A 'core ATPase', Hsp70-like structure is conserved in human, rat, and C. elegans STCH proteins

We have identified the rat and Caenorhabditis elegans homologues of a 'core ATPase'-encoding Hsp70-like gene, designated Stch. We observed that the human, rat, and C. elegans Stch genes have conserved a stop codon immediately distal to the sequence encoding the Hsp70 ATPase domain. This results in the functional equivalent of an N-terminal, proteolytically cleaved fragment of Hsc70/BiP. Each homologue contains a hydrophobic signal sequence, demonstrates striking identity within the Hsp70 ATPase domain, and retains a similar C-terminal sequence (STCH specific cluster III) that is unique among Hsp70 proteins and which truncates the peptide binding domain. In addition, we have identified an internal 35-aa region that is homologous to the minimal sequence of the Hip chaperone co-factor that is required for direct binding to the ATPase domain of Hsp70. Adjacent to this region, the rat and human STCH protein sequences diverge within a short internal 'insertion' sequence that interrupts the ATPase subdomain between the phosphate-2 and adenosine ATP-binding sites. We have also demonstrated that both human and rat Stch are constitutively produced and are induced by the calcium ionophore A23187, but not by heat shock. The recognition that the truncated 'core ATPase' structure of the STCH molecule is conserved in human, rat, and C. elegans tissues suggests an important role for this unique member of the membrane-bound Hsp70 family.