Abstract
Previous studies indicate that two proteins, Mmm1p and Mdm10p, are required to link mitochondria to the actin cytoskeleton of yeast and for actin-based control of mitochondrial movement, inheritance and morphology. Both proteins are integral mitochondrial outer membrane proteins. Mmm1p localizes to punctate structures in close proximity to mitochondrial DNA (mtDNA) nucleoids. We found that Mmm1p and Mdm10p exist in a complex with Mdm12p, another integral mitochondrial outer membrane protein required for mitochondrial morphology and inheritance. This interpretation is based on observations that 1) Mdm10p and Mdm12p showed the same localization as Mmm1p; 2) Mdm12p, like Mdm10p and Mmm1p, was required for mitochondrial motility; and 3) all three proteins coimmunoprecipitated with each other. Moreover, Mdm10p localized to mitochondria in the absence of the other subunits. In contrast, deletion of MMM1 resulted in mislocalization of Mdm12p, and deletion of MDM12 caused mislocalization of Mmm1p. Finally, we observed a reciprocal relationship between the Mdm10p/Mdm12p/Mmm1p complex and mtDNA. Deletion of any one of the subunits resulted in loss of mtDNA or defects in mtDNA nucleoid maintenance. Conversely, deletion of mtDNA affected mitochondrial motility: mitochondria in cells without mtDNA move 2-3 times faster than mitochondria in cells with mtDNA. These observations support a model in which the Mdm10p/Mdm12p/Mmm1p complex links the minimum heritable unit of mitochondria (mtDNA and mitochondrial outer and inner membranes) to the cytoskeletal system that drives transfer of that unit from mother to daughter cells.