Enhanced phosphorylation and dephosphorylation of a histone-like protein in response to hyperosmotic and hypoosmotic conditions.

Placement of endothelial cells under hypoosmotic or hyperosmotic conditions results in the reduction and increase, respectively, in the phosphorylation of a M(r) = 16,500 protein (P17). The changes were dose-dependent with a 3.3 +/- 0.3-fold increase occurring at 485 mosm/kg H2O and negligible phosphorylation observed at 202 mosm/kg H2O. The phosphorylation and dephosphorylation were rapid and prolonged; modified phosphorylation levels were maintained as long as the anisotonic conditions were present. However, return to isotonic medium reversed the phosphorylation back to normal within 1 h. Cellular fractionation studies showed that P17 was associated only with the nuclear compartment under isotonic, hypertonic, or hypotonic conditions. Two forms of P17 with pI values of 9.2 and 9.6 were resolved by isoelectric focusing; both forms showed enhanced phosphorylation by hyperosmotic treatment. Phosphorylation occurred on serines exclusively. These studies demonstrate that a nuclear protein with characteristics similar to histones is affected by cell shrinkage or swelling through changes in its phosphorylation state.