MINOS1/Mio10, a conserved mitochondrial protein, is required for mitochondrial inner membrane organization and cristae morphology. MINOS1/Mio10 is a novel constituent of the mitofilin/Fcj1 complex of the inner membrane, linking the morphology phenotype of the mutant to the activity of the mitochondrial inner membrane organizing complex.

Histone H3 trimethylation of lysine 9 (H3K9me3) and proteins of the heterochromatin protein 1 (HP1) family are hallmarks of heterochromatin, a state of compacted DNA essential for genome stability and long-term transcriptional silencing. The mechanisms by which H3K9me3 and HP1 contribute to chromatin condensation have been speculative and controversial. Here we demonstrate that human HP1β is a prototypic HP1 protein exemplifying most basal chromatin binding and effects. These are caused by dimeric and dynamic interaction with highly enriched H3K9me3 and are modulated by various electrostatic interfaces. HP1β bridges condensed chromatin, which we postulate stabilizes the compacted state. In agreement, HP1β genome-wide localization follows H3K9me3-enrichment and artificial bridging of chromatin fibres is sufficient for maintaining cellular heterochromatic conformation. Overall, our findings define a fundamental mechanism for chromatin higher order structural changes caused by HP1 proteins, which might contribute to the plastic nature of condensed chromatin.

SUMOylation, an essential posttranslational protein modification, is involved in many eukaryotic cellular signaling pathways. The identification of SUMOylated proteins is difficult, because SUMOylation sites in proteins are hard to predict, SUMOylated protein states are transient in vivo and labile in vitro, only a small substrate fraction is SUMOylated in vivo, and identification tools for natively SUMOylated proteins are rare. To solve these problems, we generated knock-in mice expressing His 6 -HA-SUMO1. By anti-HA immunostaining, we show that SUMO1 conjugates in neurons are only detectable in nuclei and annulate lamellae. By anti-HA affinity purification, we identified several hundred candidate SUMO1 substrates, of which we validated Smchd1, Ctip2, TIF1γ, and Zbtb20 as novel substrates. The knock-in mouse represents an excellent mammalian model for studies on SUMO1 localization and screens for SUMO1 conjugates in vivo.synapse | affinity purification S UMOylation is a conserved posttranslational protein modification in eukaryotes, akin to ubiquitylation, and can affect the localization, interactions, function, or stability of substrates (1). SUMOylation processes participate in many cellular signaling pathways, where they intersect with other posttranslational regulatory processes such as phosphorylation, ubiquitylation, or acetylation. Consequently, many cellular processes, from nuclear transport to neuronal synaptic transmission, are controlled by SUMOylation (2), and key SUMOylation substrates or altered SUMOylation are involved in many diseases, from cancer to neurological disorders (3).Reflecting the prominent nuclear role of SUMOylation, most known SUMOylation substrates are nuclear proteins (1, 2). However, SUMOylation appears to play a much more pervasive regulatory role in cells; recent studies have shown SUMOylation of ion channels, membrane-bound receptors, solute carriers, and mitochondrial or neuronal scaffolding and signaling proteins (4-6), leading to the notion that SUMOylation is a core regulatory process in all cellular subcompartments. This triggered substantial activities to develop tools for the discovery and validation of SUMOylation substrates in cells, which has proven difficult.Mammalian genomes contain four SUMO genes, encoding SUMO1, SUMO2, SUMO3 (7, 8), and SUMO4, of which SUMO4 is poorly characterized (9). SUMO2 and SUMO3 are almost identical, whereas SUMO1 is 50% homologous to SUMO2 and SUMO3. The 3D structure of SUMOs is similar to that of ubiquitin (10), and like ubiquitylation, SUMOylation involves an E1 activating enzyme, E2 conjugating enzymes, and E3 ligases (1,2,7,8).SUMOs are conjugated to lysine residues, often within a ϕKxD/E motif (ϕ, hydrophobic residue; x, any amino acid) (1), but in general SUMO acceptor lysines cannot be predicted, which prevents the identification of SUMO substrates by bioinformatics (1). Further, SUMOylated states of proteins are transient in vivo and labile in vitro because of isopeptidases that revert SUMOylation (1), and usually only a small f...

DNA and histone modifications direct the functional state of chromatin and thereby the readout of the genome. Candidate approaches and histone peptide affinity purification experiments have identified several proteins that bind to chromatin marks. However, the complement of factors that is recruited by individual and combinations of DNA and histone modifications has not yet been defined. Here, we present a strategy based on recombinant, uniformly modified chromatin templates used in affinity purification experiments in conjunction with SILAC-based quantitative mass spectrometry for this purpose. On the prototypic H3K4me3 and H3K9me3 histone modification marks we compare our method with a histone N-terminal peptide affinity purification approach. Our analysis shows that only some factors associate with both, chromatin and peptide matrices but that a surprisingly large number of proteins differ in their association with these templates. Global analysis of the proteins identified implies specific domains mediating recruitment to the chromatin marks. Our proof-of-principle studies show that chromatin templates with defined modification patterns can be used to decipher how the histone code is read and translated.