Polyphosphate (polyP) occurs ubiquitously in cells, but its functions are poorly understood and its synthesis has only been characterized in bacteria. Using x-ray crystallography, we identified a eukaryotic polyphosphate polymerase within the membrane-integral vacuolar transporter chaperone (VTC) complex. A 2.6 angstrom crystal structure of the catalytic domain grown in the presence of adenosine triphosphate (ATP) reveals polyP winding through a tunnel-shaped pocket. Nucleotide- and phosphate-bound structures suggest that the enzyme functions by metal-assisted cleavage of the ATP gamma-phosphate, which is then in-line transferred to an acceptor phosphate to form polyP chains. Mutational analysis of the transmembrane domain indicates that VTC may integrate cytoplasmic polymer synthesis with polyP membrane translocation. Identification of the polyP-synthesizing enzyme opens the way to determine the functions of polyP in lower eukaryotes.

The Xenopus protein Maskin has been previously identified and characterized in the context of its role in translational control during oocyte maturation. Maskin belongs to the TACC protein family. In other systems, members of this family have been shown to localize to centrosomes during mitosis and play a role in microtubule stabilization. Here we have examined the putative role of Maskin in spindle assembly and centrosome aster formation in the Xenopus egg extract system. Depletion and reconstitution experiments indicate that Maskin plays an essential role for microtubule assembly during M-phase. We show that Maskin interacts with XMAP215 and Eg2, the Xenopus Aurora A kinase in vitro and in the egg extract. We propose that Maskin and XMAP215 cooperate to oppose the destabilizing activity of XKCM1 therefore promoting microtubule growth from the centrosome and contributing to the determination of microtubule steady-state length. Further more, we show that Maskin localization and function is regulated by Eg2 phosphorylation.

Sirtuins are NAD(+)-dependent protein deacetylases that connect metabolism and cellular homeostasis. Here we show that the nuclear Sirtuin SIRT7 targets PAF53, a subunit of RNA polymerase I (Pol I). Acetylation of PAF53 at lysine 373 by CBP and deacetylation by SIRT7 modulate the association of Pol I with DNA, hypoacetylation correlating with increased rDNA occupancy of Pol I and transcription activation. SIRT7 is released from nucleoli in response to different stress conditions, leading to hyperacetylation of PAF53 and decreased Pol I transcription. Nucleolar detention requires binding of SIRT7 to nascent pre-rRNA, linking the spatial distribution of SIRT7 and deacetylation of PAF53 to ongoing transcription. The results identify a nonhistone target of SIRT7 and uncover an RNA-mediated mechanism that adapts nucleolar transcription to stress signaling.