Do the utrophin tandem calponin homology domains bind F-actin in a compact or extended conformation?

Tandem calponin-homology (CH) domains play an important role in the actin-binding function of many spectrin superfamily proteins. Crystal structures from several of these proteins have suggested a flexibility between these domains, and the manner in which these domains bind to F-actin has been the subject of some controversy. A recent paper has used electron microscopy and three-dimensional reconstruction to examine the complex of the utrophin tandem CH domain with F-actin. In contrast to our previously published study, a closed conformation of the two calponin-homology domains was suggested in the new work. We show here that the new results can be explained by incomplete binding of utrophin to actin, heterogeneity in the mode of binding, and angular disorder in F-actin. We conclude that helical averaging applied to disordered filaments is responsible for their results, and that approaches designed to separate out homogeneous subsets within such filamentous complexes offer many advantages.     

The utrophin actin-binding domain binds F-actin in two different modes: implications for the spectrin superfamily of proteins

Utrophin, like its homologue dystrophin, forms a link between the actin cytoskeleton and the extracellular matrix. We have used a new method of image analysis to reconstruct actin filaments decorated with the actin-binding domain of utrophin, which contains two calponin homology domains. We find two different modes of binding, with either one or two calponin-homology (CH) domains bound per actin subunit, and these modes are also distinguishable by their very different effects on F-actin rigidity. Both modes involve an extended conformation of the CH domains, as predicted by a previous crystal structure. The separation of these two modes has been largely dependent upon the use of our new approach to reconstruction of helical filaments. When existing information about tropomyosin, myosin, actin-depolymerizing factor, and nebulin is considered, these results suggest that many actin-binding proteins may have multiple binding sites on F-actin. The cell may use the modular CH domains found in the spectrin superfamily of actin-binding proteins to bind actin in manifold ways, allowing for complexity to arise from the interactions of a relatively few simple modules with actin.     

Ankyrin repeat and SOCS box 3 (ASB3) mediates ubiquitination and degradation of tumor necrosis factor receptor II

Ankyrin repeat and SOCS box (ASB) family members have a C-terminal SOCS box and an N-terminal ankyrin-related sequence of variable repeats belonging to the SOCS superfamily. While SH2-domain-bearing SOCS proteins are mainly involved in the negative feedback regulation of the protein tyrosine kinase-STAT pathway in response to a variety of cytokines, the roles of ASB family members remain largely unknown. To investigate ASB functions, we screened for ASB3-interacting factors by using antibody array technology and identified tumor necrosis factor receptor II (TNF-R2) as an ASB3 binding target. ASB3 expression and activities are required for (i) TNF-R2 ubiquitination both in vivo and in vitro, (ii) TNF-R2 proteolysis via the proteasome pathway, and (iii) the inhibition of TNF-R2-mediated Jun N-terminal protein kinase (JNK) activation. While the ankyrin repeats of ASB3 interact with the C-terminal 37 amino acids of TNF-R2, the SOCS box of ASB3 is responsible for recruiting the E3 ubiquitin ligase adaptors Elongins-B/C, leading to TNF-R2 ubiquitination on multiple lysine residues within its C-terminal region. Downregulation of ASB3 expression by a small interfering RNA inhibited TNF-R2 degradation and potentiated TNF-R2-mediated cytotoxicity. The data presented here implicate ASB3 as a negative regulator of TNF-R2-mediated cellular responses to TNF-alpha by direct targeting of TNF-R2 for ubiquitination and proteasome-mediated degradation.     

The bacterial protein SipA polymerizes G-actin and mimics muscle nebulin

SipA is a Salmonella protein delivered into host cells to promote efficient bacterial entry, which is essential for pathogenicity. SipA exerts its function by binding F-actin, resulting in the stabilization of F-actin and the stimulation of the bundling activity of fimbrin. Here we show that under low salt conditions where spontaneous nucleation and polymerization of actin do not occur, SipA induces extensive polymerization. We have used electron microscopy and a method for helical image analysis to visualize the complex of actin with the actin-binding fragment of SipA. The SipA fragment binds to actin as a tubular molecule extending approximately 95 A. The main sites of SipA binding on actin involve sequence insertions that are not present in the bacterial homolog of actin, MreB, suggesting a mechanism for preventing SipA from interacting with bacterial MreB filaments. Remarkably, the pattern of SipA binding, which connects subunits on opposite actin strands and explains the stabilization of F-actin, is similar to that shown for a fragment of the giant muscle protein nebulin. We suggest that SipA is a bacterial structural mimic of muscle nebulin and nebulin-like proteins in non-muscle cells that are involved in the regulation of the actin-based cytoskeleton.     

Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Continuing enrichment of atmospheric CO₂ may change plant community composition, in part by altering the availability of other limiting resources including soil water, nutrients, or light. The combined effects of CO₂ enrichment and altered resource availability on species flowering remain poorly understood. We quantified flowering culm and ramet production and biomass allocation to flowering culms/ramets for 10 years in C₄‐dominated grassland communities on contrasting soils along a CO₂ concentration gradient spanning pre‐industrial to expected mid‐21st century levels (250–500 μl/L). CO₂ enrichment explained up to 77% of the variation in flowering culm count across soils for three of the five species, and was correlated with flowering culm count on at least one soil for four of five species. In contrast, allocation to flowering culms was only weakly correlated with CO₂ enrichment for two species. Flowering culm counts were strongly correlated with species aboveground biomass (AGB; R² = .34–.74), a measure of species abundance. CO₂ enrichment also increased soil moisture and decreased light levels within the canopy but did not affect soil inorganic nitrogen availability. Structural equation models fit across the soils suggested species‐specific controls on flowering in two general forms: (1) CO₂ effects on flowering culm count mediated by canopy light level and relative species AGB (species AGB/total AGB) or by soil moisture effects on flowering culm count; (2) effects of canopy light level or soil inorganic nitrogen on flowering and/or relative species AGB, but with no significant CO₂ effect. Understanding the heterogeneity in species responses to CO₂ enrichment in plant communities across soils in edaphically variable landscapes is critical to predict CO₂ effects on flowering and other plant fitness components, and species potential to adapt to future environmental changes.     

Abnormal estrous cyclicity after disruption of endothelial and inducible nitric oxide synthase in mice.

The roles of nitric oxide (NO) and nitric oxide synthase (NOS) in reproduction were studied by examining the estrous cycle of wild-type (WT) mice, inducible NOS (iNOS)-, and endothelial NOS (eNOS)-knockout mice. We observed an average estrous cycle of 4.8 +/- 0.2 days in WT mice. While we observed no significant influence of iNOS deficiency on cycle length, eNOS-knockout females showed a significantly longer estrous cycle (6.6 +/- 0.6 days; p < 0.03) than WT females, due to an extension of diestrus (p < 0.03). There was no influence of iNOS deficiency on ovulation rate compared with that in WT females; however, eNOS-knockout mice showed a significant reduction (p < 0.05) in ovulatory efficiency relative to WT or iNOS-knockout females. In contrast to WT females, in which the highest level of estradiol (E2) was observed at 1500 h of proestrus, iNOS-knockout females reached a peak of E2 at 1830 h of proestrus. In eNOS-knockout females, the peak of E2 occurred at 1830 h, as in iNOS-knockout mice; however, E2 levels were 5-fold and 3-fold higher (p < 0.05) than levels observed in WT and iNOS-knockout females, respectively. There was no effect of genotype on the plasma LH concentrations at proestrus. On the first day of diestrus, eNOS-knockout females showed significantly higher plasma E2 and progesterone levels (p < 0.05) relative to WT and iNOS-knockout females. The dysfunction in cyclicity, ovulation rate, ovarian morphology, and steroidogenesis in eNOS-knockout female mice strongly supports the concept that eNOS/NO plays critical roles in ovulation and follicular development.     

Subunit mass fingerprinting of mitochondrial complex I

We have employed laser induced liquid bead ion desorption (LILBID) mass spectrometry to determine the total mass and to study the subunit composition of respiratory chain complex I from Yarrowia lipolytica. Using 5-10 pmol of purified complex I, we could assign all 40 known subunits of this membrane bound multiprotein complex to peaks in LILBID subunit fingerprint spectra by comparing predicted protein masses to observed ion masses. Notably, even the highly hydrophobic subunits encoded by the mitochondrial genome were easily detectable. Moreover, the LILBID approach allowed us to spot and correct several errors in the genome-derived protein sequences of complex I subunits. Typically, the masses of the individual subunits as determined by LILBID mass spectrometry were within 100 Da of the predicted values. For the first time, we demonstrate that LILBID spectrometry can be successfully applied to a complex I band eluted from a blue-native polyacrylamide gel, making small amounts of large multiprotein complexes accessible for subunit mass fingerprint analysis even if they are membrane bound. Thus, the LILBID subunit mass fingerprint method will be of great value for efficient proteomic analysis of complex I and its assembly intermediates, as well as of other water soluble and membrane bound multiprotein complexes.