The Orange Carotenoid Protein of Cyanobacteria - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 7

FIGURE 1.2 In situ localization of the OCP-green l uorescence protein (GFP) fusion protein: Immunogold
labeling of a thin section of OCP-GFP transformed Synechocystis PCC6803; OCP-GFP cells were labeled
with a polyclonal antibody against the GFP coupled to 10 nm gold particles. Bar = 0.5 μm.
in l uorescence quenching was faster in ΔIsiA cells than in WT cells. This is explained by the
relationship between the quenching of l uorescence and the concentration of the OCP: In iron-
starved WT Synechocystis PCC6803 cells, the concentration of the OCP is higher than in the pres-
ence of iron, and in iron-starved ΔIsiA cells the concentration is even higher (Wilson et al. 2007).
In all cyanobacterial strains containing OCP-like genes that have been tested, the full-length OCP
is present and the NPQ mechanism is induced by blue light, suggesting that this photoprotective
mechanism is widespread in cyanobacteria (Boulay et al. 2008a). Additional details about this blue-
light-induced NPQ mechanism are described in Karapetyan 2007, Kirilovsky 2007, Bailey and
Grossman 2008.
1.3 THE OCP: PRIMARY TO QUATERNARY STRUCTURE
The crystal structure of the OCP from Arthrospira maxima has been solved to 2.1 Å resolution
(Kerfeld et al. 2003). It is composed of two domains and the carotenoid, 3′-hydroxyechinenone,
spans both. The carotenoid is almost completely buried within the protein; only 3.4% of the pigment
surface is accessible to solvent (see Figure 1.3a). The OCP is a dimer in solution; the intermolecular
interactions are largely mediated by hydrogen bonding among the N-terminal 30 amino acids, as
shown in Figure 1.3b
The N-terminal domain of the OCP is an orthogonal alpha-helical bundle, subdivided into two
four-helix bundles (Figure 1.3a and c). These subdomains are composed of discontinuous segments
of the polypeptide chain (gray and white in Figure 1.3c). To date, the OCP N-terminal domain
is the only known protein structure with this particular fold (Pfam 09150). The hydroxyl terminus
of the 3′-hydroxyechinenone is nestled between the two bundles. The C-terminal domain (dark