Osmotic Stress Protein 94 (Osp94)

Preservation of cell viability and function in the hyperosmolar environment of the renal medulla is a complex process that requires selective gene expression. We have identified a new member of the heat shock protein (hsp) 70 superfamily that is up-regulated in renal inner medullary collecting duct cells (mIMCD3 cells) during exposure to hyperosmotic NaCl stress. Known as osmotic stress protein 94, or Osp94, this 2935-base pair cDNA encodes an 838-amino acid protein that shows greatest homology to the recently discovered hsp110/SSE gene subfamily. Like the hsps, Osp94 has a putative amino-terminal ATP-binding domain and a putative carboxyl-terminal peptide-binding domain. The in vitro translated Osp94 product migrated as a 105-110-kDa protein on SDS-polyacrylamide gel electrophoresis. In mIMCD3 cells, Osp94 mRNA expression was greatly up-regulated by hyperosmotic NaCl or heat stress. In mouse kidney, Osp94 mRNA expression paralleled the known corticomedullary osmolality gradient showing highest expression in the inner medulla. Moreover, inner medullary Osp94 expression was increased during water restriction when osmolality is known to increase. Thus, Osp94 is a new member of the hsp110/SSE stress protein subfamily and likely acts as a molecular chaperone.