Actin is good at long division

F-actin helps mitochondria divide by polymerizing on the organelles, Li et al. show.

The GTPase Drp1 forms spirals around mitochondria to cut the organelles in two. Studies suggest that actin also has a role in mitochondrial division and recruitment of Drp1. The mechanisms, however, remain unclear.

Li et al. found that F-actin polymerizes on the outer mitochondrial membrane in cultured cells but doesn’t extend into the organelles. When the researchers spurred mitochondria to divide by putting them under stress, F-actin amassed on the organelles. However, latrunculin B, which prevents actin polymerization, curtailed this accumulation. Actin therefore gathers on mitochondria when they are ready to split.

The proteins cortactin and cofilin and the Arp2/3 complex—all of which spur actin to branch— also collect on mitochondria, Li et al. found. Depleting any of these factors led to extra-long mitochondria but didn’t alter the rate of organelle fusion, suggesting that actin branching promotes mitochondrial division.

In cells lacking Drp1, extra cortactin, cofilin, and Arp2/3 accumulated on mitochondria. In cells lacking either cortactin or cofilin, mitochondria carried excess Drp1. Mitochondria are abnormally long in both types of cells, suggesting that Drp1 accumulation and F-actin polymerization are necessary for mitochondrial fission. But how actin polymerization helps Drp1 cleave mitochondria remains unknown.