Structural basis of client specificity in mitochondrial membrane-protein chaperones

Chaperones are essential for assisting protein folding, and for transferring poorly soluble proteins to their functional locations within cells. Hydrophobic interactions drive promiscuous chaperone-client binding, but our understanding how additional interactions enable client specificity is sparse. Here we decipher what determines binding of two chaperones (TIM8{middle dot}13, TIM9{middle dot}10) to different integral membrane proteins, the all-transmembrane mitochondrial carrier Ggc1, and Tim23 which has an additional disordered hydrophilic domain. Combining NMR, SAXS and molecular dynamics simulations, we determine the structures of Tim23/TIM8{middle dot}13 and Tim23/TIM9{middle dot}10 complexes. TIM8{middle dot}13 uses transient salt bridges to interact with the hydrophilic part of its client, but its interactions to the trans-membrane part are weaker than in TIM9{middle dot}10. Consequently, TIM9{middle dot}10 is outcompeting TIM8{middle dot}13 in binding hydrophobic clients, while TIM8{middle dot}13 is tuned to few clients with both hydrophilic and hydrophobic parts. Our study exemplifies how chaperones fine-tune the balance of promiscuity vs. specificity.