Allosteric signaling and a nuclear exit strategy: binding of UL25/UL17 heterodimers to DNA-Filled HSV-1 capsids

Abstract

UL25 and UL17 are two essential minor capsid proteins of HSV-1, implicated in DNA packaging and capsid maturation. We 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 quasiequivalent sites or on unfilled capsids. Using SDS-PAGE and mass spectrometry to analyze wild-type capsids, UL25 null capsids, and denaturant-extracted capsids, we conclude that (1) the C-capsid-specific component is a heterodimer of UL25 and UL17, and (2) 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 structural changes induced on the outer surface as pressure builds up inside the capsid during DNA packaging. Its binding may signal that the C-capsid is ready to exit the nucleus.

Figure 1. Alternative maturation pathways for the HSV-1 procapsid

The procapsid surface shell is made up of hexamers (light blue subunits) and pentamers (mid-blue) of UL19, coordinated by triplexes, which are heterotrimers of UL18 and UL38. Its inner shell consists of the scaffolding proteins, UL26.5 and UL26, which is a fusion of the protease VP24 with VP21, an extended version of UL26.5. A single dodecamer of UL6 is present at the portal vertex. In maturation, the protease is released by autoproteolysis and processes the C-termini of the scaffolding proteins at the interface between the inner and outer shells. The surface shell then switches from round to polyhedral in shape, and UL35 binds to the outer tips of UL19 hexamers. The capsids are represented schematically in cross-sections that convey some but not all of their salient features.

Figure 2. The CCSC is present on C-capsids but not on UL25-null A-capsids

Surface renderings of cryo-EM reconstructions are shown in B (C-capsid) and A (UL25-null A-capsid). Triplexes are green; hexons (UL19 hexamers with six VP26 molecules around their outer tip) are light blue; pentons (UL19 pentamers) are mid-blue. The CCSC in B is purple. The capsids are viewed along a 2-fold axis of icosahedral symmetry. C shows a blow-up of the region around a penton, presenting five different views of the CCSC molecules clustered around this vertex. Bar (B, E) = 20 nm. D and E show central sections of the two capsids. High density is dark. The UL25-null A-capsid is empty; the C-capsid contains nested shells of DNA (Booy et al., 1991). Arrows in E point to the CCSC which is sampled in this plane. F shows the optimal fit of a UL25 fragment into the CCSC.

Figure 3. The CCSC is visible at only one of six quasi-equivalent sites on the HSV-1 C-capsid

A & B show icosahedral sections through the C-capsid at radii of 61 nm and 53 nm, respectively, along the 3-fold axis. In them, the sampling surface maintains the same radial position relative to the capsid shell (as opposed to the capsid center). Section A is rescaled so that features in it line up with underlying features in section B. Section A samples the CCSC (yellow arrow), whereas section B samples the triplexes as well as the capsomer protrusions, just above the floor. In C are mapped the positions of the four kinds of capsomers and six kinds of triplexes on one triangular facet. Each CCSC (one is marked in red) overlies a pair of triplexes (Tc, Ta). Other quasi-equivalent sites may be found by substituting another kind of triplex for Tc. There are 320 triplexes, but maximum CCSC binding would correspond to 160 copies, because one CCSC occludes two triplexes. The CCSC is seen on C-capsids at only one of the six possible sites, at 50% occupancy. This occupancy may increase in virions, whose UL25 content is higher than that of C-capsids (Thurlow et al., 2006). Bar (B) = 15 nm.

Figure 4. The CCSC is removed from C-capsids by 0.5 M guanidine.HCl and is absent from wild-type A-capsids

A, D: two sections from a C-capsid reconstruction in which the CCSC is sampled (arrow). B, E: corresponding sections from C-capsids, after treatment with 0.5 Guan.HCl. Note the absence of both the DNA and of the CCSC. C, F: corresponding sections from a reconstruction of wild-type A-capsids. Bar = 10 nm.

Figure 5. Identification of minor protein components of HSV-1 capsids

At left, C-capsids (C), wildtype A-capsids (A), and virions (V) are compared by SDS-PAGE with Coomassie Blue staining. Equal amount of protein were loaded in each lane. The major capsid proteins, UL19, UL38 and UL18 contribute strongly staining bands. The weak bands, c1, c-2, and c-3 were identified by mass spectrometry as UL17, UL6, and UL25, respectively. The v-1 band was identified as UL36. At right, wild-type A-capsids are compared with A-capsids from a UL25-null mutant. Again, c1, c-2, and c-3 are marked. The mutant capsids do not contain UL25, as confirmed by Western blotting (McNab et al., 1998). The very faint band running just below the UL25 position in the UL25-null mutant lane represents an unknown contaminant.

Figure 6. Changes affecting the CCSC binding site on maturation of the HSV-1 procapsid

The regions of capsid surface surrounding the CCSC binding site are compared in cryo-EM reconstructions at ~ 2 nm resolution of the procapsid (A – from Cheng et al., 2002) and the C-capsid (B). In both cases, the two triplexes and one UL19 subunit that form the binding site for a single representative CCSC are shaded green and blue, respectively; this CCSC is purple in panel B and the UL35 subunit that tops the interacting UL19 subunit (but is not present on the procapsid) is gold. As reference points, the 5-fold axis of the adjacent UL19 penton is marked “5”, and a neighboring P-hexon with P. Bar = 10 nm.