The functional conservation of SSR1 homologs in plants

AtSSR1 (Arabidopsis thaliana short and swollen root 1) is a plant-specific gene that encodes a mitochondrial protein containing TPR (tetratricopeptide repeat) domains, and was shown to be required for maintaining mitochondrial function. However, the evolution of its function in the plant lineage is not known. In this paper, SSR1 homologs were cloned from seven representative plant species ranging from lower to higher plants. Their structural and functional conservation were characterized in detail. The results demonstrated that most of the SSR1 homologs are predicted to have mitochondrial localization except for the one from Physcomitrella patens and all of them possess various numbers of TPR domains. Upon introduction into the Arabidopsis ssr1-2 knock-out mutant, all SSR1 homologs were capable of fully rescuing the short-root and stress hypersensitive phenotype of the mutant. In addition, in vitro pull-down analysis showed that similar to AtSSR1, the selected SSR1 homologs were also able to interact with AtHSCA2 and AtISU1, two components of mitochondrial iron-sulfur cluster assembly pathway, suggesting that SSR1 homologs from various plant species are functionally conserved. Despite the conserved function, different SSR1 homologs shared relatively low sequence identity with AtSSR1. Instead, their 3D structures display a common feature of a globular shape, mainly dominated by α-helices and two β-sheets embedded in the center. Taken together, our results suggest that SSR1, as a plant-specific mitochondrial protein, might have complete functionality in plant development and stress response already in the early stage of plant evolution.