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J. Biol. Chem., Vol. 283, Issue 28, 19293-19300, July 11, 2008

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Biogenesis of Phycobiliproteins

III. CpcM IS THE ASPARAGINE METHYLTRANSFERASE FOR PHYCOBILIPROTEIN β-SUBUNITS IN CYANOBACTERIA^*

Crystal A. Miller, Heidi S. Leonard¹, Ivan G. Pinsky, Brandy M. Turner², Shervonda R. Williams, Leon Harrison, Jr., Ariane F. Fletcher, Gaozhong Shen, Donald A. Bryant, and Wendy M. Schluchter³

From the Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70148 and the Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802

All phycobiliproteins contain a conserved, post-translational modification on asparagine 72 of their β-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-³H]S-adenosylmethionine and recombinant CpcM. CpcM methylated the β-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 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.

Received for publication, April 8, 2008 , and in revised form, May 14, 2008.

^* This work was supported by National Science Foundation (NSF) Grants MCB-0133441 (to W. M. S.) and MCB-0077586 and MCB-0519743 (to D. A. B.). This is Paper III in the series "Mutational Analysis of Primosome Assembly Sites." Ref. 25 is Paper II in the series. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1–4.

¹ Supported by funds from the LA Board of Regents in Grant LEQSF (1999–2002)-RD-A-5 (to W. M. S.).

² Supported by funds from the Undergraduate Mentoring in Environmental Biology program at the NSF (Grant UMEB0405263).

³ To whom correspondence should be addressed: Dept. of Biological Sciences, University of New Orleans, New Orleans, LA 70148. Tel.: 504-280-7194; Fax: 504-280-6121; E-mail: wschluch{at}uno.edu.

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