The rat-liver microsomal AP endonuclease. The endoplasmic reticulum is presented as a net thrown into the cytosol to capture newly synthesized karyophilic proteins.

Although most of the rat-liver AP (apurinic/apyrimidinic) endonuclease is in chromatin, some activity is found in microsomes. A quantitative assay of the microsomal AP endonuclease is described. The enzyme is a peripheral membrane protein that is located on the outside surface of microsomes. All the binding sites on the microsomes appear to have the same affinity for the AP endonuclease, suggesting the presence of receptors for the enzyme. The AP endonuclease is displaced from its membrane attachment by submicromolar concentrations of the karyophilic signal of SV-40 T antigen. The AP endonuclease receptors are likely to be on the cytosolic side of the endoplasmic reticulum. It is suggested that binding of the protein to these receptors might be the first step of the transport mechanism that enables the AP endonuclease to penetrate into the nucleus. The same mechanism utilizing the same receptors might be used by other karyophilic proteins, including SV-40 T antigen.