86 research outputs found
Introduction : Réflexions sur la construction des identités dans des régions frontalières
Le 1er mai 2004, 59 ans après la fin de la seconde guerre mondiale, 10 nouveaux pays (dont 8 issus de l’ancien bloc soviétique ou de l’ancienne Yougoslavie) sont entrés dans l’Union européenne. Ainsi, d’une fédération d’états occidentaux, l’Union européenne s’est transformée en une « nouvelle Europe ». Avec l’entrée dans l’Union, l’une des frontières les plus disputées entre l’Est et l’Ouest est devenue une frontière interne entre partenaires. Le fer est devenu de la soie. Quinze ans auparava..
A Chaperonin Subunit with Unique Structures Is Essential for Folding of a Specific Substrate
Type I chaperonins are large, double-ring complexes present in bacteria (GroEL),
mitochondria (Hsp60), and chloroplasts (Cpn60), which are involved in mediating
the folding of newly synthesized, translocated, or stress-denatured proteins. In
Escherichia coli, GroEL comprises 14 identical subunits and
has been exquisitely optimized to fold its broad range of substrates. However,
multiple Cpn60 subunits with different expression profiles have evolved in
chloroplasts. Here, we show that, in Arabidopsis thaliana, the
minor subunit Cpn60β4 forms a heterooligomeric Cpn60 complex with
Cpn60α1 and Cpn60β1–β3 and is specifically required for the
folding of NdhH, a subunit of the chloroplast NADH dehydrogenase-like complex
(NDH). Other Cpn60β subunits cannot complement the function of Cpn60β4.
Furthermore, the unique C-terminus of Cpn60β4 is required for the full
activity of the unique Cpn60 complex containing Cpn60β4 for folding of NdhH.
Our findings suggest that this unusual kind of subunit enables the Cpn60 complex
to assist the folding of some particular substrates, whereas other dominant
Cpn60 subunits maintain a housekeeping chaperonin function by facilitating the
folding of other obligate substrates
DNA G-segment bending is not the sole determinant of topology simplification by type II DNA topoisomerases
DNA topoisomerases control the topology of DNA. Type II topoisomerases exhibit topology simplification, whereby products of their reactions are simplified beyond that expected based on thermodynamic equilibrium. The molecular basis for this process is unknown, although DNA bending has been implicated. To investigate the role of bending in topology simplification, the DNA bend angles of four enzymes of different types (IIA and IIB) were measured using atomic force microscopy (AFM). The enzymes tested were Escherichia coli topo IV and yeast topo II (type IIA enzymes that exhibit topology simplification), and Methanosarcina mazei topo VI and Sulfolobus shibatae topo VI (type IIB enzymes, which do not). Bend angles were measured using the manual tangent method from topographical AFM images taken with a novel amplitude-modulated imaging mode: small amplitude small set-point (SASS), which optimises resolution for a given AFM tip size and minimises tip convolution with the sample. This gave improved accuracy and reliability and revealed that all 4 topoisomerases bend DNA by a similar amount: ~120° between the DNA entering and exiting the enzyme complex. These data indicate that DNA bending alone is insufficient to explain topology simplification and that the ‘exit gate’ may be an important determinant of this process
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