The chromosomal passenger complex (CPC) is a key regulator of chromosome segregation and cytokinesis. CPC functions are connected to its localization. The complex first localizes to centromeres and later associates with the central spindle and midbody. Survivin, Borealin, and INCENP are the three components of the CPC that regulate the activity and localization of its enzymatic component, the kinase Aurora B. We determined the 1.4 A resolution crystal structure of the regulatory core of the CPC, revealing that Borealin and INCENP associate with the helical domain of Survivin to form a tight three-helical bundle. We used siRNA rescue experiments with structure-based mutants to explore the requirements for CPC localization. We show that the intertwined structural interactions of the core components lead to functional interdependence. Association of the core "passenger" proteins creates a single structural unit, whose composite molecular surface presents conserved residues essential for central spindle and midbody localization.

Patients with human severe combined immunodeficiency (SCID) can be divided into those with B lymphocytes (B+ SCID) and those without (B- SCID). Although several genetic causes are known for B+ SCID, the etiology of B- SCID has not been defined. Six of 14 B- SCID patients tested were found to carry a mutation of the recombinase activating gene 1 (RAG-1), RAG-2, or both. This mutation resulted in a functional inability to form antigen receptors through genetic recombination and links a defect in one of the site-specific recombination systems to a human disease.

In metazoa, regulation of the phosphorylation state of UPF1 is crucial for nonsense-mediated mRNA decay (NMD), a process by which aberrant mRNAs containing nonsense mutations are degraded. UPF1 is targeted for dephosphorylation by three related proteins, SMG5, SMG6, and SMG7. We report here the crystal structure of the N-terminal domain of SMG7. The structure reveals that SMG7 contains a 14-3-3-like domain. Residues that bind phosphoserine-containing peptides in 14-3-3 are conserved at the equivalent positions in SMG7. Mutation of these residues impairs UPF1 binding to SMG7 in vitro and UPF1 recruitment to cytoplasmic mRNA decay foci in vivo, suggesting that SMG7 acts as an adaptor in targeting mRNAs associated with phosphorylated UPF1 for degradation. The 14-3-3 site of SMG7 is conserved in SMG5 and SMG6. These data also imply that the homologous human Est1 might have a 14-3-3 function at telomeres, and that phosphorylation events may be important for telomerase regulation.

Exosomes are complexes containing 3 0 -5 0 exoribonucleases that have important roles in processing, decay and quality control of various RNA molecules. Archaeal exosomes consist of a hexameric core of three active RNase PH subunits (ribosomal RNA processing factor (Rrp)41) and three inactive RNase PH subunits (Rrp42). A trimeric ring of subunits with putative RNA-binding domains (Rrp4/cep1 synthetic lethality (Csl)4) is positioned on top of the hexamer on the opposite side to the RNA degrading sites. Here, we present the 1.6 Å resolution crystal structure of the nine-subunit exosome of Sulfolobus solfataricus and the 2.3 Å structure of this complex bound to an RNA substrate designed to be partly trimmed rather than completely degraded. The RNA binds both at the active site on one side of the molecule and on the opposite side in the narrowest constriction of the central channel. Multiple substrate-binding sites and the entrapment of the substrate in the central channel provide a rationale for the processive degradation of extended RNAs and the stalling of structured RNAs.

Polycomb group (PcG) proteins repress transcription by modifying chromatin structure in target genes. dSfmbt is a subunit of the Drosophila melanogaster PcG protein complex PhoRC and contains four malignant brain tumour (MBT) repeats involved in the recognition of various mono-and dimethylated histone peptides. Here, we present the crystal structure of the four-MBT-repeat domain of dSfmbt in complex with a mono-methylated histone H4 peptide. Only a single histone peptide binds to the four-MBT-repeat domain. Mutational analyses show highaffinity binding with low peptide sequence selectivity through combinatorial interaction of the methyl-lysine with an aromatic cage and positively charged flanking residues with the surrounding negatively charged surface of the fourth MBT repeat. dSfmbt directly interacts with the PcG protein Scm, a related MBT-repeat protein with similar methyl-lysine binding activity. dSfmbt and Scm co-occupy Polycomb response elements of target genes in Drosophila and they strongly synergize in the repression of these target genes, suggesting that the combined action of these two MBT proteins is crucial for Polycomb silencing.