Crystal Structure and Activity of Human p23, a Heat Shock Protein 90 Co-chaperone

Abstract

p23 is a co-chaperone for the heat shock protein, hsp90. This protein binds hsp90 and participates in the folding of a number of cell regulatory proteins, but its activities are still unclear. We have solved a crystal structure of human p23 lacking 35 residues at the COOH terminus. The structure reveals a disulfide-linked dimer with each subunit containing eight ␤-strands in a compact antiparallel ␤-sandwich fold. In solution, however, p23 is primarily monomeric and the dimer appears to be a minor component. Conserved residues are clustered on one face of the monomer and define a putative surface region and binding pocket for interaction(s) with hsp90 or protein substrates. p23 contains a COOH-terminal tail that is apparently less structured and is unresolved in the crystal structure. This tail is not needed for the binding of p23 to hsp90 or to complexes with the progesterone receptor. However, the tail is necessary for optimum active chaperoning of the progesterone receptor, as well as the passive chaperoning activity of p23 in assays measuring inhibition of heat-induced protein aggregation.p23 is a ubiquitous, highly conserved protein which functions as a co-chaperone for the larger molecular chaperone, hsp90.

Gedunin Inactivates the Co-chaperone p23 Protein Causing Cancer Cell Death by Apoptosis

“…Using computational modeling studies, we showed that gedunin can be successfully docked into the p23 structure, and three amino acids (Thr-90, Ala-94, and Lys-95) possibly mediate noncovalent interactions with gedunin. The crystal structure of p23 dimer reported by Weaver et al (32) showed that the surface of p23 molecule exhibits a solvent-accessible cavity surrounded by polar amino acids, whereas the floor of the cavity contains nonpolar hydrophobic amino acids, which appear to provide a ligand-binding site on the surface of p23. Utilizing molecular modeling and docking software, we observed that gedunin docks into this ligand-binding site, forms hydrogen bonds with Thr-90 and Lys-95, and maintains hydrophobic interactions with Ala-94.…”

“…In addition, it appears that the C-terminal 35 amino acids are essential for gedunin-p23 binding, which suggests that the unstructured C-terminal tail stabilizes gedunin binding to the identified pocket. The ligand-binding cleft, along with other amino acids in this vicinity, compose the Hsp90-binding site on the surface of p23 (32,44), and gedunin binding to p23 appears to alter the topology of this region thus disrupting the p23-Hsp90 interaction. Data in Fig.…”

“…Mapping the Gedunin-binding Site on p23-To identify the binding site of gedunin on p23, we used the available structure of p23C35 (Protein Data Bank code 1EJF) (32) to perform in silico docking experiments using the residues identified by NMR to be important in celastrol binding to p23 (8). We identified a potential binding site on the surface of p23 that could reasonably accommodate gedunin binding (Fig.…”

“…The level of p23 was assessed by Western blotting using JJ3 antibody (bottom). C and D, gedunin and 7-oxo-gedunin docked into the p23 crystal structure (32). E, docking of both parent gedunin and 7-oxo-gedunin on p23.…”

Caspase-7 uses an exosite to promote poly(ADP ribose) polymerase 1 proteolysis

“…A second constraint is the requirement for the cleavage site to occupy the substrate-binding pocket. In the case of p23, the length of the C-terminal region spans 49 residues and is unstructured (34), which theoretically allows it to reach either catalytic center. However, in the model we proposed, the C-terminal binds the closest active site.…”