A novel membrane sensor controls the localization and ArfGEF activity of bacterial RalF.

The intracellular bacterial pathogen Legionella pneumophila (Lp) evades destruction in macrophages by camouflaging in a specialized organelle, the Legionella-containing vacuole (LCV), where it replicates. The LCV maturates by incorporating ER vesicles, which are diverted by effectors that Lp injects to take control of host cell membrane transport processes. One of these effectors, RalF, recruits the trafficking small GTPase Arf1 to the LCV. LpRalF has a Sec7 domain related to host ArfGEFs, followed by a capping domain that intimately associates with the Sec7 domain to inhibit GEF activity. How RalF is activated to function as a LCV-specific ArfGEF is unknown. We combined the reconstitution of Arf activation on artificial membranes with cellular expression and Lp infection assays, to analyze how auto-inhibition is relieved for LpRalF to function in vivo. We find that membranes activate LpRalF by about 1000 fold, and identify the membrane-binding region as the region that inhibits the Sec7 active site. It is enriched in aromatic and positively charged residues, which establish a membrane sensor to control the GEF activity in accordance with specific lipid environments. A similar mechanism of activation is found in RalF from Rickettsia prowazekii (Rp), with a different aromatic/charged residues ratio that results in divergent membrane preferences. The membrane sensor is the primary determinant of the localization of LpRalF on the LCV, and drives the timing of Arf activation during infection. Finally, we identify a conserved motif in the capping domain, remote from the membrane sensor, which is critical for RalF activity presumably by organizing its active conformation. These data demonstrate that RalF proteins are regulated by a membrane sensor that functions as a binary switch to derepress ArfGEF activity when RalF encounters a favorable lipid environment, thus establishing a regulatory paradigm to ensure that Arf GTPases are efficiently activated at specific membrane locations.

A corkscrew model for dynamin constriction.

Numerous vesiculation processes throughout the eukaryotic cell are dependent on the protein dynamin, a large GTPase that constricts lipid bilayers. We have combined X-ray crystallography and cryo-electron microscopy (cryo-EM) data to generate a coherent model of dynamin-mediated membrane constriction. GTPase and pleckstrin homology domains of dynamin were fit to cryo-EM structures of human dynamin helices bound to lipid in nonconstricted and constricted states. Proteolysis and immunogold labeling experiments confirm the topology of dynamin domains predicted from the helical arrays. Based on the fitting, an observed twisting motion of the GTPase, middle, and GTPase effector domains coincides with conformational changes determined by cryo-EM. We propose a corkscrew model for dynamin constriction based on these motions and predict regions of sequence important for dynamin function as potential targets for future mutagenic and structural studies.

Conformational changes of Escherichia coli sigma54-RNA-polymerase upon closed-promoter complex formation.

RNA polymerase from the mesophile Escherichia coli exists in two forms, the core enzyme and the holoenzyme. Using cryo-electron microscopy and single-particle analysis, we have obtained the structure of the complete RNA polymerase from E.coli containing the sigma54 factor within the closed-promoter complex. Comparisons with earlier reconstructions of the core enzyme and the sigma54 holoenzyme reveal the behaviour of this major variant RNA polymerase in defined functional states. The binding of DNA leads to significant conformational changes in the enzyme's catalytic subunits, apparently a necessity for the initiation of enhancer-dependent promoter-specific transcription.

Secondary structure and DNA binding by the C-terminal domain of the transcriptional activator NifA from Klebsiella pneumoniae.

The NifA protein of Klebsiella pneumoniae is required for transcriptional activation of all nitrogen fixation (nif) operons except the regulatory nifLA genes. At these operons, NifA binds to an upstream activator sequence (UAS), with the consensus TGT-N(10)-ACA, via a C-terminal DNA-binding domain (CTD). Binding of the activator to this upstream enhancer-like sequence allows NifA to interact with RNA polymerase containing the alternative sigma factor, sigma(54). The isolated NifA CTD is monomeric and binds specifically to DNA in vitro as shown by DNase I footprinting. Heteronuclear 3D NMR experiments have been used to assign the signals from the protein backbone. Three alpha-helices have been identified, based on secondary chemical shifts and medium range Halpha(i)-NH(i)( + 1), and NH(i)-NH(i)( + 1) NOEs. On addition of DNA containing a half-site UAS, several changes are observed in the NMR spectra, allowing the identification of residues that are most likely to interact with DNA. These occur in the final two helices of the protein, directly confirming that DNA binding is mediated by a helix-turn-helix motif.

