Self-assembly of multi-component mitochondrial nucleoids via phase separation

Mitochondria contain an autonomous and spatially segregated genome. The organizational unit of their genome is the nucleoid, which consists of mitochondrial DNA (mtDNA) and associated architectural proteins. Here, we show that phase separation is the primary physical mechanism for assembly and size control of the mitochondrial nucleoid (mt-nucleoid). The major mtDNA-binding protein TFAM spontaneously phase separates in vitro via weak, multivalent interactions into droplets with slow internal dynamics. TFAM and mtDNA form heterogenous, viscoelastic structures in vitro, which recapitulate the dynamics and behavior of mt-nucleoids in vivo. Mt-nucleoids coalesce into larger droplets in response to various forms of cellular stress, as evidenced by the enlarged and transcriptionally active nucleoids in mitochondria from patients with the premature aging disorder Hutchinson-Gilford Progeria Syndrome (HGPS). Our results point to phase separation as an evolutionarily conserved mechanism of genome organization.

Automated summary

Mitochondria contain an autonomous genome organized into nucleoids, and the primary physical mechanism for assembly and size control of the mitochondrial nucleoid (mt-nucleoid) is phase separation. The major mtDNA-binding protein TFAM can spontaneously phase separate into droplets with slow internal dynamics in vitro, which recapitulates the dynamics and behavior of mt-nucleoids in vivo. Mt-nucleoids coalesce into larger droplets in response to cellular stress, as seen in patients with Hutchinson-Gilford Progeria Syndrome (HGPS), pointing to phase separation as an evolutionarily conserved mechanism of genome organization.