The X-ray structure of a recombinant major urinary protein at 1.75 angstrom resolution. A comparative study of X-ray and NMR-derived structures

Major urinary proteins belong to the lipocalin family and are present in th e urine of rodents as an ensemble of isoforms with pheromonal activity. The crystal structure of a recombinant mouse MUP (rMUP) was solved by the mole cular-replacement technique and refined to an R factor and R-free of 20 and 26.5%, respectively, at 1.75 Angstrom resolution. The structure was compar ed with an NMR model and with a crystallographic structure of the wild-type form of the protein. The crystal structures determined in different space groups present significantly smaller conformational differences amongst the mselves than in comparison with NMR models. Some, but not all, of the confo rmational differences between the crystal and solution structures can be ex plained by the influence of crystallographic contacts. Most of the differen ces between the NMR and X-ray structures were found in the N-terminus and l oop regions. A number of side chains lining the hydrophobic pocket of the m olecule are more tightly packed in the NMR structure than in the crystallog raphic model. Surprisingly, clear and continuous electron density for a lig and was observed inside the hydrophobic pocket of this recombinant protein. Conformation of the ligand modelled inside the density is coherent with th e results of recent NMR experiments.