NUCLEAR IMPORT AND NUCLEOLAR ACCUMULATION OF THE HUMAN RIBOSOMAL-PROTEIN S7 DEPENDS ON BOTH A MINIMAL NUCLEAR-LOCALIZATION SEQUENCE AND AN ADJACENT BASIC REGION

In the course of the eukaryotic ribosomal biogenesis, both the nuclear import and export are involved. We have studied the nuclear and nucle olar localization of the human ribosomal protein S7. We examined the s ubcellular distribution of the S7:beta-galactosidase fusion proteins i n SAOS-2 cells. We have identified two evolutionarily conserved domain s, 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 nucle ar transport and nucleolar accumulation. Interestingly, a tetrapeptide (KRPR118)-K-115, which can act as an independent nuclear localization signal (NLS), is not sufficient for exclusively nuclear accumulation of the 57 protein if the adjacent region 98...109 is deleted. In addit ion, site-directed mutagenesis revealed that critical residues for nuc lear targeting in this tetrapeptide and in the full-length 57 protein are different. While mutation of a Pro(117) significantly impaired nuc lear import of 57, similar substitution within the tetrapeptide-NLS ha d no effect on nuclear targeting. This suggests that to function perfe ctly, proper secondary structure of the 57 nuclear targeting domain is required. (C) 1998 Academic Press.