Muhammad Manjurul Karim

Repressor binding to a dorsal regulatory site traps human eIF4E in a high cap‐affinity state

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 Kd values of 10−8 and 3×10−9 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.

A Quantitative Molecular Model for Modulation of Mammalian Translation by the eIF4E-binding Protein 1

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, Thr46 and Ser65, consistently have the most significant effects, and that phosphorylation of Ser65 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.

Clinical presentation and molecular characterization of group B rotaviruses in diarrhoea patients in Bangladesh

A total of 1106 stool samples collected from diarrhoea patients admitted to Dhaka hospital of the International Centre for Diarrhoeal Disease Research, Bangladesh, during January-December 2008 were analysed for the presence of rotavirus-specific RNA by PAGE. The group B-specific RNA migration pattern was detected in 26 patients (2.4%) and group A-specific pattern in 259 patients (23.4%). Clinical data from group A and group B rotavirus-infected patients indicated that episodes did not differ much in the prevalence of diarrhoea, number of stools, outcome or differences in gender. However, abdominal pain was more common in group B rotavirus infections (36 vs 15%, P=0.02) and the virus was responsible for more severe dehydration compared with group A-infected patients (12 vs 3%, P=0.04). Sequence analyses of VP4, VP7 and NSP2 indicated that an Indian-Bangladeshi lineage of the virus, which is different from both the prototype (Chinese) lineage and from the animal group B rotaviruses, has been circulating in Bangladesh. Continuous monitoring of group B rotaviruses both in hospitals and in the community will be helpful to determine the true burden of group B rotaviruses.

Metallothionein: a Potential Link in the Regulation of Zinc in Nutritional Immunity

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.

Prediction of the post-translational modification sites on dengue virus E protein and deciphering their role in pathogenesis

Dengue virus, a member of the flavivirus family, is a mosquito-borne viral pathogen for which any specific treatment or control of infection by vaccination is yet to be conclusive. The envelope glycoprotein, E, mediates viral entry by membrane fusion. Elucidation of post-translational modification sites in E protein followed by sequence alignment produced stretches of residues which are conserved in most of the members of flaviviruses. Presence of protein kinase A (PKA) and protein kinase G (PKG) phosphorylation sites predicts that E protein may activate PKA and PKG through phosphorylation which is responsible for inhibition of platelet activation, and thereby causing thrombocytopenia. Here, we attempt to decipher the novel role of Dengue virus E protein in pathogenesis.