Biogenesis of a Bacterial Organelle: The Carboxysome Assembly Pathway

Abstract

The carboxysome is a protein-based organelle for carbon fixation in cyanobacteria, keystone organisms in the global carbon cycle. It is composed of thousands of subunits including hexameric and pentameric proteins that form a shell to encapsulate the enzymes ribulose 1,5-bisphosphate carboxylase/oxygenase and carbonic anhydrase. Here, we describe the stages of carboxysome assembly and the requisite gene products necessary for progression through each. Our results demonstrate that, unlike membrane-bound organelles of eukaryotes, in carboxysomes the interior of the compartment forms first, at a distinct site within the cell. Subsequently, shell proteins encapsulate this procarboxysome, inducing budding and distribution of functional organelles within the cell. We propose that the principles of carboxysome assembly that we have uncovered extend to diverse bacterial microcompartments.

Light Modulates the Biosynthesis and Organization of Cyanobacterial Carbon Fixation Machinery through Photosynthetic Electron Flow

“…We chose Synechococcus as the model organism due to its superior genetic tractability and proven suitability for fluorescence imaging (Savage et al, 2010;Liu et al, 2012;Cameron et al, 2013;Cohen et al, 2014). RbcL, the large subunit of Rubisco that resides in the b-carboxysome lumen, was tagged at the C terminus with enhanced GFP (eGFP) and was visualized under confocal fluorescence microscopy to characterize the formation and positioning of carboxysomes in vivo.…”

“…On the other hand, despite potential repairing mechanism led by protein dynamics (Sutter et al, 2016), there appears to be no specific degradation pathway for carboxysomes. The stability of mature carboxysomes in vivo may be of physiological importance for the cellular metabolism (Cameron et al, 2013).…”

“…Recent studies elucidated that the b-carboxysome assembly is initiated from the packing of Rubisco enzymes, followed by the encapsulation of peripheral shell proteins (Cameron et al, 2013;Chen et al, 2013). In the model rod-shaped cyanobacterium Synechococcus elongatus PCC7942 (hereafter Synechococcus), three to four b-carboxysomes were observed to be evenly spaced along the centerline of the longitudinal axis of cells, ensuring the equal segregation of the machinery between daughter cells (Savage et al, 2010).…”

Diverse Bacterial Microcompartment Organelles

“…Recent work with the ␤-carboxysome of Synechococcus indicates that the cargo proteins (RuBisCO and CA) assemble into a procarboxysome by a process that requires CcmM (55,56). Subsequently or concomitantly, the shell forms around the cargo, pinching off excess cargo, which serves as a nucleus for biogenesis of the next carboxysome (55,56).…”

“…Recent work with the ␤-carboxysome of Synechococcus indicates that the cargo proteins (RuBisCO and CA) assemble into a procarboxysome by a process that requires CcmM (55,56). Subsequently or concomitantly, the shell forms around the cargo, pinching off excess cargo, which serves as a nucleus for biogenesis of the next carboxysome (55,56). The formation of the shell around the procarboxysome is thought to start with the binding of CcmN, after which a C-terminal peptide of CcmN recruits the additional shell components CcmK2, CcmO, and CcmL, leading to the enclosure process (55).…”