Protein translocation across the endoplasmic reticulum (ER) membrane of yeast can be inhibited by agents believed to specifically affect the transport of ATP through the membrane (Mayinger P. and Meyer D.I. (1993) EMBO J. 12, 659-666), suggesting the involvement of a translocation component in the lumen of the ER that binds ATP. We demonstrate that one of the inhibitors, 4.4'-diisothiocyanatostilbene-2.2'-disulfonic acid (DIDS), also affects the translocation of proteins into mammalian microsomes. Translocation is blocked at the point of transfer of the nascent chain from the signal recognition particle (SRP) into the ER-membrane. We also confirm that photoaffinity-labelling of microsomes with 8-azido-ATP inhibits the same early step of protein translocation. Since this step is reported to not require ATP. these results raise the possibility that, in both cases, factor(s) other than ATP-binding components of the translocation machinery are perturbed.
