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

The nucleolus is a dynamic structure that disassembles and reforms during each cell cycle around the rRNA gene clusters [1] . Nucleolus is a major nuclear substructure that coordinates many cellular activities including ribosomal production [2] , cell cycle control [3] , DNA damage repair [4] and tRNA processing [5] . In order to get into the nucleus, nucleolar proteins must be targeted to the nucleus through the nuclear pore complex (NPC) [6] . Small molecules can move freely from the cytoplasm through the NPC, but transport of proteins larger than 40 kDa is mediated by specific amino acid sequences, referred as nuclear localization signal (NLS) [7, 8] . A classic NLS contains a cluster of basic amino acids, typically composed of lysines (K) or arginines (R), organized in either a single stretch, the monopartite NLS ((K/R)4-6), or in two stretches, the bipartite NLS, where two small clusters are separated by a few amino acids ((K/R)2X10-12(K/R)3) [9] . Unlike the nucleus, the nucleolus is a membrane-free nuclear structure. Nucleolar localization of proteins is mediated typically by functional interaction with nucleolar core components, such as ribosomal DNA, RNA and proteins, yet in most of the cases depends on nucleolar localization sequences (NoLSs) [10] . These kinds of signals have been found in cellular and in viral nucleolar proteins [11] . NoLSs and NLSs have very similar amino acid compositions (a high prevalence of basic residues in both cases) and in some proteins the same region can both target proteins across the nuclear envelope and cause proteins to accumulate in the nucleolus (for example, UTP20 is reported to contain overlapping NLS and NoLS near its C-terminus) [12] . However, in others proteins the NoLS cause proteins to accumulate in the nucleolus but are unable to mediate nuclear envelop translocation. In these cases the proteins usually also contain a NLS (for instance, PPP1R11 contain two different signals: a NoLS-only signal and a NLS-only signal [8] . Because the nucleolus is not a membranebound structure, it is presumed that nucleolar accumulation occurs via interaction with already-established nucleolar components. Therefore, members of a subset of the nucleolar proteome act as building blocks of the nucleolus around rDNA genes, and it is formed in an incremental manner [13] . Consequently, many nucleolar proteins may accumulate in a steady-state compartment mediated by NoLS (i.e. a sequence or domain) which might interact with local high-affinity binding sites [14] .