INTERACTION BETWEEN MITOCHONDRIA AND THE ACTIN CYTOSKELETON IN BUDDING YEAST REQUIRES 2 INTEGRAL MITOCHONDRIAL OUTER-MEMBRANE PROTEINS, MMM1P AND MDM10P

Transfer of mitochondria to daughter cells during yeast cell division is essential for viable progeny. The actin cytoskeleton is required fo r this process, potentially as a track to direct mitochondrial movemen t into the bud. Sedimentation assays reveal two different components r equired for mitochondria-actin interactions: (I) mitochondrial actin b inding protein(s) (mABP), a peripheral mitochondrial outer membrane pr otein(s) with ATP-sensitive actin binding activity, and (2) a salt-ine xtractable, presumably integral, membrane protein(s) required for dock ing of mABP on the organelle. mABP activity is abolished by treatment of mitochondria with high salt. Addition of either the salt-extracted mitochondrial peripheral membrane proteins (SE), or a protein fraction with ATP-sensitive actin-binding activity isolated from SE, to salt-w ashed mitochondria restores this activity. mABP docking activity is sa turable, resistant to high salt, and inhibited by pretreatment of salt -washed mitochondria with papain. Two integral mitochondrial outer mem brane proteins, Mmm1p (Burgess, S.M., M. Delannoy, and R.E. Jensen. 19 94, J. Cell Biol. 126:1375-1391) and Mdm10p, (Sogo,,F,, and M.P. Yaffe . 1994. J. Cell Biol. 126:1361-1373) are required for these actin-mito chondria interactions. Mitochondria isolated from an mmm1-1 temperatur e-sensitive mutant or from an mmm10 deletion mutant show no mABP activ ity and no mABP docking activity. Consistent with this, mitochondrial motility in vivo in mmm1-1 and mdm10 Delta mutants appears to be actin independent. Depolymerization of F-actin using latrunculin-A results in loss of long-distance, linear movement and a fivefold decrease in t he velocity of mitochondrial movement. Mitochondrial motility in mmm1- 1 and mdm10 Delta mutants is indistinguishable from that in latrunculi n-A-treated wild-type cells. We propose that Mmm1p and Mdm10p are requ ired for docking of mABP on the surface of yeast mitochondria and coup ling the organelle to the actin cytoskeleton.