A Heat Shock Protein 48 (HSP48) Biomolecular Condensate Is Induced during Dictyostelium discoideum Development

The social amoeba Dictyostelium discoideum 's proteome contains a vast array of simple sequence repeats, providing a unique model to investigate proteostasis. Upon conditions of cellular stress, D. discoideum undergoes a developmental process, transitioning from a unicellular amoeba to a multicellular fruiting body. Little is known about how proteostasis is maintained during D. discoideum 's developmental process. Here, we have identified a novel α-crystallin domain-containing protein, heat shock protein 48 (HSP48), that is upregulated during D. discoideum development. HSP48 functions in part by forming a biomolecular condensate via its highly positively charged intrinsically disordered carboxy terminus. In addition to HSP48, the highly negatively charged primordial chaperone polyphosphate is also upregulated during D. discoideum development, and polyphosphate functions to stabilize HSP48. Upon germination, levels of both HSP48 and polyphosphate dramatically decrease, consistent with a role for HSP48 and polyphosphate during development. Together, our data demonstrate that HSP48 is strongly induced during Dictyostelium discoideum development. We also demonstrate that HSP48 forms a biomolecular condensate and that polyphosphate is necessary to stabilize the HSP48 biomolecular condensate. IMPORTANCE During cellular stress, many microbes undergo a transition to a dormant state. This includes the social amoeba Dictyostelium discoideum that transitions from a unicellular amoeba to a multicellular fruiting body upon starvation. In this work, we identify heat shock protein 48 (HSP48) as a chaperone that is induced during development. We also show that HSP48 forms a biomolecular condensate and is stabilized by polyphosphate. The findings here identify Dictyostelium discoideum as a novel microbe to investigate protein quality control pathways during the transition to dormancy.