Constitutive, translation-independent opening of the protein-conducting channel in the endoplasmic reticulum

Secretory and membrane proteins are translocated into the endoplasmic reticulum (ER) through a translocon assembled as a tetramer of Sec61 protein-conducting channels (PCC). How the opening of the PCCs in the tetramer is regulated through the protein translocation cycle is poorly understood. In this study, the permeability of PCCs in native ER membranes to small molecules was measured using fluorescence and electrophysiological techniques. Although the PCCs were closed at 4A degrees C, they were constitutively open at physiological temperatures in the absence of protein translation or a bound ribosome. The open PCCs occurred in clusters that are likely to correspond to the simultaneous opening of three or four PCCs in a translocon. The binding of 60S subunits to a ribosome-free membrane increased the number of open PCCs but did not increase the single-channel conductance. The translation-independent, constitutive opening of Sec61 PCCs provides new insight into the role of the translocon in the transport of small molecules across the ER membrane.