The highly diverged trypanosomal MICOS complex is organized in a nonessential integral membrane and an essential peripheral module

The mitochondrial contact site and cristae organization system (MICOS) mediates the formation of cristae, invaginations in the mitochondrial inner membrane. The highly diverged MICOS complex of the parasitic protist Trypanosoma brucei consists of nine subunits. Except for two Mic10‐like and a Mic60‐like protein, all subunits are specific for kinetoplastids. Here, we determined on a proteome‐wide scale how ablation of individual MICOS subunits affects the levels of the other subunits. The results reveal co‐regulation of TbMic10‐1, TbMic10‐2, TbMic16 and TbMic60, suggesting that these nonessential, integral inner membrane proteins form an interdependent network. Moreover, the ablation of TbMic34 and TbMic32 reveals another network consisting of the essential, intermembrane space‐localized TbMic20, TbMic32, TbMic34 and TbMic40, all of which are peripherally associated with the inner membrane. The downregulation of TbMic20, TbMic32 and TbMic34 also interferes with mitochondrial protein import and reduces the size of the TbMic10‐containing complexes. Thus, the diverged MICOS of trypanosomes contains two subcomplexes: a nonessential membrane‐integrated one, organized around the conserved Mic10 and Mic60, that mediates cristae formation, and an essential membrane‐peripheral one consisting of four kinetoplastid‐specific subunits, that is required for import of intermembrane space proteins.     

Reduced X-linked diversity in derived populations of house mice

Contrasting patterns of X-linked vs. autosomal diversity may be indicative of the mode of selection operating in natural populations. A number of observations have shown reduced X-linked (or Z-linked) diversity relative to autosomal diversity in various organisms, suggesting a large impact of genetic hitchhiking. However, the relative contribution of other forces such as population bottlenecks, variation in reproductive success of the two sexes, and differential introgression remains unclear. Here, we survey 13 loci, 6 X-linked and 7 autosomal, in natural populations of the house mouse (Mus musculus) subspecies complex. We studied seven populations of three different subspecies, the eastern house mouse M. musculus castaneus, the central house mouse M. m. musculus, and the western house mouse M. m. domesticus, including putatively ancestral and derived populations for each. All populations display lower diversity on the X chromosomes relative to autosomes, and this effect is most pronounced in derived populations. To assess the role of demography, we fit the demographic parameters that gave the highest likelihood of the data using coalescent simulations. We find that the reduction in X-linked diversity is too large to be explained by a simple demographic model in at least two of four derived populations. These observations are also not likely to be explained by differences in reproductive success between males and females. They are consistent with a greater impact of positive selection on the X chromosome, and this is supported by the observation of an elevated K(A) and elevated K(A)/K(S) ratios on the rodent X chromosome. A second contribution may be that the X chromosome less readily introgresses across subspecies boundaries.     

BcSAK1, a stress-activated mitogen-activated protein kinase, is involved in vegetative differentiation and pathogenicity in Botrytis cinerea

The gene bcsak1, encoding a mitogen-activated protein kinase (MAPK) of Botrytis cinerea, was cloned and characterized. The protein has high homology to the yeast Hog1 and to corresponding MAPKs from filamentous fungi, but it shows unique functional features. The protein is phosphorylated under osmotic stress, specific fungicides, and oxidative stress mediated by H(2)O(2) and menadione. Northern blot analyses indicate that only a subset of typical oxidative stress response genes is regulated by BcSAK1. In contrast to most other fungal systems, Deltabcsak1 mutants are significantly impaired in vegetative and pathogenic development: they are blocked in conidia formation, show increased sclerotial development, and are unable to penetrate unwounded plant tissue. These data indicate that in B. cinerea the stress-activated MAPK cascade is involved in essential differentiation programs.     

Spatiotemporal modeling of first and second wave outbreak dynamics of COVID-19 in Germany

The COVID-19 pandemic has kept the world in suspense for the past year. In most federal countries such as Germany, locally varying conditions demand for state- or county-level decisions to adapt to the disease dynamics. However, this requires a deep understanding of the mesoscale outbreak dynamics between microscale agent models and macroscale global models. Here, we use a reparameterized SIQRD network model that accounts for local political decisions to predict the spatiotemporal evolution of the pandemic in Germany at county resolution. Our optimized model reproduces state-wise cumulative infections and deaths as reported by the Robert Koch Institute and predicts the development for individual counties at convincing accuracy during both waves in spring and fall of 2020. We demonstrate the dominating effect of local infection seeds and identify effective measures to attenuate the rapid spread. Our model has great potential to support decision makers on a state and community politics level to individually strategize their best way forward during the months to come.

