Solution Structure of a Recombinant Mouse Major Urinary Protein

Major urinary proteins (MUPs) form an ensemble of protein isoforms which are expressed and secreted by sexually mature male mice only. They belong to the lipocalin superfamily and share with other members of this family the capacity to bind hydrophobic molecules, some of which are odorants. MUPs, either associated with or free of their natural ligands, play an important role in the reproductive cycle of these rodents by acting as pheromones. In fact, they are able to interact with receptors in the vomeronasal organ of the female mice, inducing hormonal and physiological responses by an as yet unknown mechanism. In order to investigate the structural and dynamical features of these proteins in solution, one of the various wild-type isoforms (rMUP: 162 residues) was cloned and subsequently isotopically labeled. The complete 1H, 13C and 15N resonance assignments of that isoform, achieved by using a variety of multi-dimensional heteronuclear NMR experiments, has been reported recently. Here we describe the refined high-resolution three-dimensional solution structure of rMUP in the native state, obtained by a combination of distance geometry and energy minimization calculations based on 2362 NOE-derived distance restraints. A comparison with the crystal structure of the wild-type MUPs reveals, aside from minor differences, a close resemblance in both secondary structure and overall topology. The secondary structure of the protein consists of eight antiparallel beta-strands forming a single beta-sheet and an alpha-helix in the C-terminal region. In addition, there are several helical and hairpin turns distributed throughout the protein sequence, mostly connecting the beta-strands. The tertiary fold of the beta-sheet creates a beta-barrel, common to all members of the lipocalin superfamily. The shape of the beta-barrel resembles a calyx, lined inside by mostly hydrophobic residues that are instrumental for the binding and transport of small non-polar ligand molecules.