The U-box E3 ubiquitin ligase CHIP (C terminus of Hsc70-interacting protein) 
binds Hsp90 and/or Hsp70 via its tetratricopeptide repeat (TPR), facilitating 
ubiquitination of the chaperone-bound client proteins. Mechanisms that regulate 
the activity of CHIP are, at present, poorly understood. We previously reported 
that Ca(2+)/S100 proteins directly associate with the TPR proteins, such as 
Hsp70/Hsp90-organizing protein (Hop), kinesin light chain, Tom70, FKBP52, CyP40, 
and protein phosphatase 5 (PP5), leading to the dissociation of the interactions 
of the TPR proteins with their target proteins. Therefore, we have hypothesized 
that Ca(2+)/S100 proteins can interact with CHIP and regulate its function. GST 
pulldown assays indicated that Ca(2+)/S100A2 and S100P bind to the TPR domain 
and lead to interference with the interactions of CHIP with Hsp70, Hsp90, HSF1, 
and Smad1. In vitro ubiquitination assays indicated that Ca(2+)/S100A2 and S100P 
are efficient and specific inhibitors of CHIP-mediated ubiquitination of Hsp70, 
Hsp90, HSF1, and Smad1. Overexpression of S100A2 and S100P suppressed 
CHIP-chaperone complex-dependent mutant p53 ubiquitination and degradation in 
Hep3B cells. The association of the S100 proteins with CHIP provides a 
Ca(2+)-dependent regulatory mechanism for the ubiquitination and degradation of 
intracellular proteins by the CHIP-proteasome pathway.