Molecular complexity of the major urinary protein system of the Norway rat,Rattus norvegicus

Major urinary proteins (MUP) are the major component of the urinary protein fraction in house mice (Musspp.) and rats (Rattusspp.). The structure, polymorphism and functions of these lipocalins have been well described in the western European house mouse (Mus musculus domesticus), clarifying their role in semiochemical communication. The complexity of these roles in the mouse raises the question of similar functions in other rodents, including the Norway rat,Rattus norvegicus. Norway rats express MUPs in urine but information about specific MUP isoform sequences and functions is limited. In this study, we present a detailed molecular characterization of the MUP proteoforms expressed in the urine of two laboratory strains, Wistar Han and Brown Norway, and wild caught animals, using a combination of manual gene annotation, intact protein mass spectrometry and bottom-up mass spectrometry-based proteomic approaches. Detailed sequencing of the proteins reveals a less complex pattern of primary sequence polymorphism than the mouse. However, unlike the mouse, rat MUPs exhibit added complexity in the form of post-translational modifications including phosphorylation and exoproteolytic trimming of specific isoforms. The possibility that urinary MUPs may have different roles in rat chemical communication than those they play in the house mouse is also discussed.