     

Lipid‐polymorphism of plant thylakoid membranes. Enhanced non‐bilayer lipid phases associated with increased membrane permeability

Earlier experiments, using ³¹P‐NMR and time‐resolved merocyanine fluorescence spectroscopy, have shown that isolated intact, fully functional plant thylakoid membranes, in addition to the bilayer phase, contain three non‐bilayer (or non‐lamellar) lipid phases. It has also been shown that the lipid polymorphism of thylakoid membranes can be characterized by remarkable plasticity, i.e. by significant variations in ³¹P‐NMR signatures. However, changes in the lipid‐phase behaviour of thylakoids could not be assigned to changes in the overall membrane organization and the photosynthetic activity, as tested by circular dichroism and 77 K fluorescence emission spectroscopy and the magnitude of the variable fluorescence of photosystem II, which all showed only marginal variations. In this work, we investigated in more detail the temporal stability of the different lipid phases by recording ³¹P‐NMR spectra on isolated thylakoid membranes that were suspended in sorbitol‐ or NaCl‐based media. We observed, at 5°C during 8 h in the dark, substantial gradual enhancement of the isotropic lipid phases and diminishment of the bilayer phase in the sorbitol‐based medium. These changes compared well with the gradually increasing membrane permeability, as testified by the gradual acceleration of the decay of flash‐induced electrochromic absorption changes and characteristic changes in the kinetics of fast chlorophyll a‐fluorescence transients; all variations were much less pronounced in the NaCl‐based medium. These observations suggest that non‐bilayer lipids and non‐lamellar lipid phases play significant roles in the structural dynamics and functional plasticity of thylakoid membranes.     

Benno Parthier (1932–2019)

Plant Molecular Biology -     

Characterization of a highly diverged mitochondrial ATP synthase Fo subunit in Trypanosoma brucei

The mitochondrial F₁F_o ATP synthase of the parasite Trypanosoma brucei has been previously studied in detail. This unusual enzyme switches direction in functionality during the life cycle of the parasite, acting as an ATP synthase in the insect stages, and as an ATPase to generate mitochondrial membrane potential in the mammalian bloodstream stages. Whereas the trypanosome F₁ moiety is relatively highly conserved in structure and composition, the F_o subcomplex and the peripheral stalk have been shown to be more variable. Interestingly, a core subunit of the latter, the normally conserved subunit b, has been resistant to identification by sequence alignment or biochemical methods. Here, we identified a 17 kDa mitochondrial protein of the inner membrane, Tb927.8.3070, that is essential for normal growth, efficient oxidative phosphorylation, and membrane potential maintenance. Pull-down experiments and native PAGE analysis indicated that the protein is both associated with the F₁F_o ATP synthase and integral to its assembly. In addition, its knockdown reduced the levels of F_o subunits, but not those of F₁, and disturbed the cell cycle. Finally, analysis of structural homology using the HHpred algorithm showed that this protein has structural similarities to F_o subunit b of other species, indicating that this subunit may be a highly diverged form of the elusive subunit b.     

Missense mutations in SGLT1 cause glucose-galactose malabsorption by trafficking defects

Glucose-galactose malabsorption (GGM) is an autosomal recessive disorder caused by defects in the Na+/glucose cotransporter (SGLT1). Neonates present with severe diarrhea while on any diet containing glucose and/or galactose [1]. This study focuses on a patient of Swiss and Dominican descent. All 15 exons of SGLT1 were screened using single stranded conformational polymorphism analyses, and aberrant PCR products were sequenced. Two missense mutations, Gly318Arg and Ala468Val, were identified. SGLT1 mutants were expressed in Xenopus laevis oocytes for radiotracer uptake, electrophysiological experiments, and Western blotting. Uptakes of [14C]alpha-methyl-d-glucoside by the mutants were 5% or less than that of wild-type. Two-electrode voltage-clamp experiments confirmed the transport defects, as no noticeable sugar-induced current could be elicited from either mutant [2]. Western blots of cell protein showed levels of each SGLT1 mutant protein comparable to that of wild-type, and that both were core-glycosylated. Presteady-state current measurements indicated an absence of SGLT1 in the plasma membrane. We suggest that the compound heterozygote missense mutations G318R and A468V lead to GGM in this patient by defective trafficking of mutant proteins from the endoplasmic reticulum to the plasma membrane.