title: A multiprotein bicarbonate dehydration complex essential to carboxysome function in cyanobacteria.
abstract: Carboxysomes are proteinaceous biochemical compartments that constitute the enzymatic "back end" of the cyanobacterial CO2-concentrating mechanism. These protein-bound organelles catalyze HCO3- dehydration and photosynthetic CO2 fixation. In Synechocystis sp. strain PCC6803 these reactions involve the beta-class carbonic anhydrase (CA), CcaA, and Form 1B ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The surrounding shell is thought to be composed of proteins encoded by the ccmKLMN operon, although little is known about how structural and catalytic proteins integrate to form a functional carboxysome. Using biochemical activity assays and molecular approaches we have identified a catalytic, multiprotein HCO3- dehydration complex (BDC) associated with the protein shell of Synechocystis carboxysomes. The complex was minimally composed of a CcmM73 trimer, CcaA dimer, and CcmN. Larger native complexes also contained RbcL, RbcS, and two or three immunologically identified smaller forms of CcmM (62, 52, and 36 kDa). Yeast two-hybrid analyses indicated that the BDC was associated with the carboxysome shell through CcmM73-specific protein interactions with CcmK and CcmL. Protein interactions between CcmM73 and CcaA, CcmM73 and CcmN, or CcmM73 and itself required the N-terminal gamma-CA-like domain of CcmM73. The specificity of the CcmM73-CcaA interaction provided both a mechanism to integrate CcaA into the fabric of the carboxysome shell and a means to recruit this enzyme to the BDC during carboxysome biogenesis. Functionally, CcaA was the catalytic core of the BDC. CcmM73 bound H14CO3- but was unable to catalyze HCO3- dehydration, suggesting that it may potentially regulate BDC activity.
authors.affiliation: Department of Biology, University of Toronto, Mississauga, Mississauga, Ontario L5L 1C6, Canada.
authors.forename: Swan S-W
authors.initials: SS
authors.lastname: Cot
authors.forename: Anthony K-C
authors.initials: AK
authors.lastname: So
authors.forename: George S
authors.initials: GS
authors.lastname: Espie
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D019281:Dimerization; D015964:Gene Expression Regulation, Bacterial; D046912:Multiprotein Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015388:Organelles / Q000378:metabolism*; D012273:Ribulose-Bisphosphate Carboxylase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013329:Structure-Activity Relationship; D046939:Synechocystis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
chemical_list: D001426:Bacterial Proteins; D001639:Bicarbonates; D046912:Multiprotein Complexes; D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase