Net Results of Nucleolar Dynamics

The studies recapped here strengthen the view that proteins with no suspected role in ribosome biogenesis or RNA processing can reside in the nucleolus. Their sequestration and the potential exclusion of other proteins may be key for multiple aspects of cell cycle and transcriptional regulation. Such orchestration of retention and exclusion may provide the means for rapid cellular responses that could be independent of direct interaction with transcriptional and translational regulatory circuits. Are there other candidates for such control suggested by the current studies?In the case of the mitotic exit network, clear localization of other genetically implicated proteins will be important (see Figure 1Figure 1). Are they in or out? Is the protein Nan1 dynamically localized, and what might its essential function be? For Cdc14, a nucleolar role had previously been suggested because in cdc14 mutants nucleolar segregation appears aberrant (Granot and Snyder 1991xGranot, D and Snyder, M. Cell Motil. Cytoskel. 1991; 20: 47–54Crossref | PubMedSee all ReferencesGranot and Snyder 1991), but the nature of such a normal nucleolar function is not known. Further, in net1 mutants, the major nucleolar antigen Nop1 appears dispersed (Straight et al. 1999xStraight, A.F, Shou, W, Dowd, G.J, Turck, C.W, Deshaies, R.J, Johnson, A.D, and Moazed, D. Cell. 1999; 97: 245–256Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesStraight et al. 1999). Does this indicate true disruption of nucleolar integrity or simple loss of tethering? In situ hybridization studies of rDNA should answer this question and help determine whether Net1 is a general structural tether of nucleolar proteins. Indeed, it is not yet known whether Net1 binds rDNA directly or is localized to the nucleolus through other protein–protein interactions.Nucleolar targeting and accumulation sequences seem complex and are likely to be combinatorial (Melese and Xue 1995xMelese, T and Xue, Z. Curr. Opin. Cell Biol. 1995; 7: 319–324Crossref | PubMed | Scopus (209)See all ReferencesMelese and Xue 1995). It is noteworthy that among the proteins described here, we observe that Net1 has quite convincing serine/acidic patches that contribute to nucleolar accumulation of other proteins, such as Nsr1 (reviewed in Melese and Xue 1995xMelese, T and Xue, Z. Curr. Opin. Cell Biol. 1995; 7: 319–324Crossref | PubMed | Scopus (209)See all ReferencesMelese and Xue 1995). Net1 is especially closely related to Tof2, a protein identified as a topoisomerase-I interacting factor (Park and Sternglanz 1999xPark, H and Sternglanz, R. Yeast. 1999; 15: 35–41Crossref | PubMed | Scopus (43)See all ReferencesPark and Sternglanz 1999). These proteins share greatest similarity in their N-terminal sequences, but Tof2 also has the telltale serine/acidic patch at its C terminus. Whether Tof2 is nucleolar has not yet been reported, nor has it been determined whether Net1 and Tof2 have redundant or related functions, a question that will be addressed through evaluation of net1 tof2 double mutant phenotypes.net1 mutants have rDNA silencing defects (Straight et al. 1999xStraight, A.F, Shou, W, Dowd, G.J, Turck, C.W, Deshaies, R.J, Johnson, A.D, and Moazed, D. Cell. 1999; 97: 245–256Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesStraight et al. 1999), but whether this effect is solely a function of the loss of Sir2 localization is not yet known. As with Tof2, it will be useful to evaluate net1 sir2 double mutants and explore effects of NET1 on rDNA recombination. Meiotic functions have not been reported for Net1, but it may have a role in pachytene if Sir2 meiotic and mitotic tethering are similar. It is known that cdc14 mutants have meiotic defects. Whether these effects are regulated via nucleolar sequestration and release, and whether Cdc14 may function in meiotic checkpoints should prove interesting indeed. In the PCH screen, the fourth gene uncovered was DOT1/(PCH1), which had previously been identified for its dosage-dependent effects on rDNA, telomeric, and mating-type silencing (Singer et al. 1998xSinger, M.S, Kahana, A, Wolf, A.J, Meisinger, L.L, Peterson, S.E, Goggin, C, Mahowald, M, and Gottschling, D.E. Genetics. 1998; 150: 613–632PubMedSee all ReferencesSinger et al. 1998). Questions focusing on DOT1 may now establish whether its localization and other properties parallel those of PCH2 and SIR2, or what its distinct functions may be.Among yeast silencing proteins, it is worth recalling that a gene family defined by similarity to SIR2 is broadly conserved, spanning prokaryotic and eukaryotic kingdoms (Brachmann et al. 1995xBrachmann, C.B, Sherman, J.M, Devine, S.E, Cameron, E.E, Pillus, L, and Boeke, J.D. Genes Dev. 1995; 9: 2888–2902Crossref | PubMedSee all ReferencesBrachmann et al. 1995). Despite such broad conservation, in no case is a molecular mechanism known for function of Sir2 or any of its homologs. Whether these proteins function enzymatically, structurally, or in some other capacity remains a key question. Better understanding of Sir2’s newly defined interactions with Net1, Cdc14, Pch2, and perhaps Nan1 may provide clues.In recent years, evocative social metaphors have tagged the nucleolus as a ribosome “factory” and telomeres and heterochromatin as “bad neighborhoods” for gene expression. These new studies, demonstrating the convergence of silencing and nucleolar proteins, instead support the idea that the nucleolus and other silenced or heterochromatic regions of the nucleus may in fact be privileged regulatory sites. This point is further developed by considering potential mechanisms of cellular aging (reviewed in Sinclair et al. 1998xSinclair, D.A, Mills, K, and Guarente, L. Trends Biochem. Sci. 1998; 23: 131–134Abstract | Full Text | Full Text PDF | PubMed | Scopus (80)See all ReferencesSinclair et al. 1998) and data about recombination and repair proteins in the context of telomeres (see the minireview by Haber in this issue of Cell). How generally this view extends beyond yeast nuclei and nucleoli is being sorted out in especially interesting directions. Already, some clear parallels in vertebrates have been established for the mitotic exit network. A number of other recent reports also place mammalian growth factors in the nucleolus (reviewed in Pederson 1998bxPederson, T. J. Cell Biol. 1998; 143: 279–281Crossref | PubMed | Scopus (82)See all ReferencesPederson 1998b). Furthermore, nucleolar localization of the tumor suppressor protein p19ARF, with concomitant sequestration of the p53 inhibitor Mdm2, is disrupted by tumor associated mutations and may be key for p53 activation (20xWeber, J.D, Taylor, L.J, Roussel, M.F, Sherr, C.J, and Bar-Sagi, D. Nat. Cell Biol. 1999; 1: 20–26Crossref | PubMedSee all References, 21xZhang, Y and Xiong, Y. Mol. Cell. 1999; 3: 579–591Abstract | Full Text | Full Text PDF | PubMed | Scopus (319)See all References). It thus seems likely that increasing appreciation of the nucleolus and elements of subnuclear localization will lead to even better understanding of biological regulatory mechanisms.