C phycocyanin as a highly attractive model system in protein crystallography Unique crystallization properties and packing diversity screening

The unique crystallization properties of the antenna protein C phycocyanin C PC from the thermophilic cyanobacterium Thermosynechococcus elongatus are reported and discussed. C PC crystallizes in hundreds of significantly different conditions within a broad pH range and in the presence of a wide variety of precipitants and additives. Remarkably, the crystal dimensions vary from a few micrometres, as used in serial crystallography, to several hundred micrometres, with a very diverse crystal morphology. More than 100 unique single crystal X ray diffraction data sets were collected from randomly selected crystals and analysed. The addition of small molecule additives revealed three new crystal packings of C PC, which are discussed in detail. The high propensity of this protein to crystallize, combined with its natural blue colour and its fluorescence characteristics, make it an excellent candidate as a superior and highly adaptable model system in crystallography. C PC can be used in technical and methods development approaches for X ray and neutron diffraction techniques, and as a system for comprehending the fundamental principles of protein crystallograph