Divergent functions of the Arabidopsis mitochondrial SCO proteins: HCC1 is essential for COX activity while HCC2 is involved in the UV-B stress response

Abstract

The two related putative cytochrome c oxidase (COX) assembly factors HCC1 and HCC2 from Arabidopsis thaliana are Homologs of the yeast Copper Chaperones Sco1p and Sco2p. The hcc1 null mutation was previously shown to be embryo lethal while the disruption of the HCC2 gene function had no obvious effect on plant development, but increased the expression of stress-responsive genes. Both HCC1 and HCC2 contain a thioredoxin domain, but only HCC1 carries a Cu-binding motif also found in Sco1p and Sco2p. In order to investigate the physiological implications suggested by this difference, various hcc1 and hcc2 mutants were generated and analyzed. The lethality of the hcc1 knockout mutation was rescued by complementation with the HCC1 gene under the control of the embryo-specific promoter ABSCISIC ACID INSENSITIVE 3. However, the complemented seedlings did not grow into mature plants, underscoring the general importance of HCC1 for plant growth. The HCC2 homolog was shown to localize to mitochondria like HCC1, yet the function of HCC2 is evidently different, because two hcc2 knockout lines developed normally and exhibited only mild growth suppression compared with the wild type (WT). However, hcc2 knockouts were more sensitive to UV-B treatment than the WT. Complementation of the hcc2 knockout with HCC2 rescued the UV-B-sensitive phenotype. In agreement with this, exposure of wild-type plants to UV-B led to an increase of HCC2 transcripts. In order to corroborate a function of HCC1 and HCC2 in COX biogenesis, COX activity of hcc1 and hcc2 mutants was compared. While the loss of HCC2 function had no significant effect on COX activity, the disruption of one HCC1 gene copy was enough to suppress respiration by more than half compared with the WT. Therefore, we conclude that HCC1 is essential for COX function, most likely by delivering Cu to the catalytic center. HCC2, on the other hand, seems to be involved directly or indirectly in UV-B-stress responses.

Arabidopsis thaliana Hcc1 is a Sco‐like metallochaperone for CuA assembly in Cytochrome c Oxidase

“…Gonz alez and coworkers have shown that AtSco1 (formerly Hcc1) is essential for CoxII function, while Hcc2, lacking metal binding residues, was not essential. Here we show that AtSco1 is a specific metallochaperone for CoxII, accounting for the impact of the deletion of the Sco gene in the plant phenotype [38,39].…”

Interplay between cytochrome c and gibberellins during Arabidopsis vegetative development

“…Plants that lack detectable Complex IV activity, obtained by cultivation of immature seeds, have severe phenotypic alterations (Dahan et al ., ), showing the importance of CYTc‐dependent respiration for post‐germinative development. In a similar way, homozygous mutants in the Complex IV assembly factor gene HCC1 rescued by expressing the HCC1 cDNA from the embryo‐specific ABI3 promoter do not grow beyond the seedling stage (Steinebrunner et al ., ). The importance of Complex I for post‐germinative growth was also shown using knockout mutants in the gene that encodes the NDUFV1 subunit, which lack detectable Complex I activity (Kühn et al ., ).…”

The cytochrome c oxidase biogenesis factor AtCOX17 modulates stress responses in Arabidopsis

“…The imaging procedure and settings are described in Steinebrunner et al . (). For co‐localization analyses, intensity‐scatter plots were generated with the ‘Coloc 2’ and ‘Colocalization Threshold’ plug‐ins of the Fiji software (Schindelin et al ., ).…”

“…Roots of transgenic seedlings (7 to 10 d old) containing the 35S::AtCOX17-1-mRFP construct in the mt-gk background selected on hygromycin were analysed using the Zeiss (Jena, Germany) confocal laser scanning microscope 780. The imaging procedure and settings are described in Steinebrunner et al (2014). For co-localization analyses, intensity-scatter plots were generated with the 'Coloc 2' and 'Colocalization Threshold' plug-ins of the Fiji software (Schindelin et al, 2012).…”