Mmm1p, a mitochondrial outer membrane protein, is connected to mitochondrial DNA (mtDNA) nucleoids and required for mtDNA stability

In the yeast Saccharomyces cerevisiae, mitochondria form a branched, tubula r reticulum in the periphery of the cell. Mmm1p is required to maintain nor mal mitochondrial shape and in mmm1 mutants mitochondria form large, spheri cal organelles. To further explore Mmm1p function, we examined the localiza tion of a Mmm1p-green fluorescent protein (GFP) fusion in living cells. We found that Mmm1p-GFP is located in small, punctate structures on the mitoch ondrial outer membrane, adjacent to a subset of matrix-localized mitochondr ial DNA nucleoids. We also found that the temperature-sensitive mmm1-1 muta nt was defective in transmission of mitochondrial DNA to daughter cells imm ediately after the shift to restrictive temperature. Normal mitochondrial n ucleoid structure also collapsed at the nonpermissive temperature with simi lar kinetics. Moreover, we found that mitochondrial inner membrane structur e is dramatically disorganized in mmm1 disruption strains. We propose that Mmm1p is part of a connection between the mitochondrial outer and inner mem branes, anchoring mitochondrial DNA nucleoids in the matrix.