the  and  subunits of pcc were co - expressed in e. coli , with a his - tag on the  subunit . crystals were obtained by the microbatch method under oil , and the structures were determined by the molecular replacement method . frozen hydrated human pcc particles at 70 g / ml concentration were imaged at 50,000 magnification in a 100 kv cryo - electron microscope . a featureless gaussian oval was used to obtain a low resolution ( 40  ) model from negative stain electron microscope images . a 15  resolution 3d reconstruction was obtained from ~10,000 cryo - em particle images , using the structure from the negative stain images as the initial model . site - specific and deletion mutants were designed based on the structural information , and their effects on the formation of the holoenzyme was assessed by nickel affinity chromatography . the catalytic activity of pcc was determined by a coupled enzyme assay , monitoring the hydrolysis of atp . full methods and any associated references are available in the online version of the paper at www.nature.com/nature . the  and  subunits of pcc were co - expressed in e. coli , with a his - tag on the  subunit . crystals were obtained by the microbatch method under oil , and the structures were determined by the molecular replacement method . frozen hydrated human pcc particles at 70 g / ml concentration were imaged at 50,000 magnification in a 100 kv cryo - electron microscope . a featureless gaussian oval was used to obtain a low resolution ( 40  ) model from negative stain electron microscope images . a 15  resolution 3d reconstruction was obtained from ~10,000 cryo - em particle images , using the structure from the negative stain images as the initial model . site - specific and deletion mutants were designed based on the structural information , and their effects on the formation of the holoenzyme was assessed by nickel affinity chromatography . the catalytic activity of pcc was determined by a coupled enzyme assay , monitoring the hydrolysis of atp . full methods and any associated references are available in the online version of the paper at www.nature.com/nature .