The Art of Bioconjugation: Functionalization of VNPs - Viral Nanoparticles: Tools for Materials Science and Biomedicine - page 91

In the case of CPMV, the first attempt to generating a CPMV Cys-added
mutant exploited insertion of the Cys residues in the highly solvent-
exposed

b b

,
2002b) (Fig. 4.12). The mutants were found highly reactive toward thiol-
selective chemistries. On the downside, the particles show strong tendency
to form interparticle aggregates via the formation of disulfide bonds in the
absence of high concentrations of reducing agents. After 2-3 months of
storage, the particles undergo such extensive interparticle linkage that the
virus solution can turn into a viscous “glue” (Fig. 4.12) (Steinmetz

B- C loop at amino acid position 25 of the S protein (Wang et al.

et al. ,

2007), thus limiting the usefulness of this particular mutant.

Figure 4.12 Left panel: location of the Cys residue in the solvent-exposed βB-βC

loop at amino acid position 25 in the CPMV capsid. CPMV consists of small (S, in blue)
and large (L, in green) subunits. The surface-exposed βB-βC loop (highlighted in
yellow) lies on the S subunit and it is composed of the following nine amino acids:
Thr, Pro, Pro, Ala, Pro, Glu, Ser, Asp, and Val. The CPMV Cys-added mutant contains
a point mutation: exchange of a Ser at amino acid position 25 on the S subunit with
a Cys residue (highlighted as a sphere). Right panel: aggregation of mutant CPMV
particles displaying a cysteine residue in the solvent-exposed

b

B-

b

C loop.

Efforts have been made toward “smart” mutant design, and a large
collection of Cys-added CPMV mutants has been evaluated (reviewed in
Strable & Finn, 2009). As a general design rule, the Cys residue should be
inserted on the solvent-exposed surface at a site that is somewhat buried
on the particle surface. This facilitates reactivity toward thiol-selective
chemistries, but avoids undesired aggregation efects.
Cys-added versions of CCMV, CPMV, FHV, MS2, Ad, TMV, and M13
(Destito

et al. , 2009; Khalil et al. , 2007a; Klem et al. , 2003; Kreppel et al. , 2005;

Miller , 2002b) have been made and

utilized as building blocks for the construction of nanocircuits (Blum

et al. , 2007; Peabody, 2003; Wang et al.

et al. ,