Protease which deconjugates SUMO from some substrate proteins. Has isopeptidase but not SUMO-processing activity (By similarity). Collaborates with ubql-1 in the export of ubiquitinated proteins from the nucleus to the cytoplasm (PubMed:29666234). Expressed in the pharynx, hypodermis, intestine, head neuron and tail neuron. Worms lacking both POST and ubql-1 have a shorter lifespan and display an accumulation of ubiquitinated proteins in the nucleus. Belongs to the DeSI family.