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DrosophilaSmaug and its orthologs comprise a family of mRNA repressor proteins that exhibit various functions during animal development. Smaug proteins contain a characteristic RNA-binding sterile-α motif (SAM) domain and a conserved but uncharacterized N-terminal domain (NTD). Here, we resolved the crystal structure of the NTD of the human SAM domain-containing protein 4A (SAMD4A, a.k.a. Smaug1) to 2.0 Å resolution, which revealed its composition of a homodimerization D-subdomain and a subdomain with similarity to a PHAT domain. Furthermore, we show thatDrosophilaSmaug directly interacts with theDrosophilagermline inducer Oskar and with the Hedgehog signaling transducer Smoothened through its D-PHAT domain. We determined the crystal structure of the D-PHAT domain of Smaug in complex with a Smoothened α-helical peptide to 1.61 Å resolution. The peptide binds within a groove that is formed by both the D- and PHAT subdomains. Structural modeling supported by experimental data suggested that an α-helix within the disordered region of Oskar binds to the D-PHAT domain in a mode similar to Smoothened. Together, our data uncover the N-terminal D-PHAT domain of Smaug as peptide-binding domain.

Characterizing protein-protein interactions (PPIs) is fundamental for understanding biochemical processes. Many methods have been established to identify and study direct PPIs; however, screening and investigating PPIs involving large and poorly soluble proteins remain challenges. As a result, we developed ReLo, a simple cell culture-based method to detect and investigate interactions in a cellular context. ReLo allows both the identification of protein domains that mediate the formation of complexes and screening of interfering point mutations. ReLo is sensitive to drugs that mediate or interfere with a specific interaction. Furthermore, protein conformation- and protein arginine methylation-dependent interactions can be studied. Importantly, ReLo can be used for specifically detecting direct but not indirect PPIs and can be applied for describing the binding topology of subunits within multiprotein complexes. Because of these attributes, ReLo is a simple, quick and versatile tool for identifying and studying binary PPIs, as well as for characterizing multisubunit complexes.

The cohesin ring complex is required for numerous chromosomal transactions including sister chromatid cohesion, DNA damage repair and transcriptional regulation. How cohesin engages its chromatin substrate has remained an unresolved question. We show here, by determining a crystal structure of the budding yeast cohesin HEAT-repeat subunit Scc3 bound to a fragment of the Scc1 kleisin subunit and DNA, that Scc3 and Scc1 form a composite DNA interaction module. The Scc3-Scc1 subcomplex engages double-stranded DNA through a conserved, positively charged surface. We demonstrate that this conserved domain is required for DNA binding by Scc3-Scc1 in vitro, as well as for the enrichment of cohesin on chromosomes and for cell viability. These findings suggest that the Scc3-Scc1 DNA-binding interface plays a central role in the recruitment of cohesin complexes to chromosomes and therefore for cohesin to faithfully execute its functions during cell division.

Condensin protein complexes coordinate the formation of mitotic chromosomes and thereby ensure the successful segregation of replicated genomes. Insights into how condensin complexes bind to chromosomes and alter their topology are essential for understanding the molecular principles behind the large-scale chromatin rearrangements that take place during cell divisions. Here, we identify a direct DNA-binding site in the eukaryotic condensin complex, which is formed by its Ycg1Cnd3 HEAT-repeat and Brn1Cnd2 kleisin subunits. DNA co-crystal structures reveal a conserved, positively charged groove that accommodates the DNA double helix. A peptide loop of the kleisin subunit encircles the bound DNA and, like a safety belt, prevents its dissociation. Firm closure of the kleisin loop around DNA is essential for the association of condensin complexes with chromosomes and their DNA-stimulated ATPase activity. Our data suggest a sophisticated molecular basis for anchoring condensin complexes to chromosomes that enables the formation of large-sized chromatin loops.

