The objective of the proposed research is to determine which of the classes of ribosomes which may be isolated from liver cells (free ribosomes, ribosomes bound to the endoplasmic reticulum and perinuclear ribosomes) are involved in the synthesis of nuclear proteins, and to investigate the mechanism involved in the transport of these proteins from the site of synthesis to their ultimate location in the nucleus. To determine which class or classes of ribosomes produce nuclear proteins, sensitive immunological techniques will be used to detect specific nuclear proteins among the products of synthesis of each class of ribosomes. Several approaches will be employed to investigate the transport of these proteins into the nucleus. By pulse labeling and puromycin release studies, we will attempt to determine whether the newly synthesized nuclear proteins are discharged across a membrane or whether they are, instead, released into the soluble space of the cytoplasm. Newly synthesized nuclear proteins will be compared with the corresponding proteins obtained from the nucleus to detect postsynthetic modification which could be involved in the transport process. Finally, we will attempt to define the requirements for transport by use of inhibitors of energy metabolism and of protein and RNA synthesis.