Role for cER and Mmr1p in Anchorage of Mitochondria at Sites of Polarized Surface Growth in Budding Yeast

Mitochondria accumulate at neuronal and immunological synapses [1, 2] and yeast bud tips [3] and associate with the ER during phospholipid biosynthesis, calcium homeostasis, and mitochondria! fission [4, 5]. Here we show that mitochondria are associated with cortical ER (cER) sheets [6, 7] underlying the plasma membrane in the bud tip and confirm that a deletion in YPT11, which inhibits cER accumulation in the bud tip [8], also inhibits bud tip anchorage of mitochondria [9]. Time-lapse imaging reveals that mitochondria are anchored at specific sites in the bud tip. Mmr1p, a member of the DSL1 family of tethering proteins, localizes to punctate structures on opposing surfaces of mitochondria and cER sheets underlying the bud tip and is recovered with isolated mitochondria and ER. Deletion of MMR1 impairs bud tip anchorage of mitochondria without affecting mitochondrial velocity or cER distribution. Deletion of the phosphatase PTC1 results in increased Mmr1p phosphorylation, mislocalization of Mmr1p, defects in association of Mmr1p with mitochondria and ER, and defects in bud tip anchorage of mitochondria. These findings indicate that Mmr1p contributes to mitochondrial inheritance as a mediator of anchorage of mitochondria to cER sheets in the yeast bud tip and that Ptc1p regulates Mmr1p phosphorylation, localization, and function.