8b12
cryo-EM structure of carboxysomal mini-shell: icosahedral assembly from CsoS4A/1A and CsoS2 co-expression (T = 9)
Structural highlights
FunctionCSOSA_HALNC The major shell protein of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, ccbL-ccbS) is sequestered (PubMed:16535117, PubMed:7934888). Assembles into hexamers which make sheets that form the facets of the polyhedral carboxysome (PubMed:17518518). The shell probably limits the diffusion of CO(2) into and out of the carboxysome (Probable). Molecular modeling shows the central pore of this protein is selectively permeable to anions such as HCO(3) rather than CO(2) or O(2) (Probable). There are estimated to be 2970 CsoS1A/CsoS1C proteins per carboxysome (the proteins differ by only 1 residue) (Ref.5).[1] [2] [3] [4] [5] [6] Unlike beta-carboxysomes, alpha-carboxysomes (Cb) can form without cargo protein. CsoS2 is essential for Cb formation and is also capable of targeting foreign proteins to the Cb. The Cb shell assembles with the aid of CsoS2; CsoS1A, CsoS1B and CsoS1C form the majority of the shell while CsoS4A and CsoS4B form vertices. CsoS1D forms pseudohexamers that probably control metabolite flux into and out of the shell.[7] [8] Publication Abstract from PubMedCarboxysomes are a paradigm of self-assembling proteinaceous organelles found in nature, offering compartmentalisation of enzymes and pathways to enhance carbon fixation. In alpha-carboxysomes, the disordered linker protein CsoS2 plays an essential role in carboxysome assembly and Rubisco encapsulation. Its mechanism of action, however, is not fully understood. Here we synthetically engineer alpha-carboxysome shells using minimal shell components and determine cryoEM structures of these to decipher the principle of shell assembly and encapsulation. The structures reveal that the intrinsically disordered CsoS2 C-terminus is well-structured and acts as a universal "molecular thread" stitching through multiple shell protein interfaces. We further uncover in CsoS2 a highly conserved repetitive key interaction motif, [IV]TG, which is critical to the shell assembly and architecture. Our study provides a general mechanism for the CsoS2-governed carboxysome shell assembly and cargo encapsulation and further advances synthetic engineering of carboxysomes for diverse biotechnological applications. Intrinsically disordered CsoS2 acts as a general molecular thread for alpha-carboxysome shell assembly.,Ni T, Jiang Q, Ng PC, Shen J, Dou H, Zhu Y, Radecke J, Dykes GF, Huang F, Liu LN, Zhang P Nat Commun. 2023 Sep 7;14(1):5512. doi: 10.1038/s41467-023-41211-y. PMID:37679318[9] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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