduced mitochondrial calcium uptake capacity. Fibroblasts from patients with COX assembly defects due to mutations in TACO1 or COX10 also showed reduced mitochondrial membrane potential and impaired calcium uptake capacity, both of which were rescued by expression of the respective wild-type cDNAs. Deletion of fmp32, a homolog of CCDC90A in Saccharomyces cerevisiae, an organism that lacks an MCU, also produces a COX deficiency, demonstrating that the function of CCDC90A is evolutionarily conserved. We conclude that CCDC90A plays a role in COX assembly and does not directly regulate MCU.

Mitochondrial calcium uniporter regulator 1 (MCUR1) regulates the calcium threshold for the mitochondrial permeability transition

“…Instead, a potential mechanism is that MCUR1 bridges the uniporter and MPT complexes, although it is not established whether these interactions involve other scaffolding proteins. This bridging hypothesis fits with prior suggestions that MCUR1 may assemble inner membrane complexes (26). Other hypotheses for MCUR1 function, such as an effect on the MPT Ca 2+ sensor, are not excluded by this model but are difficult to investigate, because MPT Ca 2+ sensing and matrix Ca 2+ buffering are poorly understood.…”

“…not due to siRNA off-target inhibition, as MCUR1 overexpression restored Ca 2+ sensitivity. A potential discrepancy between our results and those published previously (25,26) was the absence of a Ca 2+ uptake phenotype after MCUR1 knockdown, which we hypothesized may reflect the choice of different cell lines or the length of knockdown induced by siRNA (acute) versus shRNA (chronic). To explore this further, we produced cell lines that stably expressed an shRNA targeting MCUR1, leading to clonal lines with intermediate (∼70%, shMCUR1-1) or more severe loss of MCUR1 (∼80%, shMCUR1-2) compared with control (shGFP) (Fig.…”

“…IV assembly factor (25,26). In our S2R+ cell lines, human PPIF, MCU, and MCUR1 targeted mitochondria (Fig.…”

The molecular era of the mitochondrial calcium uniporter

“…In one study, CCDC90A was shown to co-immunoprecipitate with MCU 43 , although another study characterizing MCU-interacting proteins did not identify CCDC90A 23 . However, CCDC90A has an orthologue in yeast (which lacks uniporter activity) that was recently shown to be important for the assembly of cytochrome c oxidase (complex IV) 44 . Taken together, in our opinion, the data provide strong evidence that the involvement of CCDC90A in uniporter activity is indirect, probably through effects on the respiratory chain, which generates the driving potential for mitochondrial calcium transport.…”

The EF-Hand Ca2+ Binding Protein MICU Choreographs Mitochondrial Ca2+ Dynamics in Arabidopsis

“…The recent identification of several protein families making up the mitochondrial Ca 2+ uniporter complex (MCUC) in mammals, including the pore-forming MCU (MCU and MCUb;Baughman et al, 2011;De Stefani et al, 2011;Raffaello et al, 2013), and the regulatory, associated MICUs (MICU1, 2, and 3; Perocchi et al, 2010;Plovanich et al, 2013), EMRE (Sancak et al, 2013), and MCUR1/CCDC90A (although potentially not directly; Mallilankaraman et al, 2012b;Paupe et al, 2015), has spurred intense research across the biomedical disciplines Mallilankaraman et al, 2012a;Csordás et al, 2013;Hoffman et al, 2013;Marchi et al, 2013;Pan et al, 2013;Raffaello et al, 2013;Kovács-Bogdán et al, 2014;Logan et al, 2014;Patron et al, 2014;Wang et al, 2014). While some genes of the complex components appear not to be present in plants (e.g., MCUb and EMRE), others have multiple homologs (six for the functional poreforming subunit MCU in Arabidopsis [Stael et al, 2012] and maize [Zea mays; Meng et al, 2015] and two for MCUR1/CCDC90A in Arabidopsis), suggesting potential functional modification and/or differentiation.…”