Probing the Interaction of Huntingtin Exon-1 Polypeptides with the Chaperonin Nanomachine GroEL

Huntington's disease arises from polyQ expansion within the exon-1 region of huntingtin (htt^ex1 ), resulting in an aggregation-prone protein that accumulates in neuronal inclusion bodies. We investigate the interaction of various htt^ex1 constructs with the bacterial analog (GroEL) of the human chaperonin Hsp60. Using fluorescence spectroscopy and electron and atomic force microscopy, we show that GroEL inhibits fibril formation. The binding kinetics of htt^ex1 constructs with intact GroEL and a mini-chaperone comprising the apical domain is characterized by relaxation-based NMR measurements. The lifetimes of the complexes range from 100 to 400 micros with equilibrium dissociation constants (K_D ) of ∼1-2 mM. The binding interface is formed by the N-terminal amphiphilic region of htt^ex1 (which adopts a partially helical conformation) and the H and I helices of the GroEL apical domain. Sequestration of monomeric htt^ex1 by GroEL likely increases the critical concentration required for fibrillization.