RNA helicases are the largest group of enzymes in eukaryotic RNA metabolism. The DEXD͞H-box putative RNA helicases form the helicase superfamily II, whose members are defined by seven highly conserved amino acid motifs, making specific targeting of selected members a challenging pharmacological problem. The translation initiation factor eIF4A is the prototypical DEAD-box RNA helicase that works in conjunction with eIF4B and eIF4H and as a subunit of eIF4F to prepare the mRNA template for ribosome binding, possibly by unwinding the secondary structure proximal to the 5 m 7 GpppN cap structure. We report the identification and characterization of a small molecule inhibitor of eukaryotic translation initiation that acts in an unusual manner by stimulating eIF4A-associated activities. Our results suggest that proper control of eIF4A helicase activity is necessary for efficient ribosome binding and demonstrate the feasibility of selectively targeting DEADbox RNA helicases with small molecules.chemical biology ͉ DEAD-box helicase ͉ pateamine T he ribosome recruitment step of translation initiation is ratelimiting and an important regulatory point whereby cellular environmental cues (e.g., amino acid starvation, mitogenic stimulation, and hypoxia) are linked to the process of translation (1). Two distinct pathways exist for recruitment of the ribosome to the mRNA template. One mechanism is cap-dependent and is facilitated by the presence of the 5Ј cap structure (m 7 GpppN, where N is any nucleotide) on the mRNA. It is catalyzed by the eIF4 class of translation initiation factors and involves the recruitment of ribosomes near the 5Ј end of the mRNA template (1). The second mode involves ribosome recruitment in a cap-independent fashion to an internal ribosome entry site (IRES). Initiation factor requirement for internal ribosome binding varies among IRESes, with some not requiring any factors (2).Preparation of the mRNA template for cap-dependent ribosome recruitment is achieved by eIF4F, eIF4A, eIF4B, eIF4H, and ATP hydrolysis (1). The eIF4F complex is comprised of three subunits: (i) eIF4E, which binds the mRNA cap structure in an ATP-independent fashion; (ii) eIF4A, an RNA helicase that exhibits RNA-dependent ATPase activity and ATPstimulated RNA binding activity (3); and (iii) eIF4G, a modular scaffold that mediates mRNA binding of the 43S preinitiation complex through interactions with eIF3. eIF4B, and eIF4H cooperate with eIF4A to make its helicase activity more processive (4, 5). eIF4A exists as a free form (referred to herein as eIF4A f ) and as a subunit of eIF4F (eIF4A c ) and is thought to cycle through the eIF4F complex during initiation (6-8). When localized in the eIF4F complex, eIF4A c is Ϸ20-fold more efficient as an RNA helicase than when found alone (4, 9), leading to the proposal that eIF4A c is the functional helicase for translation initiation (10). The helicase activity of eIF4F (via eIF4A c ) is thought to unwind local secondary structure in the 5Ј UTR of mRNAs to facilitate cap-dependent ribosome...

No abstract

As part of our ongoing search for novel and bioactive compounds from New Zealand marine organisms, we investigated the extracts of the sponge Darwinella oxeata. NMR-guided fractionation led to the isolation of nine new nitrogenous spongian diterpenes, oxeatamide A (1), iso-oxeatamide A (2), oxeatamides B-G (3-8), and oxeatamide A 23-methyl ester (9), as well as two known compounds, membranolides C and D (10, 11).

No abstract

Examination of the New Zealand sponge Raspailia agminata resulted in the isolation of five members of a novel family of glycolipids, agminosides A-E (1-5). These large and complex molecules contain up to six partially acetylated glucose residues. The chromatographic separation of these compounds was a challenge due to the similarity of the congeners and their lack of a chromophore. MS-guided isolation followed by extensive NMR analysis and chemical derivatization eventually led to the purification and identification of 1-5.