No abstract

Mammalian shelterin proteins POT1 and TPP1 form a stable heterodimer that protects chromosome ends and regulates telomerase-mediated telomere extension. However, how POT1 interacts with TPP1 remains unknown. Here we present the crystal structure of the C-terminal portion of human POT1 (POT1C) complexed with the POT1-binding motif of TPP1. The structure shows that POT1C contains two domains, a third OB fold and a Holliday junction resolvase-like domain. Both domains are essential for binding to TPP1. Notably, unlike the heart-shaped structure of ciliated protozoan Oxytricha nova TEBPα–β complex, POT1–TPP1 adopts an elongated V-shaped conformation. In addition, we identify several missense mutations in human cancers that disrupt the POT1C–TPP1 interaction, resulting in POT1 instability. POT1C mutants that bind TPP1 localize to telomeres but fail to repress a DNA damage response and inappropriate repair by A-NHEJ. Our results reveal that POT1 C terminus is essential to prevent initiation of genome instability permissive for tumorigenesis.

Ribonuclease P (RNase P) is a universal ribozyme responsible for processing the 5′-leader of pre–transfer RNA (pre-tRNA). Here, we report the 3.5-angstrom cryo–electron microscopy structures of Saccharomyces cerevisiae RNase P alone and in complex with pre-tRNAPhe. The protein components form a hook-shaped architecture that wraps around the RNA and stabilizes RNase P into a “measuring device” with two fixed anchors that recognize the L-shaped pre-tRNA. A universally conserved uridine nucleobase and phosphate backbone in the catalytic center together with the scissile phosphate and the O3′ leaving group of pre-tRNA jointly coordinate two catalytic magnesium ions. Binding of pre-tRNA induces a conformational change in the catalytic center that is required for catalysis. Moreover, simulation analysis suggests a two-metal-ion SN2 reaction pathway of pre-tRNA cleavage. These results not only reveal the architecture of yeast RNase P but also provide a molecular basis of how the 5′-leader of pre-tRNA is processed by eukaryotic RNase P.

The first example of a naturally occurring thiopyranchromenone, preussochromone A (1), and five other new chromone derivatives, preussochromones B-F (2-6), were isolated from solid cultures of an endolichenic fungus, Preussia africana. The structures of 1-6 were established primarily by NMR experiments, and 2 and 4 were further confirmed by X-ray crystallography. The absolute configurations of 1 and 2 were determined by the application of electronic circular dichroism (ECD), whereas those of C-5 in 3, C-6 in 4, and the 6,7-diol in 5 were deduced via the CD data of the in situ formed [Rh₂(OCOCF₃)₄] complex, the modified Mosher method, and Snatzke's method, respectively. Compounds 1 and 3 showed significant cytotoxicity against A549 cells.

Pestaloficiols F-L (1-7), new isoprenylated chromone derivatives including one heterodimer (7), have been isolated from a scale-up fermentation extract of the plant endophytic fungus Pestalotiopsis fici. The structures of these compounds were elucidated primarily by NMR and MS methods. The absolute configurations of 1 and 4 were assigned using the modified Mosher method. Compounds 1-3, 5, and 6 displayed inhibitory effects on HIV-1 replication in C8166 cells, whereas 4-7 showed cytotoxic activity against the human tumor cell lines HeLa and MCF7.

New N-hydroxypyridones, militarinones E (1) and F (2), phenylhydrazones, farylhydrazones A (3) and B (4), a quinazolinone, 2-(4-hydroxybenzyl)quinazolin-4(3H)-one (5), and the known militarinones A (6) and B (7) were isolated from cultures of the Cordyceps-colonizing fungus Isaria farinosa. The structures of 1-5 were elucidated by spectroscopic methods, and 3 was confirmed by X-ray crystallography. The absolute configuration of the C-4' secondary alcohol in 1 was deduced via the circular dichroism data of the in situ formed [Rh(2)(OCOCF(3))(4)] complex. Compounds 1 and 6 showed significant cytotoxicity against A549 cells, whereas 7 was active against Staphylococcus aureus, Streptococcus pneumoniae, and Candida albicans.

Chloropupukeanolides A (1) and B (2), unprecedented spiroketal peroxides, and chloropupukeanone A (3), three highly functionalized metabolites featuring a chlorinated pupukeanane core, were isolated from an endophytic fungus Pestalotiopsis fici, with 1 showing significant anti-HIV-1 and cytotoxic effects.

The new pyrrolidinones, rigidiusculamides A-D (1-4), have been isolated from the crude extract of the ascomycete fungus Albonectria rigidiuscula. The structures of these compounds were elucidated primarily by NMR experiments. The absolute configuration of the 3,4-diol moieties in 1 and 4 was assigned using Snatzke's method. Compounds 1 and 2 showed modest cytotoxicity against the human tumor cell lines HeLa and MCF-7.