Positive diversifying selection in avian Mx genes

Mx proteins are interferon-induced GTPases that confer antiviral activities against RNA viruses. We analysed the molecular evolution of the Mx gene in birds using data on interspecific divergence in anseriform and galliform birds, and on intraspecific diversity in commercial chicken lines, local Chinese chicken breeds as well as in the mallard. The overall ratio of non-synonymous to synonymous substitution was unusually high, 0.80, indicating relaxed constraint or positive selection. Evidence for the latter was provided by that a total of 11–18 codons were found to have evolved under positive selection. The great majority of these codons are located in a region unique to birds at the N-terminal end of the Mx protein. We found an excess of non-synonymous polymorphisms relative to synonymous variants in all comparisons. This, together with positive Tajima’s D values in the local Chinese chicken breeds and in the mallard suggests that balancing selection is acting in avian Mx genes. As such, Mx mimics the major histocompatibility complex system, indicating that heterozygous individuals are better off withstanding pathogen attack. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. S.B. and L.Q. contributed equally to this work.     

Differential interference with Pythium ultimum sporangial activation and germination by Enterobacter cloacae in the corn and cucumber spermospheres

Differential protection of plants by Enterobacter cloacae was studied by investigating early sensing and response behavior of Pythium ultimum sporangia toward seeds in the presence or absence of E. cloacae. Ten percent of P. ultimum sporangia were activated within the first 30 min of exposure to cucumber seeds. In contrast, 44% of the sporangia were activated as early as 15 min after exposure to corn seeds with over 80% sporangial activation by 30 min. Germ tubes emerged from sporangia after 2.5 and 1.0 h in the cucumber and corn spermospheres, respectively. Seed application of the wild-type strain of E. cloacae (EcCT-501R3) reduced sporangial activation by 45% in the cucumber spermosphere, whereas no reduction was observed in the corn spermosphere. Fatty acid transport and degradation mutants of E. cloacae (strains EcL1 and Ec31, respectively) did not reduce sporangial activation in either of the spermospheres. Although wild-type or mutant strains of E. cloacae failed to reduce seed colonization incidence, pathogen biomass on cucumber seeds was reduced in the presence of E. cloacae strains EcCT-501R3 and Ec31 by 4 and 8 h after sowing, respectively. By 12 h, levels of P. ultimum on cucumber seeds treated with E. cloacae EcCT-501R3 did not differ from levels on noninoculated seeds. On corn seeds, P. ultimum biomass was not affected by the presence of any E. cloacae strain. When introduced after sporangial activation had occurred, E. cloacae failed to reduce P. ultimum biomass on cucumber seeds compared with that on nontreated seeds. Also, increasing numbers of sporangia used to inoculate seeds yielded increased pathogen biomass at each sampling time. This indicates a direct link between the level of seed-colonizing biomass of P. ultimum and the number of activated and germinated sporangia in the spermosphere, suggesting that E. cloacae suppresses P. ultimum seed infections by reducing sporangial activation and germination within the first 30 to 90 min after sowing.     

Lysosomal α-D-mannosidase

-mannosidosis in the human is an autosomal recessive lysosomal storage disease caused by a deficiency of lysosomal -D-mannosidasea actvity. Lysosomal -D-mannosidase is involved in the catabolism of N-linked glycoproteins through the sequential degradation of high-mannose, hybrid and complex oligosaccharides. This review is focused on human, mouse, bovine and feline genes coding for lysosomal -D-mannosidase. In particular the exon-intron structure of the genes, their promoters, and the identification of mutations causing the disease have been examined. The construction, by homologous recombination, of a mouse model of -mannosidosis is reported.     

Knock-in of integrin beta 1D affects primary but not secondary myogenesis in mice

