Rvb1 and Rvb2 are highly conserved and essential eukaryotic AAA+ proteins linked to a wide range of cellular processes. AAA+ proteins are ATPases associated with diverse cellular activities and are characterized by the presence of one or more AAA+ domains. These domains have the canonical Walker A and Walker B nucleotide binding and hydrolysis motifs. Rvb1 and Rvb2 have been found to be part of critical cellular complexes: the histone acetyltransferase Tip60 complex, chromatin remodelling complexes Ino80 and SWR-C, and the telomerase complex. In addition, Rvb1 and Rvb2 are components of the R2TP complex that was identified by our group and was determined to be involved in the maturation of box C/D small nucleolar ribonucleoprotein (snoRNP) complexes. Furthermore, the Rvbs have been associated with mitotic spindle assembly, as well as phosphatidylinositol 3-kinase-related protein kinase (PIKK) signalling. This review sheds light on the potential role of the Rvbs as chaperones in the assembly and remodelling of these critical complexes.

Rvb1 and Rvb2 are essential AAA+ proteins that interact together during the assembly and activity of diverse macromolecules including chromatin remodelers INO80 and SWR-C, and ribonucleoprotein complexes including telomerase and snoRNPs. ATP hydrolysis by Rvb1/2 is required for function; however, the mechanism that drives substrate remodeling is unknown. Here we determined the architecture of the yeast Rvb1/2 dodecamer using cryoelectron microscopy and identify that the substrate-binding insertion domain undergoes conformational changes in response to nucleotide state. 2D and 3D classification defines the dodecamer flexibility, revealing distinct arrangements and the hexamer-hexamer interaction interface. Reconstructions of the apo, ATP, and ADP states identify that Rvb1/2 undergoes substantial conformational changes that include a twist in the insertion-domain position and a corresponding rotation of the AAA+ ring. These results reveal how the ATP hydrolysis cycle of the AAA+ domains directs insertion-domain movements that could provide mechanical force during remodeling or helicase activities.

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Cytoplasmic genomes typically lack recombination, implying that genetic hitch-hiking could be a predominant force structuring nucleotide polymorphism in the chloroplast and mitochondria. We test this hypothesis by analysing nucleotide polymorphism data at 28 loci across the chloroplast and mitochondria of the outcrossing plant Arabidopsis lyrata, and compare patterns with multiple nuclear loci, and the highly selfing Arabidopsis thaliana. The maximum likelihood estimate of the ratio of effective population size at cytoplasmic relative to nuclear genes in A. lyrata does not depart from the neutral expectation of 0.5. Similarly, the ratio of effective size in A. thaliana is close to unity, the neutral expectation for a highly selfing species. The results are thus consistent with neutral organelle polymorphism in these species or with comparable effects of hitch-hiking in both cytoplasmic and nuclear genes, in contrast to the results of recent studies on gynodioecious taxa. The four-gamete test and composite likelihood estimation provide evidence for very low levels of recombination in the organelles of A. lyrata, although permutation tests do not suggest that adjacent polymorphic sites are more closely linked than more distant sites across the two genomes, suggesting that mutation hotspots or very low rates of gene conversion could explain the data.