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

An important aspect in the characterization of NOA36 NoLS is to clarify whether this sequence is only a NoLS-signal, or would act both as a NoLS and NLS signal. In this sense our data are somehow contradictory: on the one hand, the fusion of NOA36 NoLS to the yeast exopolyphophatase is sufficient to transport this cytoplasmic protein to the nucleoli of HeLa cells (Flag-NoLS-scPPX1 construct, 50.4 kDa, Fig. 5A ). This would indicate that the amino terminal sequence of NOA36 could be a joint NoLS-NLS region. On the other hand, the full length NOA36-eGFP fusion protein (63.7 kDa) normally localizes in the cytoplasm of HeLa and CHO cells, although shows also nucleolar localization in a small fraction (less than 1%) of CHO transfected cells [35] . This fact could indicate that this sequence would not be sufficient to transport a protein larger than the nuclear pore exclusion size (,40 kDa). Because of its molecular weight (36 kDa), NOA36 should not need an active transport facilitated by specific soluble carrier proteins in order to get into the nucleus.