Identification of cytosolic factors required for nuclear location sequence-mediated binding to the nuclear envelope

Nuclear protein import can be separated into two distinct steps: binding to the nuclear pore complex followed by translocation to the nuclear interior. A previously identified nuclear location sequence (NLS) receptor and a 97-kD protein purified from bovine erythrocytes reconstitute the binding step in a permeabilized cell assay. Binding to the envelope is specific for a functional SV-40 large T antigen NLS and is not ATP or temperature dependent. Modification of p97 with N-ethylmaleimide (NEM) decreases binding to the pore, but interestingly, NEM treatment of the NLS receptor does not. Nuclear envelope binding is inhibited by wheat germ agglutinin suggesting a possible mechanism for the inhibition of transport by the lectin.