Sliding-layer conformational change limited by the quaternary structure of plant RuBisCO

RuBisCO, D-ribulose-l,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39), converts carbon dioxide to sugar in the first step of photosynthesis ^1,2. In plants and some bacteria, this enzyme has an L ₈S₈ structure, where L is the large catalytic subunit and S is the small subunit of unknown function. The molecule resembles a keg 105 Å along the 4-fold axis and 132 Å in diameter at the widest point of the keg^3-6. Here we describe the quaternary structure of RuBisCO from N. tabacum, the first L₈S₈ type known from an X-ray crystallographic study at near-atomic resolution (3 Å). The structure shows that all eight L subunits are elongated along the 4-fold axis so that the molecule cannot be simply described as layers of subunits, as it had been from studies by electron microscopy^5,6. The structure, with its elongated and inter-digitated L subunits, is evidence against a large, sliding-layer conformational change in plant RuBisCO, as proposed recently in Nature for the same enzyme from Alcaligenes eutrophus⁷.