Assembly and disassembly of the nucleolus during the cell cycle

Abstract

The nucleolus is a large nuclear domain in which transcription, maturation and assembly of ribosomes take place. In higher eukaryotes, nucleolar organization in three sub-domains reflects the compartmentation of the machineries related to active or inactive transcription of the ribosomal DNA, ribosomal RNA processing and assembly with ribosomal proteins of the two (40S and 60S) ribosomal subunits. The assembly of the nucleoli during telophase/early G(1) depends on pre-existing machineries inactivated during prophase (the transcription machinery and RNP processing complexes) and on partially processed 45S rRNAs inherited throughout mitosis. In telophase, the 45S rRNAs nucleate the prenucleolar bodies and order the dynamics of nucleolar assembly. The assembly/disassembly processes of the nucleolus depend on the equilibrium between phosphorylation/dephosphorylation of the transcription machinery and on the RNP processing complexes under the control of the CDK1-cyclin B kinase and PP1 phosphatases. The dynamics of assembly/disassembly of the nucleolus is time and space regulated.

Keywords: CDK1/PP1; PNBs; cell cycle; nucleolar assembly; nucleolus; rRNA processing; traffic in living cells.

Figure 1

Schematic illustration of nucleolar disassembly at the beginning of mitosis. In the G₂ phase of the cell cycle, the nucleolus is composed of 3 structures corresponding to different steps of ribosome biogenesis: (1) the fibrillar center or FC (white octagon) were the ribosomal genes (rDNAs) are localized, (2) the dense fibrillar component or DFC (D in green) corresponding to transcription of the 47S rRNAs and the early steps of the rRNA processing generating the small ribosome subunits and (3) the granular component or GC (G in red) corresponding to late steps of rRNA processing generating the large ribosome subunits. The nuclear envelope (double black rings) is at the periphery of the nucleus surrounding the chromatin (blue). At the beginning of prophase (prophase a), rDNA transcription is still active (white octagon) and the nucleolar processing proteins from the D and G components leave the nucleolus (processing disruption). At the end of prophase (prophase b), rDNA transcription is extinct (grey octagon), the nucleolar processing proteins from the D and G components are dispersed around condensing chromosomes (not shown) and the nuclear envelope is disrupted. In Hela cells the total duration of the prophase is of ≈30 min.

Figure 2

Schematic illustration of nucleolar assembly at the end of mitosis. In telophase transcription of the rDNAs is activated (white octagons) in several NORs (nucleolar organizing regions), whereas the early and late nucleolar processing complexes are located in the prenucleolar bodies (PNBs), consequently the PNBs containing both the D and G processing complexes (respectively D: green and G: red) appeared as yellow structures. The assembly of the nuclear envelope around chromatin is not complete. In early-G₁ a, transcription and early processing take place and generate the DFC (D: green), the late processing complexes are in PNBs (red), and the nuclear envelope is assembled at the periphery of the nucleus (double black rings). In early-G₁ b, the late processing complexes are recruited around the DFC (D) forming the granular component (G: red) and the incipient nucleoli form a single nucleolus. In early-G₁ b, a few PNBs are still present and cytokinesis is not complete (not shown). In Hela cells the duration of the telophase is ≈ 10 min, early-G₁ a is ≈20 min and early-G₁ b is ≈90 min.

Figure 3

Schematic illustration of the traffic of nucleolar processing proteins during nucleolar assembly in incipient nucleoli. The traffic of processing proteins tagged with photoactivatable green fluorescent proteins (PAGFP) activated by laser (PA) was analyzed using fibrillarin for DFC marker and using PAGFP-Nop52 for GC marker. The pool of PAGFP-fibrillarin activated in one incipient nucleolus (green) migrates to the other incipient nucleoli but not to PNBs. The pool of PAGFP-Nop52 activated in one incipient nucleolus (green) migrates to the other incipient nucleoli and also to PNBs. This explains the progressive formation of GC controlled by PNBs as described in reference .