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Figure 3.
FIGURE 3. Comparison of the processivity factor-peptide
structures of HSV UL42, HCMV UL44, and human PCNA. A, the
association of HSV UL42 (Protein Data Bank code 1DML) with the
UL30 peptide (orange) is primarily stabilized by interactions
between the C-terminal helix of the peptide and a groove on the
left side of the connector loop (red in each panel). His^1228
and Arg^1229 from the C-terminal helix of the peptide are
hydrogenbonded to Arg^64 and Gln^171, respectively, of UL42. B,
in contrast, the HCMV UL54 peptide (blue) makes significant
interactions with UL44 on the right side of the connector loop.
This interaction depends largely on three hydrophobic residues
from the peptide that bind to a hydrophobic crevice on UL44. The
side chains of Leu^1227 and Phe^1231 of UL54 are required for
the association with UL44. Although the HSV UL30 peptide has an
aromatic residue (Phe^1211) that packs into an analogous crevice
on UL42 (see inset in A), this interaction is not essential. C,
like UL44, PCNA (Protein Data Bank code 1AXC [PDB]
) contains a hydrophobic crevice on the right side of the
connector loop that binds to a peptide from its respective
binding partner, p21^WAF1/CIP1 (green). Similar to the UL54
peptide, the p21^WAF1/CIP1 peptide buries three hydrophobic
residues in the crevice. The connector loop from each
processivity factor forms an antiparallel  -sheet with its
respective peptide. In each inset, the processivity factor is
gray; the connector loop is red; and the peptide backbone is tan.
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