Mitochondrial regulation in Dictyostelium discoideum

Proteolysis is increasingly documented as a method of regulation of mitochondrial function. Our studies of rhomboidfamily proteins’ roles in organelles show that this is also the case in the social amoeba Dictyostelium discoideum, in which four of these membrane-bound, evolutionarily ubiquitous, serine proteases are found. Rhomboid proteases act on disparate substrates in different organisms so far studied, but their mode of action is conserved: their location in the membrane means that their membrane-tethered substrates can act in signalling upon release, or be activated, by rhomboid-mediated cleavage. Among eukaryotic rhomboids is the mitochondrial protease ‘PARL’, which ensures the maintenance of the structural and functional integrity of mitochondria and plastids, but we have found that other Dictyostelium rhomboids also affect the organelle. Studying the development and behaviour of Dictyostelium, a microbial model organism with a complex life cycle that includes uni- and multicellular stages, allowed investigation of the role of rhomboids in unicellular vegetative growth, multicellular development and sporulation, phagocytosis, and response to the environment.

We found that two rhomboid-null mutants gave rise to changes in development, rhmA altering the response to chemoattractants and demonstrating decreased motility in general, whereas rhmB null cells had slower growth rates with decreased response to folic acid. RhmA, although located in the contractile vacuole, affects the ultrastructure of mitochondria, and RhmB-GFP fusions protein was localised to the mitochondrion. qPCR analysis revealed RhmA and RhmB transcript levels peaking during the multicellular growth phase and transcriptional networks suggest the Dictyostelium rhmA is regulated along with the orthologues of Saccharomyces cerevisiae mitochondrial rhomboid substrates.