The physiological ligand of this protein is not really known
as yet - it occurs only in the urine of male rats, and several
compounds bind to it that cause nephropathy.
There is extensive literature available (see MedLine) since
the protein is interesting to toxicologists.
The 2.5 A structure is supposedly without any bound ligand,
although we see a clear "blob" of unexplained density in
the ligand-binding cavity.