Identification of the nucleolar targeting signal of human angiogenin

Angiogenin is endocytosed by subconfluent endothelial cells, translocated to the nucleus and accumulates in the nucleolus. It also localizes into the nucleolus of digitonin-permeabilized endothelial cells. The peptide RRRGL corresponding to residues 31-35 of human angiogenin specifically targets non-nuclear carrier proteins such as albumin, an anti-human nucleolus monoclonal antibody and R33A angiogenin to the nucleolus of permeabilized endothelial cells. Proteins conjugated with a "mutant" peptide, RRAGL, are not imported. Fluorescein isothiocyanate-conjugated RRRGL is also rapidly imported into the nucleus and localized to the nucleolus, whereas the "mutant" peptide is not. Residue R33 is essential for nuclear translocation and R31 and R32 appear to modulate this process. Thus, 31RRRGL35 is a nuclear localization signal responsible for the nucleolar targeting of human angiogenin.