Intrinsically disordered CsoS2 acts as a general molecular thread for α-carboxysome shell assembly
Carboxysomes are a paradigm of self-assembling proteinaceous organelles found in nature, offering compartmentalisation of enzymes and pathways to enhance carbon fixation. In -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 engineered -carboxysome shells using minimal shell components and determined 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 uncovered in CsoS2 a remarkable 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-govern carboxysome shell assembly and cargo encapsulation and further advances synthetic engineering of carboxysomes for diverse biotechnological applications.