Inhibition of protein translocation across the endoplasmic reticulum membrane by sterols

Cholesterol and related sterols are known to modulate the physical properti es of biological membranes and can affect the activities of membrane-bound protein complexes. Here, we report that an early step in protein translocat ion across the endoplasmic reticulum (ER) membrane is reversibly inhibited by cholesterol levels significantly lower than those found in the plasma me mbrane. By UV-induced chemical cross-linking we further show that high chol esterol levels prevent cross-linking between ribosome-nascent chain complex es and components of the Sec61 translocon, but have no effect on cross-link ing to the signal recognition particle. The inhibiting effect on translocat ion is different between different sterols. Our data suggest that the prote in translocation machinery may be sensitive to changes in cholesterol level s in the ER membrane.