Eukaryotic translation initiation involves recognition of the 5' end of cellular mRNA by the cap-binding complex known as eukaryotic initiation factor 4F (eIF4F). Initiation is a key point of regulation in gene expression in response to mechanisms mediated by signal transduction pathways. We have investigated the molecular interactions underlying inhibition of human eIF4E function by regulatable repressors called 4E-binding proteins (4E-BPs). Two essential components of eIF4F are the cap-binding protein eIF4E, and eIF4G, a multi-functional protein that binds both eIF4E and other essential eIFs. We show that the 4E-BPs 1 and 2 block the interaction between eIF4G and eIF4E by competing for binding to a dorsal site on eIF4E. Remarkably, binding of the 4E-BPs at this dorsal site enhances cap-binding via the ventral cap-binding slot, thus trapping eIF4E in inactive complexes with high affinity for capped mRNA. The binding contacts and affinities for the interactions between 4E-BP1/2 and eIF4E are distinct (estimated K(d) values of 10(-8) and 3x10(-9) for 4E-BP1 and 2, respectively), and the differences in these properties are determined by three amino acids within an otherwise conserved motif. These data provide a quantitative framework for a new molecular model of translational regulation.

Translation initiation is a key point of regulation in eukaryotic gene expression. 4E-binding proteins (4E-BPs) inhibit initiation by blocking the association of eIF4E with eIF4G, two integral components of the mRNA cap-binding complex. Phosphorylation of 4E-BP1 reduces its ability to bind to eIF4E and thereby to compete with eIF4G. A novel combination of biophysical and biochemical tools was used to measure the impact of phosphorylation and acidic side chain substitution at each potentially modulatory site in 4E-BP1. For each individual site, we have analyzed the effects of modification on eIF4E binding using affinity chromatography and surface plasmon resonance analysis, and on the regulatory function of the 4E-BP1 protein using a yeast in vivo model system and a mammalian in vitro translation assay. We find that modifications at the two sites immediately flanking the eIF4E-binding domain, Thr 46 and Ser 65 , consistently have the most significant effects, and that phosphorylation of Ser 65 causes the greatest reduction in binding affinity. These results establish a quantitative framework that should contribute to understanding of the molecular interactions underlying 4E-BP1-mediated translational regulation.

The eukaryotic mRNA 3 poly(A) tail and the 5 cap cooperate to synergistically enhance translation. This interaction is mediated by the cap-binding protein eIF4E, the poly(A) binding protein (PABP), and eIF4G, a scaffolding protein that bridges between eIF4E and PABP to bring about the circularization of the mRNA. The translational repressor, Paip2 (PABP-interacting protein 2), inhibits translation by promoting the dissociation of PABP from poly(A). Here we report on the existence of an alternative mechanism by which Paip2 inhibits translation by competing with eIF4G for binding to PABP. We demonstrate that Paip2 can abrogate the translational activity of PABP, which is tethered to the 3 end of the mRNA. Thus, Paip2 can inhibit translation by a previously unrecognized mechanism, which is independent of its ability to disrupt PABP-poly(A) interaction.circularization ͉ translation initiation T ranslational control is an important means by which cells govern gene expression. Initiation, the rate-limiting step of translation, is often the target of translational control (1). This control involves in many circumstances the cap structure at the mRNA 5Ј end and the poly(A) tail at the mRNA 3Ј end. Although both mRNA terminal structures stimulate translation on their own, their combined translational enhancement is synergistic. Such a mechanism was demonstrated in yeast, plant, and mammalian systems (2, 3) and plays a key role during development of Xenopus, Drosophila, and mouse (4). Translational synergy was also recapitulated in vitro (5-9). Hence, the cap-poly(A) tail synergy represents a common paradigm for translational control.The mechanism by which the mRNA 3Ј poly(A) tail synergizes with the 5Ј cap to stimulate translation was first documented in yeast, where PABP was shown to interact directly with the eIF4G subunit of the cap-binding complex eIF4F (see below) (10, 11). In yeast, PABP is an essential protein because deletion of the PABP1 gene is lethal (12). However, because deletion of the eIF4G binding site in PABP causes only a mild effect on yeast cell growth (13), it is not clear that PABP-eIF4G interaction is the only mechanism by which PABP regulates translation and cell growth in yeast. In addition, Searfoss et al. (14) demonstrated that yeast Ski proteins inhibit the activity of eIF5 and eIF4B in 60S ribosomal subunit joining and that this inhibition is reversed by PABP. These results implicate an important role for PABP in 60S ribosomal subunit joining. In contrast to yeast, the expression of an eIF4G mutant that does not interact with PABP in Xenopus oocytes repressed translation of polyadenylated mRNAs and inhibited progesteroneinduced oocyte maturation (15).PABP contains four RNA-recognition motifs (RRMs) and a proline-rich C-terminal region, which is N-terminal to a highly evolutionarily conserved sequence termed PABC (see below) (16,17). eIF4F is composed of the cap-binding subunit eIF4E, an RNA-dependent ATPase͞ATP-dependent RNA helicase, eIF4A, and eIF4G (reviewed in ref. 18). The latter serves...

Nutritional immunity describes mechanisms for withholding essential transition metals as well as directing the toxicity of these metals against infectious agents. Zinc is one of these transition elements that are essential for both humans and microbial pathogens. At the same time, Zn can be toxic both for man and microbes if its concentration is higher than the tolerance limit. Therefore a "delicate" balance of Zn must be maintained to keep the immune cells surveilling while making the level of Zn either to starve or to intoxicate the pathogens. On the other hand, the invading pathogens will exploit the host Zn pool for its survival and replication. Apparently, different sets of protein in human and bacteria are involved to maintain their Zn need. Metallothionein (MT)-a group of low molecular weight proteins, is well known for its Zn-binding ability and is expected to play an important role in that Zn balance at the time of active infection. However, the differences in structural, functional, and molecular control of biosynthesis between human and bacterial MT might play an important role to determine the proper use of Zn and the winning side. The current review explains the possible involvement of human and bacterial MT at the time of infection to control and exploit Zn for their need.

Growth and productivity of rice are negatively affected by soil salinity. However, some salt-tolerant rhizosphere-inhabiting bacteria can improve salt resistance of plants, thereby augmenting plant growth and production. Here, we isolated a total of 53 plant-growth-promoting rhizobacteria (PGPR) from saline and non-saline areas in Bangladesh where electrical conductivity was measured as >7.45 and <1.80 dS/m, respectively. Bacteria isolated from saline areas were able to grow in a salt concentration of up to 2.60 mol/L, contrary to the isolates collected from non-saline areas that did not survive beyond 854 mmol/L. Among the salt-tolerant isolates, Bacillus aryabhattai, Achromobacter denitrificans, and Ochrobactrum intermedium, identified by comparing respective sequences of 16S rRNA using the NCBI GenBank, exhibited a higher amount of atmospheric nitrogen fixation, phosphate solubilization, and indoleacetic acid production at 200 mmol/L salt stress. Salt-tolerant isolates exhibited greater resistance to heavy metals and antibiotics, which could be due to the production of an exopolysaccharide layer outside the cell surface. Oryza sativa L. fertilized with B. aryabhattai MS3 and grown under 200 mmol/L salt stress was found to be favoured by enhanced expression of a set of at least four salt-responsive plant genes: BZ8, SOS1, GIG, and NHX1. Fertilization of rice with osmoprotectant-producing PGPR, therefore, could be a climate-change-preparedness strategy for coastal agriculture.