[
International Worm Meeting,
2005]
Chromosome condensation, the mechanism that transforms chromosomes from stringy masses into compact rod-shaped structures, is essential for the accurate segregation of the genetic material. We are studying how the conserved protein complex condensin condenses and segregates chromosomes. In previous work we identified the C. elegans mitotic condensin complex and showed that it is distinct from a second condensin-like complex that regulates X-linked gene expression. Condensin localized to the centromere and was required for centromere organization. Upon depletion of condensin, chromosomes were poorly condensed at prometaphase and failed to segregate at anaphase, but nevertheless attained a degree of compaction during metaphase. This raised the possibility that condensin function is integrated with additional factors that promote complete chromosome condensation. To identify additional condensin subunits and condensin-interacting proteins, immunoprecipitations (IPs) were performed with antibodies against the condensin subunit SMC-4. Sensitive mass spectrometry methods were applied to identify interacting proteins. A control IP against the kinetochore protein CeMCAK was performed, and proteins common to both IPs were eliminated as suspected contaminants. The remainder was tested for function in vivo by RNA interference (RNAi) in a strain carrying GFP-labeled chromosomes and microtubules. Time-lapse microscopy was used to assess the consequence of depleting each protein. Three interesting classes of proteins interact with SMC-4. First, we identified F55C5.4 and C29E4.2 and provide evidence that these are additional condensin subunits based on their interaction with SMC-4, their phenotype, and their immuno-localization. Second, we uncovered regulators of phosphorylation, ubiquitination, and nuclear import and are currently addressing whether these represent cell cycle regulators of condensin. Finally, we identified uncharacterized proteins with essential functions in chromosome and spindle dynamics during meiosis and mitosis. Thus, by combining sensitive biochemical methods with in vivo functional criteria, we have uncovered factors that may help us understand mechanisms of chromosome segregation.