The invention provides isolated SUMO-specific protease-like (or "SSP") domain-containing polypeptides from microorganisms, including bacteria, protozoans and yeast, including Escherichia, Salmonella, Pseudomonas, Chlamydia, Plasmodium, Trypanosma, Mesorhizobium, Rickettsia, Cryptosporidium and Candida species. The invention further provides modifications of such polypeptides, functional fragments therefrom, encoding nucleic acid molecules and specific antibodies. Also provided are methods for identifying polypeptides and compounds that associate with or modulate the activity of the SSP domain-containing polypeptides. Further provided are methods of modulating a biological activity in a cell, and treating pathological conditions, using the described nucleic acid molecules, polypeptides and compounds.