Analysis of the dominant effects mediated by wild type or R120G mutant of αB-crystallin (HspB5) towards Hsp27 (HspB1).

Several human small heat shock proteins (sHsps) are phosphorylated oligomeric chaperones that enhance stress resistance. They are characterized by their ability to interact and form polydispersed hetero-oligomeric complexes. We have analyzed the cellular consequences of the stable expression of either wild type HspB5 or its cataracts and myopathies inducing R120G mutant in growing and oxidative stress treated HeLa cells that originally express only HspB1. Here, we describe that wild type and mutant HspB5 induce drastic and opposite effects on cell morphology and oxidative stress resistance. The cellular distribution and phosphorylation of these polypeptides as well as the oligomerization profile of the resulting hetero-oligomeric complexes formed by HspB1 with the two types of exogenous polypeptides revealed the dominant effects induced by HspB5 polypeptides towards HspB1. The R120G mutation enhanced the native size and salt resistance of HspB1-HspB5 complex. However, in oxidative conditions the interaction between HspB1 and mutant HspB5 was drastically modified resulting in the aggregation of both partners. The mutation also induced the redistribution of HspB1 phosphorylated at serine 15, originally observed at the level of the small oligomers that do not interact with wild type HspB5, to the large oligomeric complex formed with mutant HspB5. This phosphorylation stabilized the interaction of HspB1 with mutant HspB5. A dominant negative effect towards HspB1 appears therefore as an important event in the cellular sensitivity to oxidative stress mediated by mutated HspB5 expression. These observations provide novel data that describe how a mutated sHsp can alter the protective activity of another member of this family of chaperones.