Abstract
UL25 and UL17 are two essential minor capsid proteins of HSV-1, implicated in DNA
packaging and capsid maturation. We have used cryo-electron microscopy to examine
their binding to capsids, whose architecture observes T=16 icosahedral geometry.
C-capsids (mature DNA-filled capsids) have an elongated two-domain molecule present
at a unique, vertex-adjacent, site that is not seen at other quasi-equivalent sites nor on
unfilled capsids. Using SDS-PAGE and mass spectrometry to analyze the protein
compositions of wild-type capsids, UL25-null capsids, and denaturant-extracted capsids,
we conclude that (i) the C-capsid-specific component is a heterodimer of UL25 and
UL17; and (ii) capsids have additional populations of UL25 and UL17 that are invisible
in reconstructions because of sparsity and/or disorder. We infer that binding of the
ordered population reflects local structural changes induced on the outer surface as
pressure builds up inside the capsid during DNA packaging. Its binding may signal that
the DNA-filled C-capsid is ready to exit the nucleus.
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