Nuclear targeting of the maize R protein requires two nuclear localization sequences

Previous genetic and structural evidence indicates that the maize R gene encodes a nuclear transcriptional activating factor. In-frame carboxyl- and amino-terminal fusions of the R gene to the reporter gene encoding beta-glucuronidase (GUS) were sufficient to direct GUS to the nucleus of the transiently transformed onion (Allium cepa) epidermal cells. Further analysis of chimeric constructs containing regions of the R gene fused to the GUS cDNA revealed three specific nuclear localization sequences (NLSs) that were capable of redirecting the GUS protein to the nucleus. Amino-terminal NLS-A (amino acids 100-109, GDRRAAPARP) contained several arginine residues; a similar localization signal is found in only a few viral proteins. The medial NLS-M (amino acids 419-428, MSERKRREKL) is a simian virus 40 large T antigen-type NLS, and the carboxyl-terminal NLS-C (amino acids 598-610, MISESLRKAIGKR) is a mating type alpha 2 type. NLSs M and C are independently sufficient to direct the GUS protein to the nucleus when it is fused at the amino terminus of GUS, whereas NLS-A fused to GUS partitioned between the nucleus and cytoplasm. Similar partitioning was observed when localization signals NLS-A and NLS-C were independently fused to the carboxy-terminal portion of GUS. A sequential deletion of the localization signals indicated that the amino-terminal and carboxyl-terminal fusions of R and GUS were redirected to the nucleus only when both NLS-A and -M, or NLS-C and -M, were present. These results indicate that multiple localization signals are necessary for nuclear targeting of this protein. The conservation of the localization signals within the alleles of R and similar proteins from other organisms is also discussed.