1. A polynucleotide encoding a protease-regulated polypeptide comprising the following operatively linked elements: (a) a protein degradation signal; (b) a polypeptide of interest; and (c) a linker disposed between (a) and (b), wherein the linker comprises a protease recognition motif that is cleaved by a protease; wherein, in the presence of the protease, cleavage of the linker protects the polypeptide of interest from proteasomal degradation; and, in the absence of the protease, activity of the polypeptide of interest is decreased.