A 22 kDa protein specific for yeast mitochondrial nucleoids is an unidentified putative ribosomal protein encoded in open reading frame YGL068W.

Mitochondrial-nucleoid (mt-nucleoid) proteins of the yeast Saccharomyces cerevisiae were separated by two-dimensional gel electrophoresis. Analysis of the N-terminal amino acid sequence showed that a 22 kDa protein which is unique in the mt-nucleoid fraction is an unidentified protein encoded in the open reading frame YGL068W and shows a homology with the ribosomal protein L7/L12 of bacteria. We named this protein Mnp1p (for the mitochondrial-nucleoid protein 1). Immunoblotting of each fraction with an anti-Mnp1p antibody during the mt-nucleoid isolation showed that Mnp1p is highly concentrated in the mt-nucleoid fraction. Immunofluorescence microscopy suggested that Mnp1p is localized to mitochondria in vivo, and a significant amount of Mnp1p is associated with the mt-nucleoids. On the other hand, Northern blotting showed that a large amount of large and small mitochondrial ribosomal RNAs was not associated with the mt-nucleoids and remained in the supernatant after the isolation of mt-nucleoids. The null mutation of MNP1 led to a respiratory-deficient phenotype, but the morphology of the mt-nucleoids in the transformants carrying the null mutation was normal. These results suggest that a significant amount of Mnp1p plays a role as a major component of the mt-nucleoids.