Volume 20 Issue 8, August 2013

The binding of stromal interaction molecule 1 (STIM1) to ORAI calcium channels is critical for store-operated calcium entry (SOCE), a calcium influx pathway conserved among nearly all vertebrate cells. Although many major steps of this pathway are well understood, crucial details regarding the mechanism of STIM1 activation remain unclear. A study in this issue provides important new insights into the conformational changes that occur during STIM1 activation.

The detailed mechanism by which the molecular motors kinesin and myosin travel along their respective protein tracks as they generate force during motile processes is still poorly understood. In a recent breakthrough, a crystal structure of kinesin in complex with tubulin illuminates the atomic-level details of a motor-track interaction, answering many questions yet leaving a number of mysteries unresolved.

Most mammalian promoters are inherently bidirectional, but transcription only elongates productively in one direction. Data presented in this paper demonstrate that at least part of the answer lies in the asymmetric distribution of polyadenylation-site sequences around human gene promoters causing termination of upstream antisense transcription.

The protein Hip interacts with chaperone Hsp70 and slows ADP dissociation from Hsp70, thus resulting in a delay in substrate release. Now crystal structures of Hip domains alone or in complex with Hsp70 nucleotide-binding domain, along with biochemical analyses, explain how Hip performs its activities.

The RNA helicase UPF1 has been implicated in various functions including nonsense-mediated decay (NMD). Transcriptome-wide analysis of UPF1-binding sites in translationally active versus inhibited cells provides evidence for translation-independent UPF1-RNA interactions and also suggests that UPF1 bound to coding sequence may be displaced by translating ribosomes and that NMD substrate selection may occur after UPF1-RNA interaction.

Eukaryotic DNA replication begins with recruitment of the ring helicase MCM2-7 by the origin recognition complex (ORC) in a reaction facilitated by initiation factors Cdc6 and Cdt1. A new cryo-EM structure of a helicase loading intermediate, the ORC–Cdc6–Cdt1–MCM2-7 complex, unexpectedly reveals both MCM2-7 and ORC hexamers encircling the DNA, and shows the arrangement of all 14 subunits within the helicase-loader complex.

How a class V myosin transports mRNA is not well understood. Single-molecule reconstitution of messenger ribonucleoprotein complexes from purified proteins and a localizing mRNA in budding yeast demonstrates that the mRNA is instrumental in ensuring a stable, processive transport complex, whereas the number of localizing elements ('zip codes') influences run length and frequency.

Given the high degree of similarity between Eph receptors, it has remained unclear how the same ligand can produce different signaling outcomes. The crystal structures of apo and ligand-bound EphA4, combined with cellular assays with chimeric ectodomains, now indicate that the specificity of signaling outcome is largely dictated by clustering properties structurally encoded within the ectodomain.

The mitochondrial inner membrane generates a proton-motive force (PMF) that drives cellular processes. Using a high-resolution fluorescence mapping approach combined with kinetic analyses, changes in the PMF are now shown to drive marked structural changes in the Tim23 channel of the TIM23 protein-translocation complex that are important for channel gating.

In immune cells, CRAC channels in the plasma membrane regulate store-operated Ca²⁺ entry in response to STIM1, a sensor protein located in the endoplasmic reticulum (ER) membrane. New biophysical assays show how dimerization of STIM1's ER-luminal domains causes extension of its cytoplasmic domains toward the plasma membrane to contact the ORAI pore and activate the channel, revealing the structural dynamics of this Ca²⁺-signaling mechanism.

In dimeric RING E3 ligases, the tail domain of the second subunit primes the transfer reaction. The crystal structure of human Tyr363-phosphorylated CBL-B in complex with ubiquitin-linked E2 UbcH5Band a peptide substrate now shows that a phosphorylated residue can act as the priming element in monomeric RING E3s.

Localization of protein to the peroxisome occurs through the recognition of specific targeting sequences (PTS1 or PTS2). The crystal structure of a yeast Pex7–Pex21 recognition complex bound to PTS2 reveals that the receptor complex forms a cavity with surfaces properties that complement the charge distribution on PTS2 and explain the conserved features of the peroxisomal targeting system.

HP1 proteins bind methylated histone H3 Lys9, a hallmark of heterochromatin, and mediate heterochromatin spreading by recruiting histone methyltransferase activities. New studies have now identified a long noncoding RNA called BORDERLINE that prevents spreading of the HP1 protein Swi6 and histone H3 Lys9 methylation beyond the pericentromeric repeat region of fission yeast chromosome 1.

Combined with kinetic analyses, the high resolution structure of kinesin-1 bound to a tubulin dimer offers a much-anticipated view of the motor-microtubule interface that illuminates the key step of neck-linker docking and of the structural basis for microtubule-accelerated ATP hydrolysis and motility.

The general transcription factor TFIID comprises TATA-binding protein (TBP) and TBP-associated factors (TAFs). The high-resolution structure of yeast TBP in complex with yeast TAF1 containing both transcriptionally activating and repressing regions reveals detailed and specific molecular patterns of interactions with TBP and their significance for transcriptional regulation.

Initiation factors eIF1 and eIF1A are key determinants of ribosomal scanning and initiation-codon selection during translation initiation. The structure of Tetrahymena thermophila 40S ribosome in complex with eIF1 and eIF1A reveals the conformational changes that accompany initiation-factor binding and provides new insights into the mechanism of start-codon recognition.