SPLIT UBIQUITIN AS A SENSOR OF PROTEIN INTERACTIONS IN-VIVO

We describe an assay for in vivo protein interactions. Protein fusions containing ubiquitin, a 76-residue, single-domain protein, are rapidl y cleaved in vivo by ubiquitin-specific proteases, which recognize the folded conformation of ubiquitin. When a C-terminal fragment of ubiqu itin (C-ub) is expressed as a fusion to a reporter protein, the fusion is cleaved only if an N-terminal fragment of ubiquitin (N-ub) is also expressed in the same cell. This reconstitution of native ubiquitin f rom its fragments, detectable by the in vivo cleavage assay, is not ob served with a mutationally altered N-ub. However, if C-ub and the alte red N-ub are each linked to polypeptides that interact in vivo, the cl eavage of the fusion containing C-ub is restored, yielding a generally applicable assay for kinetic and equilibrium aspects of in vivo prote in interactions. This method, termed USPS (ubiquitin-based split-prote in sensor), makes it possible to monitor a protein-protein interaction as a function of time, at the natural sites of this interaction in a living cell.