Nuclear Import and Nucleolar Accumulation of the Human Ribosomal Protein S7 Depends on both a Minimal Nuclear Localization Sequence and an Adjacent Basic Region

Abstract

In the course of the eukaryotic ribosomal biogenesis, both the nuclear import and export are involved. We have studied the nuclear and nucleolar localization of the human ribosomal protein S7. We examined the subcellular distribution of the S7:β-galactosidase fusion proteins in SAOS-2 cells. We have identified two evolutionarily conserved domains, both of which are necessary for S7 nuclear and nucleolar targeting: amino acids 98 to 109 and 115 to 118. Out of the S7 protein context, a fragment 98 … 118, containing these domains, is sufficient for nuclear transport and nucleolar accumulation. Interestingly, a tetrapeptide115KRPR118, which can act as an independent nuclear localization signal (NLS), is not sufficient for exclusively nuclear accumulation of the S7 protein if the adjacent region 98 … 109 is deleted. In addition, site-directed mutagenesis revealed that critical residues for nuclear targeting in this tetrapeptide and in the full-length S7 protein are different. While mutation of a Pro117significantly impaired nuclear import of S7, similar substitution within the tetrapeptide-NLS had no effect on nuclear targeting. This suggests that to function perfectly, proper secondary structure of the S7 nuclear targeting domain is required.