A SUMO-Groucho Q domain fusion protein: characterization and in vivo Ulp1-mediated cleavage.

We describe here a system for the expression and purification of small ubiquitin-related modifier (SUMO) fusion proteins, which often exhibit dramatically increased solubility and stability during expression in bacteria relative to unfused proteins. The vector described here allows expression of a His-tagged protein of interest fused at its N-terminus to SUMO. Using this vector, we have produced a polypeptide consisting of SUMO fused to the Q domain of Drosophila Groucho in a concentrated soluble form. Hydrodynamic analysis shows that, consistent with previous studies on full-length Groucho, the fusion protein forms an elongated tetramer, as well as higher order oligomers. After expressing a protein as a fusion to SUMO, it is often desirable to cleave the SUMO off of the fusion protein using a SUMO-specific protease such as Ulp1. To facilitate such processing, we have constructed a dual expression vector encoding two fusion proteins: one consisting of SUMO fused to Ulp1 and a second consisting of SUMO fused to a His-tagged protein of interest. The SUMO–Ulp1 cleaves both itself and the other SUMO fusion protein in the bacterial cells prior to lysis, and the proteins retain solubility after cleavage.