Heat shock cognate protein 70 chaperone-binding site in the co-chaperone murine stress-inducible protein 1 maps to within three consecutive tetratricopeptide repeat motifs

Murine stress-inducible protein 1 (mSTI1) is a co-chaperone homologous with the human heat shock cognate protein 70 (hsc70)/heat shock protein 90 (hsp 90)-organizing protein (Hop). The concomitant interaction of mSTI1 with hsp 70 and hsp90 at its N- and C-termini respectively is mediated by the tetrat ricopeptide repeat (TPR) motifs in these regions. With the use of co-precip itation assays, we show here that the N-terminal TPR domain of mSTI1 withou t extensive flanking regions is both necessary and sufficient to mediate a specific interaction with hsc70. In contrast, other TPR-containing co-chape rones require TPR Banking regions for target substrate recognition, suggest ing different mechanisms of TPR-mediated chaperone-co-chaperone interaction s. Furthermore, the interaction between mSTI1 and hsc70 was analysed to asc ertain the effect of replacing or deleting conserved amino acid residues an d sequences within the three TPR motifs constituting the N-terminal TPR dom ain of full-length mSTI1. Replacement of a bulky hydrophobic residue in TPR 1 disrupted the interaction of mSTI1 with hsc70. A highly conserved sequenc e in TPR2 was altered by deletion or single amino acid replacement. These d erivatives retained a specific interaction with hsc70. These results are co nsistent with a model in which conserved residues within the N-terminal TPR region of mSTI1 contribute differentially to the interaction with hsc70, a nd in which TPR1 has a significant role in targeting mSTI1 to hsc70. The co ntribution of the TPR domain mutations and deletions are discussed with res pect to their effect on target substrate interactions.