Impact of Haemophilus influenzae Type B Conjugate Vaccines on Nasopharyngeal Carriage in HIV-infected Children and Their Parents From West Bengal, India.

BACKGROUND: In addition to reducing Haemophilus influenzae type b (Hib) disease in vaccinated individuals, the Hib conjugate vaccine (HibCV) has indirect effects; it reduces Hib disease in unvaccinated individuals by decreasing carriage. Human immunodeficiency virus (HIV)-infected children are at increased risk for Hib disease and live in families where multiple members may have HIV. The aim of this study is to look at the impact of 2 doses of the HibCV on nasopharyngeal carriage of Hib in HIV-infected Indian children (2-15 years) and the indirect impact on carriage in their parents. METHODS: This prospective cohort study was conducted in HIV-infected and HIV-uninfected families. Nasopharyngeal swabs were collected from children and parents before and after vaccination. HIV-infected children 2-15 years of age got two doses of HibCV and were followed up for 20 months. Uninfected children 2-5 years of age got 1 dose of HibCV as catch-up. RESULTS: 123 HIV-infected and 44 HIV-uninfected children participated. Baseline colonization in HIV-infected children was 13.8% and dropped to 1.8% (P = 0.002) at 20 months. Baseline carriage in HIV-uninfected children was 4.5% and dropped to 2.3% after vaccination (P = 0.3). HIV-infected parents had 12.3 times increased risk of Hib carriage if their child was colonized (P = 0.04) and had 9.3 times increased risk if their child had persistent colonization postvaccine (P = 0.05). No parent of HIV-uninfected children had Hib colonization at any point. Pneumococcal colonization was associated with increased Hib colonization. CONCLUSION: Making the HibCV available to HIV-infected children could interrupt Hib carriage in high-risk families.

A dynamin mutant defines a superconstricted prefission state.

Dynamin is a 100 kDa GTPase that organizes into helical assemblies at the base of nascent clathrin-coated vesicles. Formation of these oligomers stimulates the intrinsic GTPase activity of dynamin, which is necessary for efficient membrane fission during endocytosis. Recent evidence suggests that the transition state of dynamin's GTP hydrolysis reaction serves as a key determinant of productive fission. Here, we present the structure of a transition-state-defective dynamin mutant K44A trapped in a prefission state at 12.5 Å resolution. This structure constricts to 3.7 nm, reaching the theoretical limit required for spontaneous membrane fission. Computational docking indicates that the ground-state conformation of the dynamin polymer is sufficient to achieve this superconstricted prefission state and reveals how a two-start helical symmetry promotes the most efficient packing of dynamin tetramers around the membrane neck. These data suggest a model for the assembly and regulation of the minimal dynamin fission machine.

Acetylation and MAPK phosphorylation cooperate to regulate the degradation of active GATA-1.

Regulation of transcription requires mechanisms to both activate and terminate transcription factor activity. GATA-1 is a key haemopoietic transcription factor whose activity is increased by acetylation. We show here that acetylated GATA-1 is targeted for degradation via the ubiquitin/proteasome pathway. Acetylation positively signals ubiquitination, suggesting that activation by acetylation simultaneously marks GATA-1 for degradation. Promoter-specific MAPK phosphorylation then cooperates with acetylation to execute protein loss. The requirement for both modifications is novel and suggests a way by which degradation of the active protein can be specifically regulated in response to external phosphorylation-mediated signalling. As many transcription factors are activated by acetylation, we suggest that this might be a general mechanism to control transcription factor activity.