Synthetic peptides as nuclear localization signals

The nuclear envelope defines a compartment boundary which is penetrated by pores that mediate a remarkable transport process. Precursor RNAs are retained in the nucleus, while processed messenger RNA 1 , transfer RNA 2 and ribosomal subunits 3 are transported to the cytoplasm. Proteins destined for the nucleus become localized soon after synthesis and again following mitosis, while cytoplasmic proteins are excluded 4 . The process is highly specific: a single base change in vertebrate initiator tRNA Met (tRNA i met ) reduces the rate of export 20-fold 5 ; a point mutation within the simian virus 40 (SV40) large-T antigen, converting Lys 128 to Thr (ref. 6) or Asn (ref. 7), prevents import. Lys 128 lies within a short ‘signal’ sequence which, when fused to large non-nuclear proteins, causes their accumulation in nuclei 6–8 . Regions of other eukaryotic proteins also seem to contain nuclear localization signals, although a single consensus sequence has not emerged 9–13 . We report here that a synthetic peptide containing 10 residues of large-T antigen sequence serves as a nuclear localization signal when cross-linked to bovine serum albumin (BSA) or immunoglobulin G (IgG) and microinjected in Xenopus oocytes. Substitution of Thr at the position of Lys 128 in this peptide renders it six- to sevenfold less effective. The uptake of peptide-linked BSA is saturable, and the rate is diminished by co-injection of free peptide. These findings are indicative of a receptor-mediated uptake process. With the use of anti-peptide antibodies, a family of proteins is revealed in nuclear but not cytoplasmic extracts of human lymphocytes which contain large-T antigen-like sequences.