Biogenesis of phycobiliproteins: III. CpcM is the asparagine methyltransferase for phycobiliprotein β-subunits in cyanobacteria

All phycobiliproteins contain a conserved, post-translational modification on asparagine 72 of their β-subunits. Methylation of this Asn to produce γ-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-³H]S-adenosylmethionine and recombinant CpcM. CpcM methylated the β-subunits of phycobiliproteins (CpcB, ApcB, and ApcF) and did not methylate the corresponding α-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 Cys⁸². 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.