Cloning and characterization of salmon hsp90 cDNA: upregulation by thermal and hyperosmotic stress.

Accumulating evidence suggests that glucocorticoids are essential for development of hypoosmoregulatory capacity in salmon during adaptation to seawater. Heat shock protein (hsp)90 has been reported to function in signal transduction and the maturation and affinity of glucocorticoid receptors. We sought to determine whether this hsp might be upregulated by thermal and hyperosmotic stress in salmon, a species that migrates between the freshwater and marine environments. A 2625-bp cDNA cloned from a salmon cDNA library was found to code for a protein of 722 amino acids exhibiting a high degree of identity with zebra fish (92%) and human (89%) hsp90beta. Accumulation of hsp90 mRNA was observed in isolated branchial lamellae incubated under hyperosmotic conditions and in branchial lamellae of salmon exposed to hyperosmotic stress in vivo. In contrast, exposure of kidney to hyperosmotic stress in vitro and in vivo failed to elicit an increase in the quantity of hsp90 mRNA. By way of comparison, accumulation of hsp90 mRNA was observed in both branchial lamellae and kidney tissue subjected to thermal stress in vitro and in vivo. Western blot analyses of proteins isolated from tissues under identical conditions in vitro revealed that the pool of hsp90 increased with thermal stress but not with osmotic stress. The results suggest that accumulation of hsp90 mRNA in response to osmotic stress is unrelated to cellular protein denaturation and that synthesis of hsp90 may be regulated at both the level of transcription and translation.