Hop (Hsp70/Hsp90 Organizing Protein), or Stress-induced Phosphoprotein 1 (STI1) as it is also known, is a 60kDa protein that belongs to the large group of co-chaperones which regulate and assist the major chaperones. It is located in diverse cellular regions and can move between the cytoplasm and the nucleus.
It functions to reversibly link together the protein chaperones Hsp70 and Hsp90. HOP contains three tetratricopeptide repeat (TPR) domains, TPR1, TPR2a and TPR2b. Hsp70 binding has been localized to TRP1 and sp90 binding have been localized to TPR2a (1). It has also been found to modulate the chaperone activities of the linked proteins and possible interacts with other chaperones and proteins. It has also been found to participate in other complexes besides the Hsp70/Hsp90 one (2).
HOP is closely related to human 63kDa protein that is sensitive to simian virus SV40 transformation, and is related to the yeast heat-shock- responsive STI1 gene product (3, 4).