Major Urinary Proteins belong to the lipocalin family and are present in the urine of rodents as an ensemble of isoforms with pheromonal activity. The crystal structure of a recombinant mouse MUP (rMUP) was solved by molecular replacement technique and refined to R-factor (Rfree) of 20% (26.5%) at 1.75 Å resolution. The structure was compared to a NMR model and to a crystallographic structure of the wild-type form of the protein. The crystal structures determined in different space groups present significantly smaller conformational differences between themselves when compared to NMR models. Some, but not all, conformational differences between crystal and solution structures can be explained by the influence of crystallographic contacts. Most of the differences between NMR and X-ray structures have been found in the N-terminus and loop regions. A number of side-chains lining the hydrophobic pocket of the molecule are tighter packed in the NMR structure when compared to the crystallographic model. Surprisingly, clear and continuous electron density for a ligand was observed inside the hydrophobic pocket of this recombinant protein. Conformation of the ligand modelled inside the density is coherent with the results of NMR experiments.
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