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Figure 6.
Figure 6. WRN-exo hexameric ring model and dGMP-binding site,
and altered processing by the W145A mutant. (a) The WRN ring
homology model, with differently colored WRN-exo subunits, was
built by structural superimposition with the A. thaliana homolog
(PDB entry 1VK0). The active site of the exonuclease (with gray
spheres denoting metal ions) faces the center of the ring. The
central cavity of the WRN ring is large enough (about 30 Å
in diameter by 35 Å deep) to accommodate dsDNA and is
similar to that observed in Ku70/80 (ref. 49). (b) DNA
processing is altered in a WRN-exo W145A mutant. Control
reactions with DNA alone or with 10 pmol of Ku70/80 are
indicated. WRN-exo and W145A reactions contained 20 fmol of
radiolabeled dsDNA substrate, approximately 200 pmol of each WRN
nuclease variant and increasing amounts of Ku70/80 (0.06, 0.6
and 6 pmol), denoted by triangles. (c) F[o] - F[c] electron
density map of WRN-exo dGMP soak (blue, 3  ;
red, 5 ).
dGMP stacks against Trp145, consistent with this region
interacting with DNA substrate at the center of the ring. (d)
Similar internal and external dimensions of the WRN-exo hexamer
model (right) and Ku70/80 bound to DNA (left) suggest a possible
interaction mode, which would place the protruding  2-
3
loop (left face) adjacent to the Ku dimer and/or allow Ku to
provide a suitable DNA orientation.
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