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Figure 3.
Figure 3 Proposed model of the polymerase  UBZ
domain–ubiquitin complex. (A) A ribbon diagram of the
UIM–ubiquitin complex (PDB entry 1Q0W), with conserved
residues shown in stick model. The orientation of the UIM  -helix
(pink) bound to ubiquitin (green) is indicated by an arrow,
directed from the N to C terminus. (B) A ribbon diagram of the
MIU/IUIM motif (brown) bound to ubiquitin (PDB entry 2FIF), with
the orientation of the -helix
and conserved residues indicated as in (A). (C) Alignment of the
consensus sequences of the UBZ domain with MIU/IUIM and reversed
UIM. The central invariant alanine is highlighted in purple,
conserved hydrophobic residues in yellow, acidic residues in
red, zinc ligands in blue and a highly conserved glutamine
residue in grey. A serine residue at the +4 position in the UIM,
which is replaced by an aspartate in the MIU/IUIM and the UBZ
domains, is shown in blue. (D) A model of the UBZ
domain–ubiquitin complex. The C-terminal cysteine of the UBZ
domain, which was modified by the spin-label reagent MTSL
(denoted as S), and ubiquitin residues, the resonances of which
were severely attenuated during the spin-label titration, are
coloured in blue. (E) Sections of ^1H-^15N HSQC spectra of
ubiquitin, (F) in the presence of the spin-labelled UBZ domain
and (G) after addition of 5 mM dithiothreitol. Note that the
amide resonance of G75[Ub] disappears in the presence of the
spin-labelled UBZ domain (F). IUIM; inverted UIM; MIU, motif
interacting with ubiquitin; PDB, Protein Data Bank; UBZ,
ubiquitin-binding zinc finger; UIM, ubiquitin-interacting motif.
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