Abstract 2522 The Role of UbpA in Nutrient Sensing and Controlling the Growth-to-Development Transition in Dictyostelium

The ubiquitin processing protease, UbpA (a homolog of human USP5), is required for the growth-to-development transition (GDT) of the social amoeba Dictyostelium.UbpA functions in the disassembly of free polyubiquitin chains, and at the GDT, ubpA cells accumulate ubiquitin-conjugated proteins.In addition, ubpA cells misregulate genes at the GDT, particularly those required for chemotaxis, and the cells fail to aggregate unless given exogenous pulses of cAMP.We have previously shown that UbpA is required for expression of LmcB, a nutrient sensor necessary for the GDT.LmcB has predicted structural similarity to esterases and binds the cytidine deaminase, CdaA.Because cytidine deaminase (CDA) activity increases shortly after the GDT, it is possible that CdaA may play a role in inhibiting cell proliferation.Here we show a spectrophotometric analysis that demonstrated that CdaA has CDA activity on cytidine and significantly increased activity on deoxycytidine, decreased activity on the chemotherapy drug cytarabine, and substantially decreased activity on cytidine in the presence of the inhibitor tetrahydrouridine.In addition to functioning early in the GDT, UbpA appears to have broad effects.Proteasome 20S activity assays showed that proteasome activity in ubpA cells was reduced to 42% compared to wild-type levels.Protein levels of Atg8, a marker for autophagy, and VatM, a marker for lysosomal degradation, were substantially lower in ubpA cells, as shown by western analysis and immunofluorescence.These results suggest that UbpA may play a role in proteasomal and lysosomal degradation, and autophagy.This investigation will help elucidate the link between the biochemical function of UbpA and the UbpA-dependent pathways at the GDT.