Rearrangement of cortex proteins constitutes an osmoprotective mechanism in Dictyostelium

Dictyostelium responds to hyperosmotic stress of 400 mOsm by a rapid reduct ion of its cell volume to 50%, The reduced cell volume is maintained as lon g as these osmotic conditions prevail. Dictyostelium does not accumulate co mpatible osmolytes to counteract the osmotic pressure applied. Using two-di mensional gel electrophoresis, we demonstrate that during the osmotic shock the protein pattern remains unaltered in whole-cell extracts. However, whe n cells were fractionated into membrane and cytoskeletal fractions, alterat ions of specific proteins could be demonstrated. In the crude membrane frac tion, a 3-fold increase in the amount of protein was measured upon hyperosm otic stress, In the cytoskeletal fraction, the proteins DdLIM and the regul atory myosin light chain (RMLC) were shown to be regulated in the osmotic s tress response. The elongation factors eEF1 alpha (ABP50) and eEF1 beta wer e found to increase in the cytoskeletal fraction, suggesting a translationa l arrest upon hyperosmotic stress, Furthermore, the two main components of the cytoskeleton, actin and myosin II, are phosphorylated as a consequence of the osmotic shock, with a tyrosine residue as the phosphorylation site o n actin and three threonines in the case of the myosin II heavy chain.