A popular method for studying the function of a given protein is to generate and characterize a suitable model deficient for its expression. Most researchers select mouse models for this purpose and investigate proteins of interest in the tissue they are most associated with. For the prion protein (PrP), best known for its role in several invariably fatal neurodegenerative diseases of humans and animals, a natural choice, therefore, would be to undertake such studies with mouse brain samples. We recently documented the surprising observation that PrP deficiency caused a loss or enhancement of NCAM1 polysialylation, dependent on the cell model used. To identify possible causes for this disparity, we set out to systematically investigate the consequence of PrP deficiency on the global proteome in brain tissue and in four distinct cell models. Here we report that PrP deficiency has no discernible impact on the global brain proteome but causes robust but surprisingly divergent changes to the global proteomes of cell models. Our data suggest that careful investigations in cell models, rather than mouse tissue, are the way forward for elucidating the molecular function of PrP.