Crystallographic and functional studies of a plant temperature-induced lipocalin.

Arabidopsis thaliana temperature-induced lipocalin (AtTIL) is a prototypical member of plant lipocalins and participates in a variety of cellular processes, particularly stress responses. Bioinformatical and physiological studies have proposed its promiscuous ligand-binding ability, but the molecular basis is yet unclear. Here, we report the 1.9-Šcrystal structure of AtTIL in complex with heme. Spectrophotometric absorbance titration with heme yields a dissociation constant of ∼2 micromolar, indicating the relatively weak interaction between AtTIL and heme, which is confirmed by the AtTIL-heme structure. Although binding to retinal or biliverdin is not detected, such possibility can not be precluded as suggested by comparison with other lipocalin structures. These results show that AtTIL is a structural and functional homolog of the bacterial lipocalin Blc.

Crystal structures of cyanobacterial light-dependent protochlorophyllide oxidoreductase.

The reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is the penultimate step of chlorophyll biosynthesis. In oxygenic photosynthetic bacteria, algae, and plants, this reaction can be catalyzed by the light-dependent Pchlide oxidoreductase (LPOR), a member of the short-chain dehydrogenase superfamily sharing a conserved Rossmann fold for NAD(P)H binding and the catalytic activity. Whereas modeling and simulation approaches have been used to study the catalytic mechanism of this light-driven reaction, key details of the LPOR structure remain unclear. We determined the crystal structures of LPOR from two cyanobacteria, Synechocystis sp. PCC 6803 and Thermosynechococcus elongatus Structural analysis defines the LPOR core fold, outlines the LPOR-NADPH interaction network, identifies the residues forming the substrate cavity and the proton-relay path, and reveals the role of the LPOR-specific loop. These findings provide a basis for understanding the structure-function relationships of the light-driven Pchlide reduction.

Interleukin-21 up-regulates interleukin-21R expression and interferon gamma production by CD8+ cells in SHIV-infected macaques.

Interleukin-21 (IL-21) is produced primarily by CD4+ T cells and regulates immunity against human/simian immunodeficiency virus (HIV/SIV) infection. Activated CD8+ cells and their secreted interferon-gamma (IFN-γ) are crucial for the control of acute HIV/SIV infection. However, whether IL-21 can regulate IFN-γ production by CD8+ cells remains controversial. Rhesus macaques (RMs, n = 8) were infected with SHIV and the levels of plasma IL-21, IFN-γ and the frequency of peripheral blood activated T cells were measured longitudinally. Following infection with SHIV, the levels of plasma IL-21 and IFN-γ increased, peaked at 17 days postinfection and declined later. Furthermore, IL-21 induced IL-21 receptor (IL-21R) and IFN-γ, perforin, but not granmyze B, expression in CD8+ cells from four selected SHIV-infected RMs. The regulatory effect of IL-21 on CD8+ cell function appeared to be associated with increased levels of STAT3, but not STAT5, phosphorylation in CD8+ cells from SHIV-infected RMs. In parallel, treatment with soluble IL-21R/Fc, an inhibitor of IL-21-induced activation of JAK1/3 and STAT3, abrogated IL-21-induced STAT3 activation and IFN-γ production in CD8+ cells from SHIV-infected RMs in vitro. Our data indicated that IL-21 was a positive regulator of IFN-γ-secreting CD8+ cells and increased the STAT3 phosphorylation, regulating T-cell immunity against acute SHIV infection in RMs. Our findings may provide a new basis for the development of immunotherapies for the control of SHIV/HIV infection.

Circulating IL-21 levels increase during early simian-human immunodeficiency virus infection in macaques.

The cytokine interleukin-21 (IL-21) regulates viral pathogenesis in individuals infected with human and simian immunodeficiency viruses. However, because the time of initial infection with HIV in humans is rarely known, the dynamics of IL-21 production during the first weeks have not been adequately explored. In the present study, we used rhesus macaques to model the first stages of infection. Twenty-two rhesus macaques were infected rectally with simian-human immunodeficiency virus (SHIV)-1157ipd3N4, and for 12 weeks, replication of the virus, the numbers of CD4+ and CD8+ T cells, and the levels of plasma IL-21 were monitored. Our study demonstrated that plasma levels of IL-21 increased during the early phase of SHIV infection when compared with the values observed before inoculation. We conclude that IL-21 has a likely role in the immunopathogenesis of HIV/SIV/SHIV.