Chemical and Genetic Screens Hit the Target in Cytokinesis

DOI: 10.1371/journal.pbio.0020386

Published October 5, 2004

Copyright: © 2004 Public Library of Science. This is an open-access article
distributed under the terms of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.

Citation: (2004) Chemical and Genetic Screens Hit the Target in Cytokinesis.
PLoS Biol 2(12): e386.

Cytokinesis, in which newly formed daughter cells separate, is the
culmination of the cell cycle. It is necessary for normal growth and
development, and it is also a sine qua non in the pathogenesis of
cancers-cells that can't divide can't form tumors, can't metastasize, and
can't kill. Therefore, understanding the full range of proteins involved in
cytokinesis has both deep theoretical and immediate practical applications.
In this issue, Ulrike Eggert and colleagues report results from two
complementary screening approaches to identify those proteins and to
discover molecules that inhibit them.

A cell cannot divide if it lacks a protein vital for cytokinesis or if that
protein is inhibited. When that occurs, the cell retains both nuclei, and
can be quickly identified by an automated process. Working with Drosophila
cells, the first screen used almost 20,000 double-stranded RNAs,
representing virtually the entire Drosophila genome. A double-stranded RNA
pairs with, and causes the destruction of, a matching messenger RNA, thus
preventing the encoded protein from being formed, a process called RNA
interference (RNAi). The authors identified 214 proteins whose absence
prevented cytokinesis. While some of these, including actin and Myosin, were
already known to be essential for the process, others were not. The latter
included a new discovery, CG4454 (named Borealin-related or Borr), which was
found to be one of the handful of proteins deemed most critical to
cytokinesis.

The second screen also treated Drosophila cells, but this time used over
50,000 "small molecules," a catchall term for molecules small
enough to pass
easily into cells. The vast majority of drugs currently in clinical use are
small molecules. This screen revealed 50 cytokinesis inhibitors, of which
25, dubbed binucleines, were selected for further characterization. Not
surprisingly, several inhibited actin, whose role in cytokinesis is key in
contraction of the cytokinesis furrow.

For the purposes of this study, however, binucleines affecting other
proteins were even more interesting. By comparing the appearances of
binucleate cells from the small-molecule screen with those from the RNAi
screen, Eggert et al. identified one molecule and three proteins that caused
a similar phenotype, suggesting that the three proteins acted within a
single pathway, which the molecule could disrupt. One of the three proteins
was CG4454/Borr, and the researchers' results indicated it interacts with
Aurora B, an essential but still poorly understood protein that is needed
for proper division of the chromosomes. The identified binucleine will be a
valuable reagent for exploring the details of the Aurora B pathway.

While these results are from Drosophila, the insights they provide into the
cell cycle are likely to be applicable to humans as well. Equally
importantly, they provide the proof of principle for a new drug discovery
method. A major bottleneck in drug development is target
identification-determining which of the cell's thousands of proteins is the
right one to inhibit with a drug. The unique aspect of this study is the
parallel use of the two approaches-small molecules and RNAi-to provide a
"stereoscopic" view of cytokinesis and its inhibition. Working together,
they provide a set of proteins and a matched set of inhibitors, the target
and the bullet at the same time.

>From PLoS Biology
http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371/journal.pbio.0020386

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