Integrins are extracellular matrix receptors composed of alpha and beta subunits involved in cell adhesion, migration and signal transduction. The beta1 subunit has two isoforms, beta 1A ubiquitously expressed and beta 1D restricted to striated muscle. They are not functionally equivalent. Replacement of beta 1A by beta 1D (beta 1D knock-in) in the mouse leads to midgestation lethality on a 50% Ola/50% FVB background [Baudoin, C., Goumans, M. J., Mummery, C. and Sonnenberg, A. (1998). Genes Dev. 12, 1202-1216]. We crossed the beta 1D knock-in line into a less penetrant genetic background. This led to an attenuation of the midgestation lethality and revealed a second period of lethality around birth. Midgestation death was apparently not caused by failure in cell migration, but rather by abnormal placentation. The beta 1D knock-in embryos that survived midgestation developed until birth, but exhibited severely reduced skeletal muscle mass. Quantification of myotube numbers showed that substitution of beta 1A with beta 1D impairs primary myogenesis with no direct effect on secondary myogenesis. Furthermore, long-term primary myotube survival was affected in beta 1D knock-in embryos. Finally, overexpression of beta 1D in C2C12 cells impaired myotube formation while overexpression of beta 1A primarily affected myotube maturation. Together these results demonstrate for the first time distinct roles for beta1 integrins in primary versus secondary myogenesis and that the beta 1A and beta 1D variants are not functionally equivalent in this process.     

Regulation of sodium channel activity by capping of actin filaments

Ion transport in various tissues can be regulated by the cortical actin cytoskeleton. Specifically, involvement of actin dynamics in the regulation of nonvoltage-gated sodium channels has been shown. Herein, inside-out patch clamp experiments were performed to study the effect of the heterodimeric actin capping protein CapZ on sodium channel regulation in leukemia K562 cells. The channels were activated by cytochalasin-induced disruption of actin filaments and inactivated by G-actin under ionic conditions promoting rapid actin polymerization. CapZ had no direct effect on channel activity. However, being added together with G-actin, CapZ prevented actin-induced channel inactivation, and this effect occurred at CapZ/actin molar ratios from 1:5 to 1:100. When actin was allowed to polymerize at the plasma membrane to induce partial channel inactivation, subsequent addition of CapZ restored the channel activity. These results can be explained by CapZ-induced inhibition of further assembly of actin filaments at the plasma membrane due to the modification of actin dynamics by CapZ. No effect on the channel activity was observed in response to F-actin, confirming that the mechanism of channel inactivation does not involve interaction of the channel with preformed filaments. Our data show that actin-capping protein can participate in the cytoskeleton-associated regulation of sodium transport in nonexcitable cells.     

Heme and pH-dependent stability of an anionic horseradish peroxidase

Horseradish peroxidase A1 thermal stability was studied by steady-state fluorescence, circular dichroism and differential scanning calorimetry at pH values of 4, 7 and 10. Changes in the intrinsic protein probes, tryptophan fluorescence, secondary structure, and heme group environment are not coincident. The T(m) values measured from the visible CD data are higher than those measured from Trp fluorescence and far-UV CD data at all pH values showing that the heme cavity is the last structural region to suffer significant conformational changes during thermal denaturation. However ejection of the heme group leads to an irreversible unfolding behavior at pH 4, while at pH 7 and 10 refolding is still observed. This is putatively correlated with the titration state of the heme pocket. Thermal transitions of HRPA1 showed scan rate dependence at the three pH values, showing that the denaturation process was kinetically controlled. The denaturation process was interpreted in terms of the classic scheme, N<-->U-->D and fitted to far-UV CD ellipticity. A good agreement was obtained between the experimental and theoretical T(m) values and percentages of irreversibility. However the equilibrium between N and U is probably more complex than just a two-state process as revealed by the multiple T(m) values.     

Conformational switching and fibrillogenesis in the amyloidogenic fragment of apolipoprotein a-I

The N-terminal portion of apolipoprotein A-I corresponding to the first 93 residues has been identified as the main component of apolipoprotein A-I fibrils in a form of systemic amyloidosis. We have been able to characterize the process of conformational switching and fibrillogenesis in this fragment of apolipoprotein A-I purified directly from ex vivo amyloid material. The peptide exists in an unstructured form in aqueous solution at neutral pH. The acidification of the solution provokes a collapse into a more compact, intermediate state and the transient appearance of a helical conformation that rapidly converts to a stable, mainly beta-structure in the fibrils. The transition from helical to sheet structure occurs concomitantly with peptide self-aggregation, and fibrils are detected after 72 h. The alpha-helical conformation is induced by the addition of trifluoroethanol and phospholipids. Interaction of the amyloidogenic polypeptide with phospholipids prevents the switching from helical to beta-sheet form and inhibits fibril formation. The secondary structure propensity of the apolipoprotein A-I fragment appears poised between helix and the beta-sheet. These findings reinforce the idea of a delicate balance between natively stabilizing interactions and fatally stabilizing interactions and stress the importance of cellular localization and environment in the maintenance of protein conformation.