6hbc
Structure of the repeat unit in the network formed by CcmM and Rubisco from Synechococcus elongatus
Structural highlights
Function[CCMM_SYNE7] The presence of two potential DNA-binding regions suggests this protein may be a transcriptional regulator. [RBL_SYNE7] RuBisCO catalyzes two reactions: the carboxylation of D-ribulose 1,5-bisphosphate, the primary event in carbon dioxide fixation, as well as the oxidative fragmentation of the pentose substrate in the photorespiration process. Both reactions occur simultaneously and in competition at the same active site. Publication Abstract from PubMedCells use compartmentalization of enzymes as a strategy to regulate metabolic pathways and increase their efficiency(1). The alpha- and beta-carboxysomes of cyanobacteria contain ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-a complex of eight large (RbcL) and eight small (RbcS) subunits-and carbonic anhydrase(2-4). As HCO3(-) can diffuse through the proteinaceous carboxysome shell but CO2 cannot(5), carbonic anhydrase generates high concentrations of CO2 for carbon fixation by Rubisco(6). The shell also prevents access to reducing agents, generating an oxidizing environment(7-9). The formation of beta-carboxysomes involves the aggregation of Rubisco by the protein CcmM(10), which exists in two forms: full-length CcmM (M58 in Synechococcus elongatus PCC7942), which contains a carbonic anhydrase-like domain(8) followed by three Rubisco small subunit-like (SSUL) modules connected by flexible linkers; and M35, which lacks the carbonic anhydrase-like domain(11). It has long been speculated that the SSUL modules interact with Rubisco by replacing RbcS(2-4). Here we have reconstituted the Rubisco-CcmM complex and solved its structure. Contrary to expectation, the SSUL modules do not replace RbcS, but bind close to the equatorial region of Rubisco between RbcL dimers, linking Rubisco molecules and inducing phase separation into a liquid-like matrix. Disulfide bond formation in SSUL increases the network flexibility and is required for carboxysome function in vivo. Notably, the formation of the liquid-like condensate of Rubisco is mediated by dynamic interactions with the SSUL domains, rather than by low-complexity sequences, which typically mediate liquid-liquid phase separation in eukaryotes(12,13). Indeed, within the pyrenoids of eukaryotic algae, the functional homologues of carboxysomes, Rubisco adopts a liquid-like state by interacting with the intrinsically disordered protein EPYC1(14). Understanding carboxysome biogenesis will be important for efforts to engineer CO2-concentrating mechanisms in plants(15-19). Rubisco condensate formation by CcmM in beta-carboxysome biogenesis.,Wang H, Yan X, Aigner H, Bracher A, Nguyen ND, Hee WY, Long BM, Price GD, Hartl FU, Hayer-Hartl M Nature. 2019 Feb;566(7742):131-135. doi: 10.1038/s41586-019-0880-5. Epub 2019 Jan, 23. PMID:30675061[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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