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Thursday, 30 September 2004
A New Groove for the Helix-Turn-Helix Motif...summary written by Raven Hanna
Douglas S. Daniels, Tammy T. Woo, John A. Tainer
Error-free DNA replication depends on the maintenance of the correct chemical
structure of each component base. Bases with altered structures may mispair
during the replication process, causing mutations. One common chemical
alteration is the addition of an alkyl group to guanine, causing mispairing to
thiamine during replication. The resulting G-C to A-T base pair mutation can be
prevented if the protein alkylguanine-DNA alkyltransferase (AGT) locates and
removes the alkyl group prior to replication. While AGT is a useful component
of healthy cells, elevated levels in cancer cells can cause resistance to
chemotherapy treatments that include alkylating agents. Knowledge of the
structure and mechanism of AGT may lead to the development of inhibitors that
could prevent cancer cells from acquiring resistance.
The crystal structures of AGT bound to alkylated DNA fragments, both pre- and
post- catalysis, were solved by scientists collaborating from The Scripps
Research Institute, Pennsylvania State University College of Medicine, and
Johns Hopkins University. To lock down the pre-catalytic form, they used an
inactive protein with a native substrate. To acquire the post-catalytic form,
crystals were grown of the active protein crosslinked to the DNA through an
alkylated guanine analog. In the resulting structures, the protein rotates the
phosphates of the DNA backbone to cause a rotation of the alkylated base into
the protein's active site. The researchers note their surprise that the protein
accesses the DNA through the minor groove, since all previous examples of
similar proteins show binding to the more easily accessible and
information-rich major groove.
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