ZmSk2, a mitochondrial RNA processing protein, regulates maize kernel development via group II intron splicing.

Splicing of group II introns in the organelles of higher plants requires nuclear-encoded protein co-factors. However, the exact mechanisms and interactions among these auxiliary proteins are not yet established. In the present work, we characterized a maize kernel mutant, zmsk2. The growth of endosperm and embryo in the zmsk2 mutant was severely arrested during kernel development. ZmSk2 is a mitochondria targeted CRM-domain protein that functions in splicing of group II introns, particularly nad2 introns 2 and 4 and nad5 introns 2 and 4. There were no mature nad2 and nad5 transcripts in zmsk2 which was associated with aberrant assembly of mitochondrial complexes I and I + III and then with considerable decrease in the activity of NADH dehydrogenase. Protein-protein interaction experiments revealed that ZmSk2 formed homodimers and interact with Zm-mCSF1, Dek37 and PPR231. Taken together, these results indicate that ZmSk2 is a key component of mitochondrial RNA splicing complexes and contributes to mitochondrial homeostasis and maize kernel maturation.