Small Heat Shock Proteins and Diapause in the Crustacean, Artemia franciscana

Abstract

Diapause is a physiological state of metabolic depression and increased stress tolerance observed in many organisms, and especially prominent in insects. The small heat shock proteins (sHsps) mediate protein storage as well as stress tolerance and their accumulation within cells is controlled during diapause. Partially denatured proteins are bound by sHsps preventing their irreversible denaturation, and these proteins are released to Hsp70 and other molecular chaperones for either ATP-dependent folding or degradation. During diapause, which is characterized by exposure to varying levels of stress, proteins are sequestered by sHsps, thus, by enhancing sHsp synthesis the protection of other proteins is maximized. In the crustacean, Artemia franciscana, diapause occurs in encysted gastrula stage embryos (cysts), entailing a profound reduction in metabolism and extreme stress tolerance. Three sHsps, namely p26, ArHsp21 and ArHsp22, accumulate to varying levels in diapause-destined A. franciscana embryos. Experiments performed in vivo by the use of RNA interference (RNAi) demonstrate that p26 has an important role in stress tolerance while also influencing embryo development and diapause maintenance. The activities of ArHsp21 and ArHsp22 are less well defined although the former sHsp may have a minor role in stress tolerance. The data described herein reveal that the activities of sHsps during diapause are more diverse than previously thought and they contribute to the general understanding of sHsp function.