The Essential Vertebrate ABCE1 Protein Interacts with Eukaryotic Initiation Factors

Abstract

The ABCE1 gene is a member of the ATP-binding cassette (ABC) multigene family and is composed of two nucleotide binding domains and an N-terminal Fe-S binding domain. The ABCE1 gene encodes a protein originally identified for its inhibition of ribonuclease L, a nuclease induced by interferon in mammalian cells. The protein is also required for the assembly of the HIV and SIV gag polypeptides. However, ABCE1 is one of the most highly conserved proteins and is found in one or two copies in all characterized eukaryotes and archaea. Yeast ABCE1/RLI1 is essential to cell division and interacts with translation initiation factors in the assembly of the pre-initiation complex. We show here that the human ABCE1 protein is essential for in vitro and in vivo translation of mRNA and that it binds to eIF2␣ and eIF5. Inhibition of the Xenopus ABCE1 arrests growth at the gastrula stage of development, consistent with a block in translation. The human ABCE1 gene contains 16 introns, and the extremely high degree of amino acid identity allows the evolution of its introns to be examined throughout eukaryotes. The demonstration that ABCE1 plays a role in vertebrate translation initiation extends the known functions of this highly conserved protein. Translation is a highly regulated process important to development and pathologies such as cancer, making ABCE1 a potential target for therapeutics. The evolutionary analysis supports a model in which an ancestral eukaryote had large number of introns and that many of these introns were lost in non-vertebrate lineages. The induction of ribobuclease L (RNase L)2 represents an important viral defense mechanism of mammalian cells against RNA viruses (1, 2). RNase L is present in the cell in an inactive form and can be activated by interferon. Interferon causes the activation of oligoadenylate synthases producing 2Ј-5Ј-oligoadenylate. Bisbal et al. (3) described the isolation of a 68-kDa protein that binds to and inhibits RNase L and cloning of the gene. Although originally termed RNase L inhibitor (RLI) the gene is part of the ABC multigene family and its gene symbol is ABCE1. ABCE1 is induced during infection of cells with HIV-1 and an antisense construct directed against ABCE1 resulted in a reduction of viral load (4). In cell-free extracts HIV-1 gag protein can assemble into viral capsids (5). This process is ATP-dependent and was shown to require a 68-kDa protein (HP68) identified as ABCE1/RLI. This same protein also functions in the assembly of HIV-2 and SIVmac (6).Most of the ABC family genes encode large transport proteins that contain 6 -17 transmembrane domains (7). ABCE1 is one of four human ABC genes that contain only nucleotide binding domains and are therefore not likely to be transporters. Of the 48 human ABC proteins, ABCE1 is the most conserved with a single copy of the gene in every characterized eukaryote, except for Arabidopsis, which has two ABCE1-like genes. In addition, there is an ABCE1-related gene in all characterized archae but not in prokaryotes, demonstrating t...

The biological regulation of ABCE1

“…Therefore, ABCE1 protein is supposed to be the first role for eukarya development and in assembly of initiation complex during the translation process. The protein has adapted interaction with RNase L and Gag protein as a secondary function (7). In addition, ABCE1 protein has been suggested new important roles in translation termination and ribosome recycling in eukaryotes.…”

“…The protein was then demonstrated as a host factor essential for assembly of primate lentivirus capsids, especially for HIV-1 (5,6). Recent evidence indicated new important roles of ABCE1 protein in translation initiation, elongation, termination, and ribosome recycling in eukaryotes (7)(8)(9).…”

ABCE1 is essential for S phase progression in human cells

“…ABCE1 has been shown to associate with 40S ribosomal subunits and initiation factors eIF2, eIF3, and eIF5, suggesting its role in pre-initiation complex assembly. [15][16][17][18] Consistently, ABCE1 depletion results in reduced polysome content and global translation in yeast, mammalian and Drosophila cells. 10,14,[16][17][18] ABCE1 is also involved in translation termination through interaction with release factors eRF1 and eRF3, occupying the GTPase center.…”

Rustless translation

“…Studies in flies then showed that ABCE1 (known as Pixie in Drosophila) binds the 40S ribosome in an ATPdependent manner and depletion leads to an increase in in empty 80S ribosomes (Andersen and Leevers , 2007 ). ABCE1 interacts with the translation initiation factor eIF3 through the Hcr1 component in yeast (Dong et al , 2004 ), and with eIF2 and eIF5 in vertebrates (Chen et al , 2006 ). Hcr1 or eIF3 have additional functions, including also the coordination between termination and initiation, ribosomal subunit dissociation and rRNA maturation (Valasek et al , 2001 ), which made the functional interpretation of these interactions difficult.…”

“…First insights into its core function emerged, when the Baillie, Dean, Hinnebusch, Leevers and Marygold labs found ABCE1 (Rli1p) to be involved in the assembly of the translation pre-initiation machinery, to interact with the translation initiation machinery and polysomes in yeast and to be important for translation and in worms, flies and vertebrates (Dong et al , 2004 ;Zhao et al , 2004 ;Coelho et al , 2005 ;Chen et al , 2006 ). At the same time, the Lill and Hurt labs found a functional link between the iron-sulfur clusters of ABCE1 and ribosome biogenesis (Kispal et al , 2005 ;Yarunin et al , 2005 ) while we could crystallize and determine the crystal structure of the twin ABC cassette of archaeal ABCE1 bound to ADP (Karcher et al , 2005 ) ( Figure 2).…”