Insights into the architecture of the eIF2Bα/ß/δ regulatory subcomplex.

Eukaryotic translation initiation factor 2B (eIF2B), the guanine nucleotide exchange factor for the G-protein eIF2, is one of the main targets for the regulation of protein synthesis. The eIF2B activity is inhibited in response to a wide range of stress factors and diseases, including viral infections, hypoxia, nutrient starvation, and heme deficiency, collectively known as the integrated stress response. eIF2B has five subunits (α-ε). The α, ß, and δ subunits are homologous to each other and form the eIF2B regulatory subcomplex, which is believed to be a trimer consisting of monomeric α, ß, and δ subunits. Here we use a combination of biophysical methods, site-directed mutagenesis, and bioinformatics to show that the human eIF2Bα subunit is in fact a homodimer, at odds with the current trimeric model for the eIF2Bα/ß/δ regulatory complex. eIF2Bα dimerizes using the same interface that is found in the homodimeric archaeal eIF2Bα/ß/δ homolog aIF2B and related metabolic enzymes. We also present evidence that the eIF2Bß/δ binding interface is similar to that in the eIF2Bα2 homodimer. Mutations at the predicted eIF2Bß/δ dimer interface cause genetic neurological disorders in humans. We propose that the eIF2B regulatory subcomplex is an α2ß2δ2 hexamer, composed of one α2 homodimer and two ßδ heterodimers. Our results offer novel insights into the architecture of eIF2B and its interactions with the G-protein eIF2.

The role of complement in membranous nephropathy.

Membranous nephropathy (MN) describes a histopathologic pattern of injury marked by glomerular subepithelial immune deposits and collectively represents one of the most common causes of adult nephrotic syndrome. Studies in Heymann nephritis, an experimental model of MN, have established a paradigm in which these deposits locally activate complement to cause podocyte injury, culminating in cytoskeletal reorganization, loss of slit diaphragms, and proteinuria. There is much circumstantial evidence for a prominent role of complement in human MN because C3 and C5b-9 are found consistently within immune deposits. Secondary MN often shows the additional presence of C1q, implicating the classic pathway of complement activation. Primary MN, however, is IgG4-predominant and IgG4 is considered incapable of binding C1q and activating the complement pathway. Recent studies have identified the M-type phospholipase A2 receptor (PLA2R) as the major target antigen in primary MN. Early evidence hints that IgG4 anti-PLA2R autoantibodies can bind mannan-binding lectin and activate the lectin complement pathway. The identification of anti-PLA2R antibodies as likely participants in the pathogenesis of disease will allow focused investigation into the role of complement in MN. Definitive therapy for MN is immunosuppression, although future therapeutic agents that specifically target complement activation may represent an effective temporizing measure to forestall further glomerular injury.