Abstract
Members of the ubiquitous COX11 (cytochrome c oxidase 11) protein family are involved in copper delivery to the COX complex. In this work, we characterize the Arabidopsis thaliana COX11 homolog (encoded by locus At1g02410). Western blot analyses and confocal microscopy identified Arabidopsis COX11 as an integral mitochondrial protein. Despite sharing high sequence and structural similarities, the Arabidopsis COX11 is not able to functionally replace the Saccharomyces cerevisiae COX11 homolog. Nevertheless, further analysis confirmed the hypothesis that Arabidopsis COX11 is essential for COX activity. Disturbance of COX11 expression through knockdown (KD) or overexpression (OE) affected COX activity. In KD lines, the activity was reduced by ~50%, resulting in root growth inhibition, smaller rosettes and leaf curling. In OE lines, the reduction was less pronounced (~80% of the wild type), still resulting in root growth inhibition. Additionally, pollen germination was impaired in COX11 KD and OE plants. This effect on pollen germination can only partially be attributed to COX deficiency and may indicate a possible auxiliary role of COX11 in ROS metabolism. In agreement with its role in energy production, the COX11 promoter is highly active in cells and tissues with high-energy demand for example shoot and root meristems, or vascular tissues of source and sink organs. In COX11 KD lines, the expression of the plasma-membrane copper transporter COPT2 and of several copper chaperones was altered, indicative of a retrograde signaling pathway pertinent to copper homeostasis. Based on our data, we postulate that COX11 is a mitochondrial chaperone, which plays an important role for plant growth and pollen germination as an essential COX complex assembly factor.
Highlights
The cytochrome c oxidase (COX) is a crucial component of the mitochondrial respiratory chain, which is of utmost importance for providing cellular energy
To identify members of the COX11 family, the Arabidopsis Information Research (TAIR) genome database was searched for genes with sequence similarity to COX11 genes from other species
The Arabidopsis COX11 homolog is predicted to be targeted to mitochondria (0.711 and 0.93 probability according to the programs TargetP and MitoProtII, respectively), and to possess a cleavable N-terminal mitochondrial targeting signal (Figure 1A)
Summary
The cytochrome c oxidase (COX) is a crucial component of the mitochondrial respiratory chain, which is of utmost importance for providing cellular energy. The Cu ions for CuA and CuB are provided by the two copper chaperones SCO1 (for synthesis of cytochrome c oxidase 1) (Schulze and Rödel, 1988; Lode et al, 2000; Nittis et al, 2001) and COX11 (Banting and Glerum, 2006), respectively. In vitro studies with SCO1 and COX11 from S. cerevisiae indicate that the two proteins most likely receive the Cu ions from the small soluble copper chaperone COX17 located in the IMS (Horng et al, 2004). The exact role of a third conserved cysteine outside of the Cu-binding motif that is essential for COX11 function (Banting and Glerum, 2006) is less clear. S. cerevisiae cox null mutants are respiratory deficient (Tzagoloff et al, 1990; Carr et al, 2002; Banting and Glerum, 2006), in agreement with the essential contribution of COX11 for COX assembly
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