Dictyostelium macroautophagy mutants vary in the severity of their developmental defects

Macroautophagy is the major mechanism that eukaryotes use to recycle cellular components during stressful conditions. We have shown previously that the Atg12-Atg5 conjugation system, required for autophagosome formation in yeast, is necessary for Dictyostelium development. A second conjugation reaction, Aut7/Atg8 lipidation with phosphatidylethanolamine, as well as a protein kinase complex and a phosphatidylinositol 3-kinase complex are also required for macroautophagy in yeast. In this study, we characterize mutations in the putative Dictyostelium discoideum orthologues of budding yeast genes that are involved in one of each of these functions, ATG1, ATG6, and ATG8. All three genes are required for macroautophagy in Dictyostelium. Mutant amoebae display reduced survival during nitrogen starvation and reduced protein degradation during development. Mutations in the three genes produce aberrant development with defects of varying severity. As with other Dictyostelium macroautophagy mutants, development of atg1-1, atg6(-), and atg8(-) is more aberrant in plaques on bacterial lawns than on nitrocellulose filters. The most severe defect is observed in the atg1-1 mutant, which does not aggregate on bacterial lawns and arrests as loose mounds on nitrocellulose filters. The atg6(-) and atg8(-) mutants display almost normal development on nitrocellulose filters, producing multi-tipped aggregates that mature into small fruiting bodies. The distribution of a green fluorescent protein fusion of the autophagosome marker, Atg8, is aberrant in both atg1-1 and atg6(-) mutants.