We describe a method to identify cross-linked peptides from complex samples and large protein sequence databases. The advance was achieved by combining isotopically tagged cross-linkers, chromatographic enrichment, targeted proteomics, and a novel search engine called xQuest. This software reduces the search space by an upstream candidatepeptide search before the recombination step; we show that xQuest can identify cross-linked peptides from a total E. coli lysate with an unrestricted database search.

Repetitive DNA sequences, which constitute half the genome in some organisms, often undergo homologous recombination. This can instigate genomic instability due to gain or loss of DNA1. Assembly of DNA into silent chromatin is generally thought to serve as a mechanism ensuring repeat stability by limiting access to the recombination machinery2. Consistent with this notion, in the budding yeast Saccharomyces cerevisiae, stability of the highly repetitive ribosomal DNA (rDNA) sequences requires a Sir2-containing chromatin silencing complex that also inhibits transcription from foreign promoters and transposons inserted within the repeats by a process called rDNA silencing2-5. Here, we describe a protein network that stabilizes rDNA repeats of budding yeast via interactions between rDNA-associated silencing proteins and two inner nuclear membrane (INM) proteins. Deletion of either the INM or silencing proteins reduces perinuclear rDNA positioning, disrupts the nucleolus-nucleoplasm boundary, induces the formation of recombination foci, and destabilizes the repeats. In addition, artificial targeting of rDNA repeats to the INM suppresses the instability observed in cells lacking an rDNA-associated silencing protein typically required for peripheral tethering of the repeats. Moreover, in contrast to Sir2 and its associated nucleolar factors, the INM proteins are not required for rDNA silencing, indicating that Sir2-dependent silencing is not sufficient to inhibit recombination within the rDNA locus. These findings demonstrate a role for INM proteins in perinuclear chromosome localization and show that tethering to the nuclear periphery is required for rDNA repeat stability. The INM proteins studied here are conserved and have been implicated in chromosome organization in metazoans6,7. Our results therefore reveal an ancient mechanism in which interactions between INM and chromosomal proteins ensure genome stability.

The conserved TREX mRNA export complex is known to contain UAP56, Aly, Tex1, and the THO complex. Here, we carried out proteomic analysis of immunopurified human TREX complex and identified the protein CIP29 as the only new component with a clear yeast relative (known as Tho1). Tho1 is known to function in mRNA export, and we provide evidence that CIP29 likewise functions in this process. Like the known TREX components, a portion of CIP29 localizes in nuclear speckle domains, and its efficient recruitment to mRNA is both splicingand cap-dependent. We show that UAP56 mediates an ATP-dependent interaction between the THO complex and both CIP29 and Aly, indicating that TREX assembly is ATP-dependent. Using recombinant proteins expressed in Escherichia coli, we show that UAP56, Aly, and CIP29 form an ATP-dependent trimeric complex, and UAP56 bridges the interaction between CIP29 and Aly. We conclude that the interaction of two conserved export proteins, CIP29 and Aly, with UAP56 is strictly regulated by ATP during assembly of the TREX complex. . In yeast, the THO complex consists of four tightly associated subunits (Tho2, Hpr1, Mft1, and Thp1) (Piruat and Aguilera 1998;Jimeno et al. 2006). Likewise, the metazoan THO complex consists of a set of tightly associated proteins, three of which (fSAP79, fSAP35, and fSAP24; known now as THOC5, THOC6, and THOC7, respectively) do not appear to be conserved in yeast and two of which are orthologs of Tho2 (THOC2) and Hpr1 (THOC1) (Rehwinkel et al. 2004;Masuda et al. 2005). In yeast, Aguilera and coworkers (Piruat and Aguilera 1998) identified a protein known as Tho1 during the same genetic screen that they used to identify the THO complex. Subsequent characterization of Tho1 revealed that it functions in mRNP biogenesis and export, but this protein was not identified as a component of the THO/TREX complex (Piruat and Aguilera 1998;Jimeno et al. 2006). However, Tho1 is a multicopy suppressor of THO complex mutants and is recruited to mRNA in a THO complex-dependent manner (Piruat and Aguilera 1998;Jimeno et al. 2006). In humans, a counterpart of yeast Tho1 was identified based on sequence alignment (Jimeno et al. 2006). This protein, CIP29, was first reported as a cytokine-induced protein and later was linked to several cancers (Choong et al. 2001;Fukuda et al. 2002;Hashii et al. 2004;Leaw et al. 2004). Like yeast Tho1, CIP29 contains a SAF motif and binds to DNA, which led to the speculation that CIP29 functions in transcription (Aravind and Koonin 2000; Hashii et al. 2004).3 Corresponding author. E-MAIL rreed@hms.harvard.edu; FAX (617) 432-3091. Article is online at http://www.genesdev.org/cgi

Summary Previous experiments suggest a connection between the N-alpha-acetylation of proteins and the sensitivity of cells to apoptotic signals. Here, we describe a novel biochemical assay to detect the acetylation status of proteins and demonstrate that protein N-alpha-acetylation is regulated by the availability of acetyl-CoA. Because the anti-apoptotic protein Bcl-xL is known to influence mitochondrial metabolism, we reasoned that Bcl-xL may provide a link between protein N-alpha-acetylation and apoptosis. Indeed, Bcl-xL overexpression leads to a reduction in levels of acetyl-CoA and N-alpha-acetylated proteins in the cell. This effect is independent of Bax and Bak, the known binding partners of Bcl-xL. Increasing cellular levels of acetyl-CoA by addition of acetate or citrate restores protein N-alpha-acetylation in Bcl-xL-expressing cells and confers sensitivity to apoptotic stimuli. We conclude that acetyl-CoA serves as a signaling molecule that couples apoptotic sensitivity to metabolism by regulating protein N-alpha-acetylation.