Fig. 7.

Model of Pam motor action. A model based on entropic pulling, an extension of the Brownian motion model, is shown. TIM23 translocon in inner membrane (brown); translocating polypeptide (blue); Tim44 (pink); Hsp70 (gray). (i) The presequence (yellow), upon entrance into the matrix driven by the membrane potential, binds Tim44’s NTD, perhaps activating the motor. (ii) Preprotein binds in the cleft of an Hsp70, which is tethered very close to the channel exit by Tim44. (iii) This binding, in conjunction with Pam18’s J-domain (not shown), stimulates Hsp70’s ATPase activity. The conformational change results in trapping of the translocating polypeptide and (see insert) release of Hsp70 from Tim44. According to the “entropic pulling” model a force is exerted because Hsp70’s movement is restricted by the translocon and membrane (indicated by red bars). (iv) As the translocating polypeptide, with Hsp70 bound, moves away from the membrane, the force is reduced because Hsp70’s motion is no longer restricted. Another Hsp70-ATP is able to bind Tim44, starting (v) another cycle of “directed” movement