Abstract
The 90-kDa heat shock protein (HSP 90) is one of the major stress proteins in eukaryotic cells. There are at least two HSP 90 genes; and two HSP 90 isoform proteins, HSP 90 α and HSP 90 β, are expressed by them, The third member of HSP 90 family proteins, the 94-kDa glucose-regulated protein (GRP 94), is expressed in the endoplasmic reticulum, The purpose of the present study was to investigate the structures of HSP 90-family proteins and their expression in osteoblastic cells (MC 3 T 3-E 1) and in mammalian tissues. All three HSP 90-family proteins of human origin were expressed in a bacterial expression system and were purified to near homogeneity. Two kinds of polyclonal antibodies against human GRP 94 (hGRP 94) were obtained by immunization of recombinant protein and a synthetic peptide (residues 336-349) of hGRP 94 as antigens. The results of limited proteolysis of recombinant hGRP 94 with trypsin suggested three domain structures, similar to those of HSP 90 α. The expression level of HSP 90 isoforms in various rat tissues and in MC 3 T 3-E 1 cells under nonstress conditions revealed the predominance of HSP 90 β in most tissues (including oral tissues) except for the brain, testis, and reticulocytes. The expression of HSP 90 α in MC 3 T 3-E 1 cells was induced under stress conditions such as exposure to arsenate, whereas HSP 90 β and GRP 94 were not significantly increased.
These findings suggest that the three HSP 90 family proteins possess similar domain structures and that HSP 90 β is the major HSP 90 isoform expressed constitutively in the cell, whereas HSP 90 α is an inducible form.
