Overview of the Manifold VNPs Used in Nanotechnology - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 31

their size by about 20% during maturation. The matured capsid, called head
II, is 66 nm in diameter and extremely thin walled (Gertsman et al ., 2009; Lee

et al ., 2008).

Figure 2.11 HK97 assembly and morphology. (a). The 384-residue gp5 subunit

initially assembles into hexameric and pentameric oligomers, termed capsomers,
that first assemble to form the prohead I capsid (P-I). The

= 7 particle is composed
of 12 pentamers and 60 hexamers and encapsidates approximately 60 copies of gp4
protease. Expression with a defective protease produces a prohead I particle that
can be disassembled

T

into free capsomers and re-assembled when exposed
to specific chemical treatments. When active gp4 is present, particles spontaneously
mature to the 13 MDa prohead II (P-II) form after digestion of residues 2-103 from
all subunits. Cross-linking occurs in the wild-type particle after formation of the EI
state. Cross-links (isopeptide bond) form between Lys 169 and Asn 356 located on
different subunits. A cross-link-defective mutant, K169Y, expands to head I, a state
nearly identical to balloon minus the cross-links. Wild-type balloon undergoes a final
expansion step to head II in which the pentons become more protruded and form
one last class of cross-links, with a molecular topology similar to chain mail. (b).
Crystal structure of subunit D of prohead II at 3.65 Å. (c) 3.65 Å electron density map
(displayed as a solid surface) of the full prohead II capsid, contoured at,1s in Chimera.
The prohead II hexamers and pentamers are shown alongside the capsid with the
seven subunits of the viral asymmetric subunit labeled A-F for the hexamers and G
for the pentamers. (d). A calculated electron density map of the head II capsid shown
at 3.65 Å, also rendered at,1s. (e). Prohead II and head II hexamers shown tangential
to the capsid surface (rotated 90

in vitro

from view (c) and (d). Reproduced with permission
from Gertsman, I., Gan, L., Guttman, M., Lee, K., Speir, J. A., Duda, R. L., Hendrix, R. W.,
Komives, E. A., and Johnson, J. E. (2009) An unexpected twist in viral capsid maturation,
Nature

°

, 458 (7238), 646-650.