The dynamics of the alternatively spliced NOL7 gene products and role in nucleolar architecture

Three alternatively spliced forms of the human NOL7 gene coding for relatively small proteins were identified. The two horter forms were generated by intron retention events, and each isoform was differently localized within the cell. The OL7-SP1 long form (29 kD) localized to the nucleolus, SP2 was nucleoplasmic, while SP3 was distributed throughout he whole cell. NOL7-SP1 was confined to the nucleolar granular component, and during cell division disassociated from he nucleolus. Knockdown of NOL7-SP1 levels abrogated nucleolar architecture, in particular the internal regions, and educed cell proliferation. Analysis of the nucleolar dynamics of the SP1 protein during interphase showed nucleolar high inding affinity. Dissection of protein domains showed that nucleolar targeting was mediated by a unique C-terminal ucleolar localization sequence (NoLS). However, this sequence was not sufficient for conferring high binding affinity, hich required additional regions of the protein. Our analysis shows that NOL7 is important for maintaining internal ucleolar structure and cell growth rates, and that while specific protein localization can be obtained by specific short ocalization motifs, nucleolar residency through binding must be mediated by a synergistic combination of protein odules.