Hop (protein)

Hop, occasionally written HOP, is an abbreviation for is_associated_with::Hsp70-is_associated_with::Hsp90 Organizing Protein. It functions as a co-chaperone which reversibly links together the protein chaperones Hsp70 and Hsp90.

Hop belongs to the large group of is_associated_with::co-chaperones, which regulate and assist the major chaperones (mainly is_associated_with::heat shock proteins). It is one of the best studied co-chaperones of the Hsp70/Hsp90-complex. It was first discovered in is_associated_with::yeast and homologues were identified in human, mouse, rat, insects, plants, parasites, and virus. The family of these proteins is referred to as STI1 (stress inducible protein) and can be divided into yeast, plant, and animal STI1 (Hop).

Gene
The gene for human Hop is located on is_associated_with::chromosome 11q13.1 and consists of 14 is_associated_with::exons.

Structure
STI proteins are characterized by some structural features: All homologues have nine tetratricopeptide repeat (TPR) motifs, that are clustered into domains of three TPRs. The TPR motif is a very common structural feature used by many proteins and provides the ability of directing protein-protein interactions. Crystallographic structural information is available for the N-terminal TPR1 and the central TPR2A domains in complex with Hsp90 resp. Hsp70 is_associated_with::ligand is_associated_with::peptides.

Function
The main function of Hop is to link is_associated_with::Hsp70 and is_associated_with::Hsp90 together. But recent investigations indicate that it also modulates the chaperone activities of the linked proteins and possibly interacts with other chaperones and proteins. Apart from its role in the Hsp70/Hsp90 "chaperone machine" it seems to participate in other protein complexes too (for example in the is_associated_with::signal transduction complex EcR/USP and in the is_associated_with::Hepatitis B virus is_associated_with::reverse transcriptase complex, which enables the viral replication). It acts as a receptor for is_associated_with::prion proteins too. Hop is located in diverse cellular regions and also moves between the is_associated_with::cytoplasm and the nucleus.

Interactions
Hop (protein) has been shown to interact with is_associated_with::PRNP and is_associated_with::Heat shock protein 90kDa alpha (cytosolic), member A1.