Figure 1 - full size

 

Figure 1.
Figure 1. HK97 Capsid Expansion and Organization
(A) HK97 Prohead I is assembled in an expression system. Prohead II (PII) expands in vitro when perturbed by acidic pH (4.0) and changes through a series of discrete intermediates, acquiring progressively more crosslinks. Expansion Intermediates I and II (EI-I and EI-II) and all later particles, which have matured beyond the expansion checkpoint, will convert to the mature, fully crosslinked Head II (HII) upon neutralization. Particles can be expanded and trapped into specific intermediate states under defined acidification conditions. K169Y mutant procapsids follow a similar maturation trajectory, but they tend to remain as EI-II because they cannot crosslink (Ross et al., 2005). Fully expanded K169Y particles are called Head I (HI) and are similar to those of Head II, but they are not crosslinked; wild-type capsids do not convert to Head I before maturing to Head II.
(B) The HK97 mature capsid (Head II) is assembled from 420 capsid proteins that are organized as pentons (light green) and hexons (light blue). The T = 7 laevo lattice has icosahedral symmetry and is organized as 60 asymmetric units that each contain 7 gp5^* subunits. Icosahedral 5-fold (pentagon), 3-fold (triangle), and 2-fold (ellipse) axes are labeled. One icosahedral asymmetric unit, shown in dark colors, contains one whole hexon (A through F subunits) and one penton G subunit. The seven subunits are in chemically unique environments, but they adopt very similar conformations due to the high degree of quasi-equivalence.
(C) The ribbon model of one HK97 Head II gp5^* (C subunit) is colored by its four motifs: Domain A, teal; Domain P, gold; E loop, blue; and N arm, cyan. Two motifs that are described in detail in the text (spine helix and E loop β strands) are highlighted by light-blue rectangles. The 2 residues that form crosslinks (K169 and N356) are shown as red balls-and-sticks.

The above figure is reprinted by permission from Cell Press: Structure (2006, 14, 1655-1665) copyright 2006.