Red-type Rubiscos

Structural analyses of the first identified activase for red-type Rubiscos reveal key insights into Rubisco activation.


The authors propose that Cbbx (left) releases RuBP from Rubisco (right) through transient interactions with the C-terminal tail of the Rubisco large subunit (red wire). Figure courtesy of Andreas Bracher.

Red-type Rubiscos, present in photosynthetic bacteria, red algae, and phytoplankton, are responsible for most of the oceanic carbon uptake. Understanding the catalytic cycle of red-type Rubiscos could therefore aid in improving the CO2 uptake and biomass productions of photosynthetic organisms.

Rubiscos catalyze the carboxylation of Ribulose-1,5-bisphosphate (RuBP) by CO2 in the first step of carbon fixation in photosynthesis. Binding of Rubisco to RuBP in the absence of active site carbamylation creates an inactive complex that must be reactivated by Rubisco activase (Rca), which catalyzes the release of RuBP from Rubisco in an ATP-dependent manner. While Rca has been identified in green algae and plants, no Rca homolog has been identified in organisms containing red-type Rubiscos.

Mueller-Cajar and colleagues have now identified and characterized the Rubisco activase CbbX from the proteobacterium Rhodobacter sphaeroides. Like Rca, CbbX is an AAA+ protein with ATPase activity and is able to activate inhibited Rubisco in the presence of ATP. Unlike Rca, CbbX has no inherent ATPase activity in the absence of RuBP. Therefore, RuBP acts as an allosteric regulator of CbbX and ensures that the enzyme is only active during photosynthesis, when levels of RuBP are high.

Analysis of CbbX by negative stain electron microscopy revealed that CbbX forms ring-like structures similar to other AAA+ proteins in the presence of ATP and RuBP. The crystal structure of CbbX reveals a typical AAA+ fold with an N-terminal α/β domain and a C-terminal α-helical domain. The α/β domain contains the canonical Walker A and B motifs, which are important for binding ATP, and a conserved pore loop. The position of two bound sulfate ions in the α-helical domain reveals the likely binding site for RuBP.

Based on structural analyses, the authors proposed a model for Rubisco activation in which the C-terminal extension present in red-type Rubiscos, but not green-type Rubiscos, is pulled into the core of CbbX by the conserved pore loop. This pulling force opens up the active site of Rubisco and enables the inhibitory RuBP molecule to leave.

Jennifer Cable

References:

O. Mueller-Cajar et al. Structure and function of the AAA+ protein CbbX, a red-type Rubisco activase.
Nature. 479, 194-199 (2011). doi:10.1038/nature10568