The ties that bind: Num1 anchors mitochondria to cell cortex Free

As most membrane-bound organelles, such as mitochondria, cannot be generated de novo they are inherited upon cell division, and the specific cellular architecture of each eukaryotic cell type must also be re-established in daughter cells. Although there has been much investigation into many aspects of mitochondrial dynamics in recent years, little is known about how their dynamic behaviour is integrated into cellular architecture. On page 2924, Benedikt Westermann and colleagues now investigate the contribution of the cell-cortex-associated protein Num1 to mitochondrial partitioning in yeast. Using live-cell microscopy, the authors find that Num1 is required for mitochondrial attachment to the cell cortex and retention in mother cells. Next, they show that mitochondrial tips co-localise with Num1 punctae in mother cells. Further examination by electron tomography revealed points of contact between the mitochondrial outer membrane and plasma membrane invaginations, and these were not visible in the absence of Num1. The authors therefore propose that Num1 punctae represent mitochondrial cortex anchors in mother cells. Finally, the expression of chimeric plasma membrane tethers rescued mitochondrial fission defects in Δnum1 and Δmdm36 mutants, allowing the authors to conclude that these defects are caused by a defect in cell cortex attachment. These findings assign a key role to Num1 in the attachment of mitochondria to the yeast cell cortex.