STCH is a member of the stress70 chaperone family, which plays a major role in the processing of cytosolic and secretory proteins. Members of the stress70 protein chaperone family participate in protein processing events by binding denatured or misfolded peptide sequences and then releasing these polypeptide chains by an ATP-dependent mechanism. Members of this protein family possess an ATPase activity located in the highly conserved amino-terminal domain and a less well-conserved carboxy-terminal domain necessary for peptide binding. The Stch gene encodes a 60-kDa peptide that is constitutively expressed in all human cell types and shares a high degree of amino acid identity with HSP70 and BiP. The protein differs from previously identified stress70 gene products by the presence of a unique hydrophobic signal sequence and the absence of a carboxy-terminal peptide-binding domain. This results in the protein being highly enriched in the lumen of a crude cellular microsome fraction and possessing an ATPase activity that, unlike other HSP70-like proteins, is not peptide inducible. Analysis of the rat Stch cDNA demonstrates that the unique properties of this C-terminal truncated HSP70-like molecule are conserved through mammalian evolution. As with the human Stch gene, the rat cDNA encodes a hydrophobic leader sequence, andmore » the putative translation product lacks a C-terminal peptide-binding domain. 7 refs., 2 figs.« less