Efficient site-specific processing of fusion proteins by tobacco vein mottling virus protease in vivo and in vitro

Affinity tags are widely used as vehicles for the production of recombinant proteins. Yet, because of concerns about their potential to interfere with the activity or structure of proteins, it is almost always desirable to remove them from the target protein. The proteases that are most often used to cleave fusion proteins are factor Xa, enterokinase, and thrombin, yet the literature is replete with reports of fusion proteins that were cleaved by these proteases at locations other than the designed site. It is becoming increasingly evident that certain viral proteases have more stringent sequence specificity. These proteases adopt a trypsin-like fold but possess an unconventional catalytic triad in which Cys replaces Ser. The tobacco etch virus (TEV) protease is the best-characterized enzyme of this type. TEV protease cleaves the sequence ENLYFQG/S between QG or QS with high specificity. The tobacco vein mottling virus (TVMV) protease is a close relative of TEV protease with a distinct sequence specificity (ETVRFQG/S). We show that, like TEV protease, TVMV protease can be used to cleave fusion proteins with high specificity in vitro and in vivo. We compared the catalytic activity of the two enzymes as a function of temperature and ionic strength, using an MBP-NusG fusion protein as a model substrate. The behavior of TVMV protease was very similar to that of TEV protease. Its catalytic activity was greatest in the absence of NaCl, but diminished only threefold with increasing salt up to 200 mM. We found that the optimum temperatures of the two enzymes are nearly the same and that they differ only two-fold in catalytic efficiency, both at room temperature and 4 degrees C. Hence, TVMV protease may be a useful alternative to TEV protease when a recombinant protein happens to contain a sequence that is similar to a TEV protease recognition site or for protein expression strategies that involve the use of more than one protease.