Cell Cycle

Ensuring Adequate Energy Supply

Science Signaling  04 Oct 2011:
Vol. 4, Issue 193, pp. ec273
DOI: 10.1126/scisignal.4193ec273

In quiescent cells, mitochondria exist in an interconnected network that exhibits dynamic morphological changes through fission and fusion in response to cellular energy demands and stimuli. In dividing cells, mitochondrial fission is critical to ensure proper distribution of mitochondria in both cells. DRP1 is a large dynamin-related guanosine triphosphatase (GTPase) that is recruited to the mitochondria from the cytosol to mediate fission, and this is associated with phosphorylation of DRP1 by cyclin B/cyclin-dependent kinase 1 (cyclin B/CDK1) complex (see Yamano and Youle). Kashatus et al. report that the mitotic kinase Aurora A also affects mitochondrial fission by controlling the recruitment of the GTPase RALA to mitochondria. A pool of green fluorescent protein (GFP)–tagged RALA colocalized with mitochondrial markers, and immunoblotting experiments showed that this association, as well as the association of DRP1 and the RALA binding partner RALBP1 with mitochondria, was enhanced in cells expressing kinase-active, but not kinase-inactive, Aurora A. A phosphomimetic mutant of RALA also showed increased mitochondrial colocalization, but a Ser-to-Ala mutation at the site phosphorylated by Aurora A decreased the association of RALA with mitochondria. Cells expressing the kinase-active Aurora A also exhibited small, circular mitochondria, in contrast to the cells expressing the kinase-inactive Aurora A, which had long interconnected mitochondria. The long interconnected mitochondrial phenotype and a reduction in the association of DRP1 with mitochondria were also observed in cells in which RALA was knocked down. Consistent with a role in regulating mitochondrial fission during mitosis, the relative abundance of RALA, RALBP1, and DRP1 associated with mitochondria was greater in cells synchronized in mitosis than in unsynchronized cells. Knockdown of RALBP1, but not RALA, reduced the amount of phosphorylated DRP1. RALBP1 or cyclin B immunoprecipitates from mitotic cells phosphorylated purified DRP1, and RALBP1 and cyclin B coimmunoprecipitated in mitotic cells, suggesting that RALBP1 and cyclin B/CDK1 form an active complex in mitotic cells. Chemical cross-linking experiments with mitochondrial fractions from mitotic cells showed that RALA, RALBP1, DRP1, and cyclin B formed a complex. Knockdown of RALA or RALBP1 resulted in unequal partitioning of mitochondria during mitosis and the persistence of interconnected mitochondria throughout mitosis. Collectively, these data suggest that phosphorylation of RALA by Aurora A promotes the redistribution of RALA to the mitochondria, which recruits DRP1 and RALBP1, the latter of which serves as a scaffold to recruit cyclin B/CDK1 to the mitochondria to phosphorylate its substrate DRP1 and stimulate fission.

D. F. Kashatus, K.-H. Lim, D. C. Brady, N. L. K. Pershing, A. D. Cox, C. M. Counter, RALA and RALBP1 regulate mitochondrial fission at mitosis. Nat. Cell Biol. 13, 1108–1115 (2011). [PubMed]

K. Yamano, R. J. Youle, Coupling mitochondrial and cell division. Nat. Cell Biol. 13, 1026–1027 (2011). [PubMed]