PgmNr C33: Beyond condensation: novel roles for condensin in the polyploid somatic nucleus of Tetrahymena thermophila.

Authors:
R. Howard-Till; J. Loidl


Institutes
University of Vienna, MFPL, Chromosome Biology, Vienna, AT.


Abstract:

In most organisms, condensin proteins play an important role in organizing chromosomes for orderly segregation. This is also true in the ciliate Tetrahymena thermophila, where separate germline and somatic nuclei exhibit markedly different chromosomal organization and modes of division. The germline nucleus has 10 metacentric chromosomes that divide by mitosis and meiosis. The polyploid somatic nucleus divides by amitosis, a poorly understood process that results in random segregation of the chromosomes during a closed division. The Tetrahymena genome encodes one homolog each of the Smc2, Smc4, and CapG subunits of the condensin complex, two homologs of CapD, and four homologs of the kleisin (CapH/Barren). Smc2, Smc4, and CapG are shared between all the condensin complexes in the cell, but the CapD and kleisin subunits show nuclear specific localizations. As expected, condensin is required for condensation and segregation of germline chromosomes. However, previous work has also shown an unexpected requirement for condensin in the somatic nucleus. The somatic nucleus contains approximately 45 copies each of 181 chromosomes ranging in size from ~100 kb to ~2 mb. These chromosomes do not visibly condense prior to division, but condensin is clearly required for normal DNA segregation. Depletion of Smc2, CapD1 or Brn3 subunits results in aberrant divisions in which the nuclear membrane elongates and pinches off, but the chromatin is not distributed. Thus one daughter cell receives the majority of the DNA, while the other has very little. To better understand how condensin contributes to division, we have begun to investigate the organization of chromosomes in the somatic nucleus. FISH labeling of whole somatic chromosomes in wild-type cells shows distinct foci that are distributed fairly evenly throughout the nuclear space. In the absence of condensin, this distribution is disrupted, and the foci are clustered. Two color FISH of two different chromosomes shows that clusters do not overlap, indicating that chromatin is not just more dense in one part of the nucleus. These results indicate that condensin is involved in the organization of chromosomes in the somatic nucleus. Additional results indicate that a specialized condensin may also be involved in the generation of the somatic nucleus after sexual reproduction. Depletion of CapD2 during mating disrupts the process of somatic nuclear development, resulting in the retention of DNA elements that are normally eliminated. Nuclear development arrests without amplification to the normal DNA levels. Work is ongoing to determine the exact role of condensin in these processes, and promises to reveal novel roles for condensin in the generation and maintenance of an unusual polyploid nucleus.