Dissociating effect of chromophore modifications on C-phycocyanin heterohexamers

The bilin chromophores of the α or β subunit of C-phycocyanin (PC) from Mastigocladus laminosus were modified, and subsequently recombined with the respective complementary unmodified chromophores. The modifications consisted of photobleaching (350 nm) or reversible reduction of the verdin- to rubin-type chromophore(s). Recombination led to heterodimers (αβ)1, but the heterohexameric aggregation state (αβ)3 could not be obtained with the modified chromophores. Autoxidation of the reduced α-84 chromophore in such a hybrid, which occurred on standing under aerobic conditions, induced reaggregation to heterohexamers. Chemical re-oxidation of the reduced chromophores did not produce reaggregation, and it was not promoted by a 22 kDa linker peptide fragment (Gottschalk et al., Photochem. Photobiol., 54 (1991) 283), which in unmodified samples stabilized heterohexameric aggregates. Binding of the mercurial p-chloromercury-benzenesulphonate to the single free cysteine of PC near (approximately 0.4 nm) the β-84 chromophore had only a moderately destabilizing effect on the heterohexamer (αβ)3. It was concluded that the intact chromophore structure is an important factor determining the quaternary structure of biliproteins. The tendency of heterohexamer destabilization is related to the situation in phycoerythrocyanin, where photoisomerization of the violobilin chromophore of the α subunit near the heterodimer—heterodimer contact region is also responsible for aggregate destabilization (Siebzehnrübl et al., Photochem. Photobiol., 46 (1989) 753)