IDENTIFICATION OF THE NUCLEAR AND NUCLEOLAR LOCALIZATION SIGNALS OF THE PROTEIN P120 - INTERACTION WITH TRANSLOCATION PROTEIN B23

The human p120 nucleolar protein is a cell cycle-related protein that peaks during the S phase and has been shown to be associated with a be aded fibrillar structure. To study domains responsible for the nucleol ar localization of protein p120, initially deletion mutants were made that defined sequences containing the localization signals; then, fusi on genes that were composed of segments of the p120 molecule joined to the N-terminal end of the Escherichia coli beta-galactosidase were co nstructed. In the absence of the localization signals the beta-galacto sidase remained in the cytoplasm. When the identified nuclear localiza tion signal containing the amino acid sequence 99-110 (NAPRGKKRPAPG) w as fused to the beta-galactosidase, the protein localized to the nucle us. When only the identified nucleolar localization signal containing the amino acid sequence 40-57 (SKRLSSRARKRAAKRRLG) was fused to the be ta-galactosidase, the fusion protein remained in the cytoplasm. When b oth the nuclear and nucleolar localization signals were fused to the b eta-galactosidase it localized predominantly to the nucleolus. Nucleol ar protein B23, a putative ''shuttle protein,'' bound to amino acid se quence 24-56 of protein p120. Deletion analysis showed that amino acid s 187-215 of protein B23 bound to protein p120. The results suggest th at protein B23 may be part of the mechanism of protein targeting to th e nucleolus.