A role for macromolecular crowding effects in the assembly and function of compartments in the nucleus.
The mechanisms which cause macromolecules to form discrete compartments within the nucleus are not understood. Here, two ubiquitous compartments, nucleoli, and PML bodies, are shown to disassemble when K562 cell nuclei expand in medium of low monovalent cation concentration; their major proteins dispersed as seen by immunofluorescence and immunoelectron microscopy, and nucleolar transcript elongation fell by approximately 85%. These compartments reassembled and nucleolar transcription recovered in the same medium after adding inert, penetrating macromolecules (8 kDa polyethylene glycol (PEG), or 10.5 kDa dextran) to 12% w/v, showing that disassembly was not caused by the low cation concentration. These responses satisfy the criteria for crowding or volume exclusion effects which occur in concentrated mixtures of macromolecules; upon expansion the macromolecular concentration within the nucleus falls, and can be restored by PEG or dextran. These observations, together with evidence of a high concentration of macromolecules in the nucleus (in the range of 100mg/ml) which must cause strong crowding forces, suggest strongly that these forces play an essential role in driving the formation, and maintaining the function of nuclear compartments. This view is consistent with their dynamic and mobile nature and can provide interpretations of several unexplained observations in nuclear biology.
Hancock
Ronald
R
Laval University Cancer Research Centre, Hôtel-Dieu Hospital, 9 rue MacMahon, Que., P.Q., Canada G1R 2J6. ronald.hancock@crhdq.ulaval.ca
D002021:Buffers; D002466:Cell Nucleolus / Q000378:metabolism*; D002467:Cell Nucleus / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure*; D003911:Dextrans / Q000494:pharmacology; D006801:Humans; D042581:Intranuclear Space / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D020014:K562 Cells; D046911:Macromolecular Substances / Q000378:metabolism*; D008854:Microscopy, Electron; D009363:Neoplasm Proteins / Q000378:metabolism*; D009687:Nuclear Proteins / Q000378:metabolism*; D011092:Polyethylene Glycols / Q000494:pharmacology; D000071156:Promyelocytic Leukemia Protein; D014157:Transcription Factors / Q000378:metabolism*; D025521:Tumor Suppressor Proteins
D002021:Buffers; D003911:Dextrans; D046911:Macromolecular Substances; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D000071156:Promyelocytic Leukemia Protein; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; C070576:PML protein, human; D011092:Polyethylene Glycols