The monovalent ionophore monensin maintains the nuclear localization of the human stress protein hsp28 during heat shock recovery

In HeLa cells exposed to supra-optimal temperatures, the alpha-crystallin-related stress protein hsp28 is reversibly redistributed inside the nucleus and increases its level of phosphorylation and aggregation. Here, I show that, at normal temperature after a heat stress, the sodium ionophore monensin maintains the nuclear localization of hsp28 without impairing the dephosphorylation of this protein. This phenomenon is not due to a prolongation, by monensin, of the synthesis of the heat-shock proteins after the heat stress. In contrast, the potassium ionophore nonactin induces only a weak alteration in the hsp28 locale, while the calcium ionophore A23187 and the uncoupler of oxidative phosphorylation FCCP have no effect. Following the removal of monensin 15 h after the heat stress, a further incubation of the cells for at least 36 h is necessary in order to observe a redistribution of hsp28 into the cytoplasm. A large fraction of hsp28 is then observed as dense excretion granules. In control cells kept at normal temperature, monensin, like nonactin, A23187 and FCCP, does not induce the redistribution of hsp28 inside the nucleus. Taken together, these results suggest that the disruption of the Na+ active transport by monensin probably inhibits the redistribution of hsp28 in the cytoplasm after heat shock.