A new member of the hsp90 family of molecular chaperones interacts with the retinoblastoma protein during mitosis and after heat shock.
A gene encoding a new heat shock protein that may function as a molecular chaperone for the retinoblastoma protein (Rb) was characterized. The cDNA fragment was isolated by using the yeast two-hybrid system and Rb as bait. The open reading frame of the longest cDNA codes for a protein with substantial sequence homology to members of the hsp90 family. Antibodies prepared against fusions between glutathione S-transferase and portions of this new heat shock protein specifically recognized a 75-kDa cellular protein, hereafter designated hsp75, which is expressed ubiquitously and located in the cytoplasm. A unique LxCxE motif in hsp75, but not in other hsp90 family members, appears to be important for binding to the simian virus 40 T-antigen-binding domain of hypophosphorylated Rb, since a single mutation changing the cysteine to methionine abolishes the binding. In mammalian cells, Rb formed complexes with hsp75 under two special physiological conditions: (i) during M phase, when the envelope that separates the nuclear and cytoplasmic compartments broke down, and (ii) after heat shock, when hsp75 moved from its normal cytoplasmic location into the nucleus. In vitro, hsp75 had a biochemical activity to refold denatured Rb into its native conformation. Taken together, these results suggest that Rb may be a physiological substrate for the hsp75 chaperone molecule. The discovery of a heat shock protein that chaperones Rb identifies a mechanism, in addition to phosphorylation, by which Rb is regulated in response to progression of the cell cycle and to external stimuli.
Pubmed ID: 8756626 RIS Download
Amino Acid Sequence | Animals | Base Sequence | Cell Line | Cercopithecus aethiops | Cloning, Molecular | DNA, Complementary | Escherichia coli | Genes | HSP70 Heat-Shock Proteins | HSP90 Heat-Shock Proteins | Hot Temperature | Humans | Mice | Mitosis | Molecular Sequence Data | Mutagenesis, Site-Directed | Protein Denaturation | Protein Folding | Recombinant Fusion Proteins | Retinoblastoma Protein | Saccharomyces cerevisiae | Sequence Alignment | Sequence Homology, Amino Acid