Apoptosis

Mitochondrial rhomboid PARL regulates cytochrome c release during apoptosis via OPA1-dependent cristae remodelling. Cipolat, S. et al. Cell 126, 163–175 (2006)

OPA1 controls apoptotic cristae remodeling independently from mitochondrial fusion. Frezza, C. et al. Cell 126, 177–189 (2006)

Cytochrome c resides in the convoluted folds (cristae) of the mitochondrial inner membrane until it is released to amplify apoptosis. Two studies showed that cytochrome c mobilization from mitochondria is controlled by the rhomboid protease PARL and the dynamin-related protein OPA1. Cleavage of a membrane-bound form of OPA1 by mitochondrial PARL generates soluble OPA1, which forms oligomers that maintain the shape of the cristae and, thereby, the compartmentalization of cytochrome c. The pro-apoptotic BCL2-family protein BID disrupts OPA1 oligomers and widens cristae junctions, enabling cytochrome c release.

DNA repair

Visualization of Rad54, a chromatin remodelling protein, translocating on single DNA molecules. Amitani, I. et al. Mol. Cell 23, 143–148 (2006)

Rad54 is a double-stranded DNA (dsDNA)-dependent ATPase that functions in dsDNA-break repair and homologous recombination. The authors developed an optically trapped DNA–bead complex to visualize the movement of Rad54 along single molecules of dsDNA. The translocation is rapid and processive, and Rad54 binds randomly along the dsDNA and moves either upstream or downstream with a velocity that depends on the ATP concentration.

Molecular motors

Single-molecule analysis of dynein processivity and stepping behaviour. Reck-Peterson, S. L. et al. Cell 126, 335–348 (2006)

Cytoplasmic dynein drives nearly all minus-end-directed motility in eukaryotic cells. Using a recombinant dimeric dynein protein, the Vale group showed the processive motion of dynein by structural and single-molecule analysis. Both dynein motor domains of the dimer are essential for its processivity, but the tail domains and associated subunits are not. Dynein primarily takes 8-nm steps, but shows considerable variability in its step size and direction, and its stepping behaviour is most reminiscent of myosin VI.

Cytokinesis

Assembly of the cytokinetic contractile ring from a broad band of nodes in fission yeast. Wu, J.-Q. et al. J. Cell Biol. 174, 391–402 (2006)

Wu et al. showed that the cytokinetic contractile ring in fission yeast arises from a broad band of small dots or nodes, rather than from a single myosin II progenitor as previously proposed. The anillin-like protein Mid1 is required to establish this band of nodes, which also contains other proteins, and actin polymerization stimulates contractile-ring formation.