On the Quaternary Assembly of Spinach Chloroplast Thioredoxin m

Thioredoxin m from spinach chloroplast has been structurally characterized both by X-ray crystallography and by NMR. Thioredoxin m is known to be monomeric, a finding which is confirmed by the NMR results. The crystal structure of this protein, however, contains two independent molecules per asymmetric unit. This fact was interpreted as contrasting with the NMR results [Neira et al. (2001) Biochemistry 40: 15246–15256]. Based on computational and biochemical considerations, we show that the presence of two thioredoxin m molecules per asymmetric unit bears no biological significance and does not contrast with the NMR results. The non-covalent arrangement of two monomers found in the crystals represents a ‘crystallization intermediate’ formed under the conditions for crystal growth.