NOA36 Protein Contains a Highly Conserved Nucleolar Localization Signal Capable of Directing Functional Proteins to the Nucleolus, in Mammalian Cells

de Melo, Ivan S.; Jimenez-Nuñez, Maria D.; Iglesias, Concepción; Campos-Caro, Antonio; Moreno-Sanchez, David; Ruiz, Felix A.; Bolívar, Jorge

In recent years, the nucleolus has emerged as a complex structure implicated in important processes beyond its key role in ribosomal biogenesis, a point of view supported by recent proteomic analysis [48] . Although we have previously described the pro-apoptotic role of NOA36 in the mitochondrial apoptotic pathway, the role played by this protein in the nucleolus has not yet been elucidated. As a basis for this investigation, in this work we have identified the specific amino acid sequence responsible for transporting NOA36 into the nucleolus. This subnuclear structure, the nucleolus, is reformed during each cell cycle and, because it is not isolated by a membrane, proteins do not need a specific transport mechanism in order to penetrate this compartment. Although some efforts have been made in order to characterize and classify NoLs [8] , the signal that gives rise to this translocalization into the nucleolus is difficult to predict and is sometimes contained within or overlapped by the nuclear localization signal [12] . Proteins seem to accumulate in the nucleolus in a steady-state manner and, in this view, there are different mechanisms by which proteins are retained in the nucleolus: there are proteins that bind to rDNA (i.e. with DNA binding motifs) [49] or to rRNA (i.e. containing RNA binding motifs) [19] . Other proteins are anchored to the nucleolus by binding to one of those mentioned above. Some of these proteins (hub proteins) are able to interact with several others that incorporate in their sequence one or more NoLSs, and these signals are characteristically rich in basic amino acids [50, 51] . Finally, other proteins may be located in this subnuclear structure by interacting with other nucleolar proteins in a non-predictable manner. In this work, we have found that NOA36 is transported to and localized in the nucleolus as the result of a specific peptide signal, therefore this protein probably interacts with a RNA binding protein, since the nucleolar localization of NOA36 depends on RNA but not on DNA [37] .