Ubiquitin-fusion protein system: a powerful tool for ectopic protein expression in mammalian cells.

Clonal cell lines stably expressing a protein of interest are frequently used in the characterization of the potential physiological functions of mammalian proteins, but the generation of such cell lines is laborious and time-consuming. Here, we made use of the fact that fusion proteins consisting of ubiquitin linked to the N terminus of a protein of interest are efficiently processed by ubiquitin-specific proteases into their respective free proteins within eukaryotic cells, and designed an expression system that allows easy and highly efficient selection of mammalian cells ectopically expressing a protein of interest.