A genetic interaction between a ubiquitin-like protein and ubiquitin-mediated proteolysis in Dictyostelium discoideum(1)

A ubiquitination factor, NosA, is essential for cellular differentiation in Dictyostelium discoideum. In the absence of nosA, development is blocked, resulting in a developmental arrest at the tight-aggregate stage, when cells differentiate into two precursor cell types, prespore and prestalk cells. Development is restored when a second gene, encoding the ubiquitin-like protein SonA, is inactivated in nosA-mutant cells. SonA has homology over its entire length to Dsk2 from Saccharomyces cerevisiae, a ubiquitin-like protein that is involved in the assembly of the spindle pole body. Dsk2 and SonA are both stable proteins that do not seem to be subjected to degradation via the ubiquitin pathway. SonA does not become ubiquitinated and the intracellular levels of SonA are not affected by the absence of NosA. The high degree of suppression suggests that SonA rescues most or all of the defects caused by the absence of nosA. We propose that NosA and SonA act in concert to control the activity of a developmental regulator that must be deactivated for cells to cross a developmental boundary.