Human proliferation-related protein p120 interacts with HSRP1

Protein p120 is a proliferation-related nucleolar protein, which is detectable early in the G1-phase of the cell cycle and peaks early in the S-phase. Most human malignant tumor cells contain much higher levels of protein p120 than do normal resting cells. To learn more about p120-associated proteins, a yeast two-hybrid screen was carried out using protein p120 as the bait. Five positive clones were identified from 2 million clones for further analysis. Three of them encoded portions of the same protein, which had identity to human SRP1. The recombinant p120 and HSRP1 proteins produced in Sf9 cells are associated with each other, confirming the results of the yeast genetic assay. Protein SRP1 was originally characterized as a suppressor of RNA polymerase I mutations, and recently human SRP1 was identified as a receptor for nuclear localization sequences. In the present study, use of deletion mutations revealed that the binding of human SRP1 to p120 required the p120 nuclear localization signal (amino acids 96-119) and the C-terminus of human SRP1 (amino acids 453-491).