SMALL HEAT-SHOCK PROTEINS OF DROSOPHILA - DEVELOPMENTAL EXPRESSION AND FUNCTIONS

In addition to their induced expression under various stress condition s, small heat shock proteins (sHsps) are also expressed during normal development in a wide array of organisms. Members of the sHsp family o f Drosophila melanogaster are individually expressed at many stages of development, and their developmental expression contrasts sharply wit h their stress-induced expression. First, the developmental expression of sHsps is uncoordinated and each of the sHsps shows its own pattern of expression. Secondly this expression is highly regulated in a tiss ue-, cell lineage- and/or developmental stage-specific manner. An exam ple of such regulation is during male gametogenesis when Hsp23, Hsp26 and Hsp27 are expressed in the absence of stress. However the expressi on of Hsp23 is restricted to cells of the somatic lineage while Hsp27 is mainly expressed in germ line cells and in some somatic cells. Heat shock does not alter the level nor the specificity of expression of t hese sHsps in this tissue while other HSPs, i.e., Hsp22 and Hsp70, are induced through signaling via the unique heat shock transcription fac tor (HSF). The HSF is shown to be present at a much lower level in tes tes than in other tissues and shows cell-specific distribution. The hi ghly regulated expression of sHsps during development and differentiat ion is dependent on transcription factors other than the HSF. The spec ific expression patterns of individual sHsps suggest that these protei ns may fulfill distinct functions during normal development. A search for partners of sHsps suggests that these proteins may function in cel l proteolytic processes.