Membraneless Organelles and Viscosity Changes during Desiccation: A New Phase in Anhydrobiosis

Abstract

AfrLEA6 is an intrinsically disordered protein found in the anhydrobiotic cysts of Artemia franciscana. AfrLEA6 shares homology with plant seed maturation proteins (SMPs), which have been linked to the longevity of orthodox seeds in the desiccated state. Therefore, it was hypothesized that ectopically expressing AfrLEA6 will increase the desiccation tolerance of a desiccation‐sensitive Drosophila melanogaster (Kc167) cell line. Surprisingly, ectopically expressed AfrLEA6 was found to undergo a liquid‐liquid phase separation (LLPS) in the cytoplasm of Kc167 cells. The overall architecture of the protein is composed of two SMP domains connected with a glycine‐ and proline‐rich spacer to a potential protein‐binding domain. It was determined that AfrLEA6 LLPS is dependent on the SMP domains, regulated by polysome assembly and disassembly, and may best be described as a specialized biphasic stress granule. It is speculated that AfrLEA6 stress granules protect desiccation‐sensitive proteins prior to glassy‐state formation during water loss. This hypothesis was strengthened by the observation that expression of AfrLEA6 in Kc167 cells increases their desiccation tolerance. Unexpectedly, AfrLEA6 also increases intracellular viscosity during desiccation compared to control cells, which reduced plasma membrane fusion between adjacent cells and may protect against internal sheer stresses. These data agree with recently reported cellular dielectrophoretic data, that AfrLEA6 expressed in Kc167 cells underwent a shift to become more entangled when cells were exposed to osmotic stress, thereby suggesting an increase intracellular viscosity. In A. franciscana, a rapid increase in cytoplasmic viscosity at relatively high‐water contents may be key to successfully entering the anhydrobiotic state, while trehalose glass formation at relatively low‐water contents may create a more stable intracellular environment for long‐term dormancy.Support or Funding InformationSupported by NSF IOS‐1659970.