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Mol. Cell. Biol., 09 1996, 4691-4699, Vol 16, No. 9
CF Chen, Y Chen, K Dai, PL Chen, DJ Riley and WH Lee
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.
Copyright © 1996, American Society for Microbiology
A new member of the hsp90 family of molecular chaperones interacts with the retinoblastoma protein during mitosis and after heat shock
Center for Molecular Medicine, Institute of Biotechnology, University of Texas Health Science Center at San Antonio 78245, USA.
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