Summary The condensin protein complex plays a key role in the structural organization of genomes. How the ATPase activity of its SMC subunits drives large-scale changes in chromosome topology has remained unknown. Here we reconstruct, at near-atomic resolution, the sequence of events that take place during the condensin ATPase cycle. We show that ATP binding induces a conformational switch in the Smc4 head domain that releases its hitherto undescribed interaction with the Ycs4 HEAT-repeat subunit and promotes its engagement with the Smc2 head into an asymmetric heterodimer. SMC head dimerization subsequently enables nucleotide binding at the second active site and disengages the Brn1 kleisin subunit from the Smc2 coiled coil to open the condensin ring. These large-scale transitions in the condensin architecture lay out a mechanistic path for its ability to extrude DNA helices into large loop structures., Graphical Abstract, Highlights • Smc4 and Smc2 ATPase head structures reorganize upon ATP binding and dimerization • A Q-loop-mediated switch releases the Ycs4 HEAT-repeat subunit from the Smc4 head • The Smc2 head engages with the ATP-bound Smc4 head into an asymmetric heterodimer • Head dimerization releases the Brn1 kleisin from Smc2 via coiled-coil rotation, Hassler et al. report structural and functional insights into the enzymatic core of the condensin protein complex that reveal large-scale conformational changes upon ATP binding by and subsequent dimerization of its catalytic SMC head domains. These movements presumably power the condensin-mediated extrusion of DNA loops during mitotic chromosome formation.

Summary Tail-anchored (TA) proteins, defined by the presence of a single C-terminal transmembrane domain (TMD), play critical roles throughout the secretory pathway and in mitochondria, yet the machinery responsible for their proper membrane insertion remains poorly characterized. Here we show that Get3, the yeast homolog of the TA-interacting factor Asna1/Trc40, specifically recognizes TMDs of TA proteins destined for the secretory pathway. Get3 recognition represents a key decision step, whose loss can lead to misinsertion of TA proteins into mitochondria. Get3-TA protein complexes are recruited for endoplasmic reticulum (ER) membrane insertion by the Get1/Get2 receptor. In vivo, the absence of Get1/Get2 leads to cytosolic aggregation of Get3-TA complexes and broad defects in TA protein biogenesis. In vitro reconstitution demonstrates that the Get proteins directly mediate insertion of newly synthesized TA proteins into ER membranes. Thus, the GET complex represents a critical mechanism for ensuring efficient and accurate targeting of TA proteins.

Arginine (R)-based ER localization signals are sorting motifs that confer transient ER localization to unassembled subunits of multimeric membrane proteins. The COPI vesicle coat binds R-based signals but the molecular details remain unknown. Here, we use reporter membrane proteins based on the proteolipid Pmp2 fused to GFP and allele swapping of COPI subunits to map the recognition site for R-based signals. We show that two highly conserved stretches—in the β- and δ-COPI subunits—are required to maintain Pmp2GFP reporters exposing R-based signals in the ER. Combining a deletion of 21 residues in δ-COP together with the mutation of three residues in β-COP gave rise to a COPI coat that had lost its ability to recognize R-based signals, whilst the recognition of C-terminal di-lysine signals remained unimpaired. A homology model of the COPI trunk domain illustrates the recognition of R-based signals by COPI.

Condensin complexes have central roles in the three-dimensional organization of chromosomes during cell divisions, but how they interact with chromatin to promote chromosome segregation is largely unknown. Previous work has suggested that condensin, in addition to encircling chromatin fibers topologically within the ring-shaped structure formed by its SMC and kleisin subunits, contacts DNA directly. Here we describe the discovery of a binding domain for double-stranded DNA formed by the two HEAT-repeat subunits of the Saccharomyces cerevisiae condensin complex. From detailed mapping data of the interfaces between the HEAT-repeat and kleisin subunits, we generated condensin complexes that lack one of the HEAT-repeat subunits and consequently fail to associate with chromosomes in yeast and human cells. The finding that DNA binding by condensin's HEAT-repeat subunits stimulates the SMC ATPase activity suggests a multistep mechanism for the loading of condensin onto chromosomes.

