Candida albicans PPR proteins are required for the expression of respiratory Complex I subunits

Abstract

Pentatricopeptide (PPR) proteins bind RNA and are present in mitochondria and chloroplasts of Eukaryota. In fungi they are responsible for controlling mitochondrial genome expression, mainly on the posttranscriptional level. Candida albicans is a human opportunistic pathogen with a facultative anaerobic metabolism which, unlike the model yeast S. cerevisiae, possesses mitochondrially encoded respiratory Complex I (CI) subunits and does not tolerate loss of mtDNA. We characterized the function of 4 PPR proteins of C. albicans that lack orthologs in S. cerevisiae, and found that they are required for the expression of mitochondrially-encoded CI subunits. We demonstrated that these proteins localize to mitochondria and are essential to maintain the respiratory capacity of cells. Deletion of genes encoding these PPR proteins results in changes in steady state levels of mitochondrial RNAs and proteins. We demonstrated that C. albicans cells lacking CaPpr4, CaPpr11, and CaPpr13 proteins show no CI assembly, whereas the lack of CaPpr7p results in a decreased CI activity. CaPpr13p is required to maintain the bicistronic NAD4L-NAD5 mRNA, whereas the other three PPR proteins are likely involved in translation-related assembly of mitochondrially encoded CI subunits. In addition, we show that CaAep3p which is an ortholog of ScAep3p, performs the evolutionary conserved function of controlling expression of the ATP8-ATP6 mRNA. We also show that C. albicans cells lacking PPR proteins express a higher level of the inducible alternative oxidase (AOX2) which likely rescues respiratory defects and compensates for oxidative stress.