Crystallization and Characterization of Galdieria sulphuraria RUBISCO in Two Crystal Forms: Structural Phase Transition Observed in P21 Crystal Form

We have isolated ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO) from the red algae Galdieria Sulphuraria. The protein crystallized in two different crystal forms, the I422 crystal form being obtained from high salt and the P2₁ crystal form being obtained from lower concentration of salt and PEG. We report here the crystallization, preliminary stages of structure determination and the detection of the structural phase transition in the P2₁ crystal form of G. sulphuraria RUBISCO. This red algae enzyme belongs to the hexadecameric class (L8S8) with an approximate molecular weight 0.6MDa. The phase transition in G. sulphuraria RUBISCO leads from two hexadecamers to a single hexadecamer per asymmetric unit. The preservation of diffraction power in a phase transition for such a large macromolecule is rare.