SummarySuccessful segregation of chromosomes during mitosis and meiosis depends on the action of the ring-shaped condensin complex, but how condensin ensures the complete disjunction of sister chromatids is unknown. We show that the failure to segregate chromosome arms, which results from condensin release from chromosomes by proteolytic cleavage of its ring structure, leads to a DNA damage checkpoint-dependent cell-cycle arrest. Checkpoint activation is triggered by the formation of chromosome breaks during cytokinesis, which proceeds with normal timing despite the presence of lagging chromosome arms. Remarkably, enforcing condensin ring reclosure by chemically induced dimerization just before entry into anaphase is sufficient to restore chromosome arm segregation. We suggest that topological entrapment of chromosome arms by condensin rings ensures their clearance from the cleavage plane and thereby avoids their breakage during cytokinesis.

The multisubunit condensin complex is essential for the structural organization of eukaryotic chromosomes during their segregation by the mitotic spindle, but the mechanistic basis for its function is not understood. To address how condensin binds to and structures chromosomes, we have isolated from Saccharomyces cerevisiae cells circular minichromosomes linked to condensin. We find that either linearization of minichromosome DNA or proteolytic opening of the ring-like structure formed through the connection of the two ATPase heads of condensin's structural maintenance of chromosomes (SMC) heterodimer by its kleisin subunit eliminates their association. This suggests that condensin rings encircle chromosomal DNA. We further show that release of condensin from chromosomes by ring opening in dividing cells compromises the partitioning of chromosome regions distal to centromeres. Condensin hence forms topological links within chromatid arms that provide the arms with the structural rigidity necessary for their segregation.

Arginine (R)-based ER localization signals are sorting motifs that confer transient ER localization to unassembled subunits of multimeric membrane proteins. The COPI vesicle coat binds R-based signals but the molecular details remain unknown. Here, we use reporter membrane proteins based on the proteolipid Pmp2 fused to GFP and allele swapping of COPI subunits to map the recognition site for R-based signals. We show that two highly conserved stretches--in the beta- and delta-COPI subunits--are required to maintain Pmp2GFP reporters exposing R-based signals in the ER. Combining a deletion of 21 residues in delta-COP together with the mutation of three residues in beta-COP gave rise to a COPI coat that had lost its ability to recognize R-based signals, whilst the recognition of C-terminal di-lysine signals remained unimpaired. A homology model of the COPI trunk domain illustrates the recognition of R-based signals by COPI.

Cellular ion homeostasis involves communication between the cytosol and the luminal compartment of organelles. This is particularly critical for metal ions because of their toxic potential. We have identified the yeast homologue of the prokaryotic ArsA protein, the homodimeric ATPase Arr4p, as a protein that binds to the yeast intracellular CLC chloride-transport protein, Gef1p. We show that binding of Arr4p to the C terminus of Gef1p requires the presence of yeast cytosol and is sensitive to a highly specific copper chelator in vitro and in vivo. Copper alone can substitute for cytosol to support the interaction of Arr4p with the C terminus of Gef1p. The migration behavior of Arr4p in nonreducing gel electrophoresis correlates with cellular copper deficiency, repletion, or stress. Our homology model of Arr4p shows that the antimony (arsenic) metal binding site of ArsA is not conserved in Arr4p. The model suggests that a pair of cysteines, Cys285 and Cys288, is located in the interface of the Arr4p dimer. These residues are required for Arr4p homodimerization and for binding to the C terminus of Gef1p. Whereas both proteins are required for normal growth under iron-limiting conditions, they play opposite roles when copper and heat stress are combined in an alkaline environment. Under these conditions, deltagef1 cells grow much better than wild type yeast, whereas deltaarr4 cells are unable to grow. Comparison of the deltaarr4 with the deltaarr4deltagef1 strain suggests that Arr4p antagonizes the function of Gef1p.