GTP-dependent ADP-ribosylation of a 22 KDa Protein in the Endoplasmic Reticulum Membrane

Treatment of salt-stripped rough microsomal membranes from pancreas or liver with NAD and cholera toxin in the presence of GTP yields an ADP-ribosylated non-ribosomal 22 kDa protein. Membranes containing the modified protein are less active in the co-translational processing of secretory preproteins translated from isolated mRNA in a reticulocyte translation system, but signal peptidase activity is unchanged, suggesting that the 22 kDa protein is involved in the targetting or translocation of secretory proteins at the membrane of the endoplasmic reticulum.