Ratcheting in post-translational protein translocation: A mathematical model

We have developed a non-steady-state mathematical model describing post-tra nslational protein translocation across the endoplasmic reticulum membrane. Movement of the polypeptide chain through the channel in the endoplasmic r eticulum membrane is considered to be a stochastic process which is biased at the lumenal side of the channel by the binding of BiP (Kar2p), a member of the Hsp70 family of ATPases (ratcheting model). Assuming that movement o f the chain through the channel is caused by passive diffusion (Brownian ra tchet), the model describes all available experimental data. The optimum se t of model parameters indicates that the ratcheting mechanism functions at near-maximum rate, being relatively insensitive to variations of the associ ation or dissociation rate constants of BiP or its concentration. The estim ated rate constant for diffusion of a polypeptide inside the channel indica tes that the chain makes contact with the walls of the channel. Since fitti ng of the model to the data required that the backward rate constant be lar ger than the forward constant during early diffusion steps, translocation m ust occur against a force. The latter may arise, for example, from the unfo lding of the polypeptide chain in the cytosol. Our results indicate that th e ratchet can transport polypeptides against a free energy of about 25 kJ/m ol without significant retardation of translocation. The modeling also sugg ests that the BiP ratchet is optimized, allowing fast translocation to be c oupled with minimum consumption of ATP and rapid dissociation of BiP in the lumen of the ER. Finally, we have estimated the maximum hydrophobicity of a polypeptide segment up to which lateral partitioning from the channel int o the lipid phase does not result in significant retardation of translocati on. (C) 2001 Academic Press.