Mago and Y14 are core components of the exon junction complex (EJC), an assembly central to nonsense-mediated mRNA decay in humans and mRNA localization in flies. The Mago-Y14 heterodimer shuttles between the nucleus, where it is loaded onto specific mRNAs, and the cytoplasm, where it functions in translational regulation. The heterodimer is imported back into the nucleus by Importin 13 (Imp13), a member of the karyopherin-beta family of transport factors. We have elucidated the structural basis of the Mago-Y14 nuclear import cycle. The 3.35 A structure of the Drosophila Imp13-Mago-Y14 complex shows that Imp13 forms a ring-like molecule, reminiscent of Crm1, and encircles the Mago-Y14 cargo with a conserved interaction surface. The 2.8 A structure of human Imp13 bound to RanGTP reveals how Mago-Y14 is released in the nucleus by a steric hindrance mechanism. Comparison of the two structures suggests how this unusual karyopherin might function in bidirectional nucleocytoplasmic transport.

Cup is an eIF4E-binding protein (4E-BP) that plays a central role in translational regulation of localized mRNAs during early Drosophila development. In particular, Cup is required for repressing translation of the maternally contributed oskar, nanos, and gurken mRNAs, all of which are essential for embryonic body axis determination. Here, we present the 2.8 Å resolution crystal structure of a minimal eIF4E-Cup assembly, consisting of the interacting regions of the two proteins. In the structure, two separate segments of Cup contact two orthogonal faces of eIF4E. The eIF4E-binding consensus motif of Cup (YXXXXLF) binds the convex side of eIF4E similarly to the consensus of other eIF4E-binding proteins, such as 4E-BPs and eIF4G. The second, noncanonical, eIF4E-binding site of Cup binds laterally and perpendicularly to the eIF4E b-sheet. Mutations of Cup at this binding site were shown to reduce binding to eIF4E and to promote the destabilization of the associated mRNA. Comparison with the binding mode of eIF4G to eIF4E suggests that Cup and eIF4G binding would be mutually exclusive at both binding sites. This shows how a common molecular surface of eIF4E might recognize different proteins acting at different times in the same pathway. The structure provides insight into the mechanism by which Cup disrupts eIF4E-eIF4G interaction and has broader implications for understanding the role of 4E-BPs in translational regulation.

Importin13 (Imp13) is an unusual β-karyopherin that is able to both import and export cargoes in and out of the nucleus. In the cytoplasm, Imp13 associates with different cargoes such as Mago-Y14 and Ubc9, and facilitates their import into the nucleus where RanGTP binding promotes the release of the cargo. In this study, we present the 2.8 Å resolution crystal structure of Imp13 in complex with the SUMO E2-conjugating enzyme, Ubc9. The structure shows an uncommon mode of cargo-karyopherin recognition with Ubc9 binding at the N-terminal portion of Imp13, occupying the entire RanGTP-binding site. Comparison of the Imp13-Ubc9 complex with Imp13-Mago-Y14 shows the remarkable plasticity of Imp13, whose conformation changes from a closed ring to an open superhelix when bound to the two different cargoes. The structure also shows that the binding mode is compatible with the sumoylated states of Ubc9. Indeed, we find that Imp13 is able to bind sumoylated Ubc9 in vitro and suppresses autosumoylation activity in the complex.

Importin13 (Imp13) is a bidirectional karyopherin that can mediate both import and export of cargoes. Imp13 recognizes several import cargoes, which include the exon junction complex components Mago‐Y14 and the E2 SUMO‐conjugating enzyme Ubc9, and one known export cargo, the translation initiation factor 1A (eIF1A). To understand how Imp13 can perform double duty, we determined the 3.6‐Å crystal structure of Imp13 in complex with RanGTP and with eIF1A. eIF1A binds at the inner surface of the Imp13 C‐terminal arch adjacent and concomitantly to RanGTP illustrating how eIF1A can be exported by Imp13. Moreover, the 3.0‐Å structure of Imp13 in its unbound state reveals the existence of an open conformation in the cytoplasm that explains export cargo release and completes the export branch of the Imp13 pathway. Finally, we demonstrate that Imp13 is able to bind and export eIF1A in vivo and that its function is essential.

No abstract

In two stands of spruce in northern Germany colonization patterns of seven species of bark beetles were examined under field conditions. It can be shown that all species are segregated from each other by means of only a few factors. Those are stand‐specific microclimate, cover of logs with soil, bark thickness, and body size of beetles. Particularly the relation between body height and minimal bark thickness is pointed out. The lower limit of thickness of bark which can be colonized by Ips typographus (L.) is 2.5 mm. Zusammenfassung Öml;kologische Segregation bei Borkenkäfern (Col., Scolytidae) der Fichte Die Besiedlungsmuster von 7 Borkenkäferarten an der Fichte wurden an 2 Standorten in Norddeutschland unter Freilandbedingungen untersucht. Es wird gezeigt, daß wenige Faktoren allein ausreichen, die Segregation aller Arten voneinander nachzuweisen. Es handelt sich hierbei um das standortspezifische Mikroklima, Erdbedeckung der Stämme, Rindendicke und Körper‐größe der Käfer. Der Beziehung zwischen Körperhöhe und minimaler Rindendicke kommt besondere Bedeutung zu. Die Untergrenze der von Ips typographus (L.) besiedelbaren Rinde liegt bei 2,5 mm Dicke.

The maturation of nickel-dependent enzymes requires the participation of several accessory proteins. Typically the hydrolysis of nucleotides is necessary for the final metal transfer steps. The ATPase CooC has been implicated in the insertion of nickel into the Ni,Fe cluster (C cluster) of the carbon monoxide dehydrogenase from Rhodospirillum rubrum. Analysis of the amino acid sequence of CooC suggests the presence of motifs typical for the MinD family of SIMIBI class NTPases, which contain a deviant Walker A motif. The genome of the carboxidotrophic hydrogenogenic bacterium Carboxydothermus hydrogenoformans contains three open reading frames with distinct sequence homology to CooC from R. rubrum. We overproduced, isolated, and studied CooC1 from C. hydrogenoformans. As-isolated CooC1 is monomeric in the absence of ligands but dimerizes in the presence of either nickel, ADP, or ATP. CooC1 shows ATPase activity, and the ADP- and ATP-bound dimeric states are distinguished by their stability. The K8A mutant of CooC1, in which alanine replaces the signature lysine typical for the deviant Walker A motif in the MinD family, is incapable of both ATP hydrolysis and ATP-dependent dimerization. This corroborates that CooC1 is indeed a member of the MinD family and suggests an analogous dynamic equilibrium between monomeric and dimeric states. CooC proteins are involved in the insertion of nickel into carbon monoxide dehydrogenases, and we found that one CooC1 dimer binds one Ni(II) ion with nanomolar affinity. Ni-induced dimerization and the Ni(II)-CooC1 stoichiometry suggest that the Ni-binding site of CooC1 occurs in the dimer interface.