Abstract
It is generally assumed that protein clients fold following their release from chaperones instead of folding while remaining chaperone-bound, in part because binding is assumed to constrain the mobility of bound clients. Previously, we made the surprising observation that the ATP-independent chaperone Spy allows its client protein Im7 to fold into the native state while continuously bound to the chaperone. Spy apparently permits sufficient client mobility to allow folding to occur while chaperone bound. Here, we show that strengthening the interaction between Spy and a recently discovered client SH3 strongly inhibits the ability of the client to fold while chaperone bound. The more tightly Spy binds to its client, the more it slows the folding rate of the bound client. Efficient chaperone-mediated folding while bound appears to represent an evolutionary balance between interactions of sufficient strength to mediate folding and interactions that are too tight, which tend to inhibit folding.
Highlights
It is generally assumed that protein clients fold following their release from chaperones instead of folding while remaining chaperone-bound, in part because binding is assumed to constrain the mobility of bound clients
Fyn SH3 interacts with Spy to form a 1:1 complex
Using isothermal titration calorimetry (ITC), we confirmed that the Spy dimer binds native SH3 with a dissociation constant (Kd) of 50 ± 6 μM (Supplementary Fig. 1a), similar to the ~30 μM affinity that we obtained for the Spy–Im7 interaction[6]
Summary
It is generally assumed that protein clients fold following their release from chaperones instead of folding while remaining chaperone-bound, in part because binding is assumed to constrain the mobility of bound clients. We made the surprising observation that the ATP-independent chaperone Spy allows its client protein Im7 to fold into the native state while continuously bound to the chaperone. Spy allows Im7 to fold while it remains continuously in contact with the surface of the chaperone[6,7,8,9,10] This chaperone’s mechanism does not appear to be unique; there are several other intriguing examples of protein folding while bound to chaperones[11]. Increasing the affinity of client chaperone interactions slows folding—the tighter the interactions, the more folding is slowed These observations lead us to conclude that chaperone-mediated folding-while-bound represents an evolutionary balance between too tight binding, which can act to inhibit folding, and too loose an association, where the chaperone is unable to significantly affect folding or aggregation
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