Biogenesis of Phycobiliproteins

Abstract

All phycobiliproteins contain a conserved, post-translational modification on asparagine 72 of their ␤-subunits. Methylation of this Asn to produce ␥-N-methylasparagine has been shown to increase energy transfer efficiency within the phycobilisome and to prevent photoinhibition. We report here the biochemical characterization of the product of sll0487, which we have named cpcM, from the cyanobacterium Synechocystis sp. PCC 6803. Recombinant apo-phycocyanin and apo-allophycocyanin subunits were used as the substrates for assays with [methyl-3 H]Sadenosylmethionine and recombinant CpcM. CpcM methylated the ␤-subunits of phycobiliproteins (CpcB, ApcB, and ApcF) and did not methylate the corresponding ␣-subunits (CpcA, ApcA, and ApcD), although they are similar in primary and tertiary structure. CpcM preferentially methylated its CpcB substrate after chromophorylation had occurred at Cys 82 . CpcM exhibited lower activity on trimeric phycocyanin after complete chromophorylation and oligomerization had occurred. Based upon these in vitro studies, we conclude that this post-translational modification probably occurs after chromophorylation but before trimer assembly in vivo.

Phycourobilin in Trichromatic Phycocyanin from Oceanic Cyanobacteria Is Formed Post-translationally by a Phycoerythrobilin Lyase-Isomerase

“…Only after maturation can phycobiliproteins form spontaneously trimers, which are then integrated into the PBSs by interaction with specific structural proteins, so-called linker polypeptides. These post-translational modifications reactions include the covalent attachment of 1-4 chromophores to each individual apoprotein (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), methylation of an asparagine residue (21)(22)(23), and cleavage of N-terminal methionine residue (5, 24) (see UniProtKB/Swiss-Prot entry Q1XDQ2).…”

The phycobilisomes: an early requisite for efficient photosynthesis in cyanobacteria

“…PBPs may comprise more than 60 % of the total soluble cellular protein fraction, which is equivalent to almost 20 % of the total dry weight of cyanobacteria (Soni et al, 2008[ 91 ]). The assembly of PBS is initiated with chromophorylation of PBPs, followed by methylation and oligomerization, and linker protein insertion (Miller et al, 2008[ 63 ]). PBS assembly and architecture are driven to facilitate energy transfer from higher to lower energy state.…”

“…This change in conformation increases the photosynthetic efficiency by decreasing the excited state proton transfer reactions and intersystem photoisomerization (Zhao et al, 2000[ 105 ]). Since Asn72 is very close to the surface of the protein, methylation of Asn72 also restricts the approach of surrounding solvents and oxygen to the chromophore i.e., photo-oxidation in presence of very high light intensity (Miller et al, 2008[ 63 ]).…”

“…The molecular principles that underlie such molecular adaptations are only partly understood. But preliminary investigations suggest extensive inactivation of chromophore protein turn over, by which chromophore conformation and dynamics are modulated (Miller et al, 2008[ 63 ]). The CCA is grouped on the basis of PC and PE content of chromatically adapting cyanobacterial species in red and green light respectively (Kehoe, 2010)[ 42 ] into following categories:…”

A preliminary analysis of the gene all0012 of Anabaena PCC 7120

“…During the maturation of PBP, another unique posttranslational modification occurs, the methylation of N-asparagine, which is located at the 72 position (consensus numbering), and no such modification has been found on the homologous position (Swanson and Glazer, 1990;Miller et al, 2008;Shen et al, 2008). Minami et al first reported that ApcB of Anabaena cylindria contained a modified aspartate residue at position 71 (Minami et al, 1985).…”