Biology Reference
In-Depth Information
can also be bound to the interface of
adjacent coat proteins of the CCMV capsid (Allen
The lanthanides Gd
3+
and Tb
3+
., 2003).
CCMV offers 180 intrinsic metal-binding sites at the threefold axis; under
physiological conditions, Ca
et al
., 2005; Basu
et al
2+
ions are bound to these sites. Coordination of
bivalent Ca
ions involves carboxylic oxygens of three carboxylic acids (Asp/
Glu) and carbonyl oxygens of two Gln residues contributed from two adjacent
coat protein subunits (Fig. 5.1). Removal of the bivalent Ca
2+
ions at near-
neutral pH causes structural transitions into the swollen, open conformation.
This is a result of electrostatic repulsion between the negatively charged
groups involved in metal binding (Johnson & Speir, 1997; Speir
2+
et al
., 1995).
Ca
cations; binding results in
transition into the non-swollen, closed conformation. Up to 180 lanthanides
can be bound, and the interaction was found to be stable under physiological
conditions (Allen
2+
cations can be replaced with Gd
3+
or Tb
3+
., 2003).
The materials properties of VNP-lanthanide complexes have been
evaluated, and it is implied that these formulations would be excellent
candidates for further development as contrast agents for magnetic
resonance imaging (MRI) (discussed in Chapter 8).
Using a similar approach, small molecules such as fluorescent dyes and
drugs have been infused and stably entrapped in particles formed by the
plant virus RCNMV (Loo
et al
., 2005; Basu
et al
., 2008). Like CCMV, RCNMV particles allow
pH- and metal ion-dependent reversible gating (Sherman
et al
., 2006).
In the open conformation and in presence of the RNA molecules, small
positively charged molecules can freely diffuse into the interior cavity of
the particles, where they bind to the negatively charged viral nucleic acids
via electrostatic interactions. It was shown that pore opening is required to
achieve entrapment, and once entrapped the molecules are stably bound.
Pore re-opening does not release the trapped materials. A study reported
that around 80 copies of the positively charged fluorescent dye rhodamine
and up to 4300
et al
±
1300 molecules of the cytotoxic drug doxorubicin could
be infused and bound within the RCNMV capsid (Loo
., 2008). The
high loading with doxorubicin can be explained by (i) the positive charge
of the drug and (ii) its high intrinsic binding affinity for nucleic acids
(Hande, 1998). Fluorescent dye-labeled and drug-loaded particles may find
further application in biomedical imaging and drug delivery (discussed in
Chapter 8).
et al
5.1.1TriggeringEncapsidaionofMaterialsUsingRNAOperators
The translational repression (TR) operator of the bacteriophage MS2 has
been exploited to facilitate loading of MS2 with drugs (Brown
et al
., 2002;
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