Interaction of eukaryotic translation initiation factor 4G with the nuclear cap-binding complex provides a link between nuclear and cytoplasmic functions of the m7 guanosine cap

In eukaryotes the majority of mRNAs have an m7G cap that is added cotranscriptionally and that plays an important role in many aspects of mRNA metabolism. The nuclear cap-binding complex (CBC; consisting of CBP20 and CBP80) mediates the stimulatory functions of the cap in pre-mRNA splicing, 3' end formation, and U snRNA export. As little is known about how nuclear CBC mediates the effects of the cap in higher eukaryotes, we have characterized proteins that interact with CBC in HeLa cell nuclear extracts as potential mediators of its function. Using cross-linking and coimmunoprecipitation, we show that eukaryotic translation initiation factor 4G (eIF4G), in addition to its function in the cytoplasm, is a nuclear CBC-interacting protein. We demonstrate that eIF4G interacts with CBC in vitro and that, in addition to its cytoplasmic localization, there is a significant nuclear pool of eIF4G in mammalian cells in vivo. Immunoprecipitation experiments suggest that, in contrast to the cytoplasmic pool, much of the nuclear eIF4G is not associated with eIF4E (translation cap binding protein of eIF4F) but is associated with CBC. While eIF4G stably associates with spliceosomes in vitro and shows close association with spliceosomal snRNPs and splicing factors in vivo, depletion studies show that it does not participate directly in the splicing reaction. Taken together the data indicate that nuclear eIF4G may be recruited to pre-mRNAs via its interaction with CBC and accompanies the mRNA to the cytoplasm, facilitating the switching of CBC for eIF4F. This may provide a mechanism to couple nuclear and cytoplasmic functions of the mRNA cap structure

Cardiolipin stabilizes respiratory chain supercomplexes

Cardiolipin stabilized supercomplexes of Saccharomyces cerevisiae respiratory chain complexes III and IV (ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase, respectively), but was not essential for their formation in the inner mitochondrial membrane because they were found also in a cardiolipin-deficient strain. Reconstitution with cardiolipin largely restored wild-type stability. The putative interface of complexes III and IV comprises transmembrane helices of cytochromes b and c1 and tightly bound cardiolipin. Subunits Rip1p, Qcr6p, Qcr9p, Qcr10p, Cox8p, Cox12p, and Cox13p and cytochrome c were not essential for the assembly of supercomplexes; and in the absence of Qcr6p, the formation of supercomplexes was even promoted. An additional marked effect of cardiolipin concerns cytochrome c oxidase. We show that a cardiolipin-deficient strain harbored almost inactive resting cytochrome c oxidase in the membrane. Transition to the fully active pulsed state occurred on a minute time scale