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.



Close Window