Structure and function of Hip, an attenuator of the Hsp70 chaperone cycle

Area of Science:

• Molecular biology
• Protein biochemistry
• Structural biology

Background:

• Heat shock protein 70 (Hsp70) is a chaperone protein crucial for protein folding and preventing aggregation.
• Hsp70's function relies on an ATP-dependent cycle involving J-domain proteins and nucleotide exchange factors (NEFs).
• The Hsp70-interacting protein (Hip) is known to modulate Hsp70 activity, potentially by influencing substrate release.

Purpose of the Study:

• To elucidate the structural basis of Hip's interaction with Hsp70.
• To understand how Hip regulates the Hsp70 ATPase cycle.
• To explain Hip's role in enhancing Hsp70's chaperone activity.

Main Methods:

• X-ray crystallography was used to determine the structures of Hip's dimerization and tetratricopeptide repeat (TPR) domains.
• A co-crystal structure of Hip bound to Hsp70 was obtained.
• Biochemical assays were performed to assess the interplay between Hip, NEFs, and Hsp70.

Main Results:

• Crystal structures revealed the detailed interface between Hip's TPR domain and the Hsp70 ATPase domain.
• Hip forms a 'bracket' that locks ADP within Hsp70's binding cleft.
• Binding of Hip and NEFs to Hsp70 are mutually exclusive events.
• Hip binding inhibits the active cycling of Hsp70-substrate complexes.

Conclusions:

• Hip attenuates Hsp70's ATPase activity by preventing ADP release, thereby stabilizing Hsp70-substrate interactions.
• This mechanism explains how Hip enhances Hsp70's ability to prevent protein aggregation.
• Hip facilitates the targeted delivery of proteins to other cellular machinery, such as downstream chaperones or the proteasome.