Dissociating effect of chromophore modifications on C-phycocyanin heterohexamers

Abstract

The bilin chromophores of the a or /3 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 (a& but the heterohexameric aggregation state (a& could not be obtained with the modified chromophores. Autoxidation of the reduced a-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 884 chromophore had only a moderately destabilizing effect on the heterohexamer (a&. 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 a subunit near the heterodimer-heterodimer contact region is also responsible for aggregate destabilization (Siebzehnriibl et al., Photochem. Photobiol., 46 (1989) 753).

Biosynthesis of fluorescent cyanobacterial allophycocyanin trimer in Escherichia coli

“…An intact bilin structure was found to be an important factor for biliprotein aggregation. Bilin modification, such as reduction and photobleaching, prevents the formation of in vitro aggregation states higher than dimers (Fischer and Scheer 1992). Bilin, when located in the E a helix, has extensive interaction with residue sidechains in the E-F a helices (Liu et al 1999;McGregor et al 2008;Murray et al 2007;Reuter et al 1999;Schirmer et al 1987).…”

“…Bilin strongly affects the conformation of the subunits, and contributes to the stability of the higher-order structure (Fischer and Scheer 1992;Toole et al 1998). Specific lyases are responsible for bilin attachment (Fairchild and Glazer 1994;Jung et al 1995;Scheer and Zhao 2008).…”