New Mechanism Discovered For DNA Recombination And Repair

A biochemistry research team led by Dr. Andrew H.-J. Wang and Dr.
Ting-Fang Wang at the Institute of Biological Chemistry, Academia
Sinica (IBCAS), has
discovered that the RecA family recombinases function as a new type of
rotary motor proteins to repair DNA damages.



Dr. Wangs' team has recently published two structural biology articles
related to RecA family recombinases. One paper is to be published in the
online, open-access journal PLoS ONE on September 12, 2007 and the other
has been already published in the Nucleic Acids Research on Feb. 28, 2007.



Homologous recombination (HR) is a mechanism that repairs damaged DNA with
perfect accuracy, it utilizes the homologous sequence from a partner DNA
as
a template. This process involves the bringing together of 2 DNA
molecules, a search for homologous sequences, and exchange of DNA strands.



RecA family proteins are the central recombinases for HR. The family
includes prokaryotic RecA, archaeal RadA, and eukaryotic Rad51 and Dmc1.
They
have important roles in cell proliferation, genome maintenance, and
genetic diversity, particularly in higher eukaryotes. For example,
Rad51-deficient
vertebrate cells accumulate chromosomal breaks before death. Rad51 and its
meiosis-specific homolog, Dmc1, are also indispensable for meiosis, a
specialized cell cycle for production of gametes. Mammalian Rad51 and Dmc1
proteins are known to interact with tumor suppressor proteins such as
BRCA2.



Since scientists discovered RecA family proteins, it has been assumed that
RecA (and other homologs) forms only 61 right-handed filaments (six
protein
monomers per helical turn), and then hydrolyzes ATP to promote HR and
recombinational DNA repair. Whereas a controversial puzzle came out, how
the
energy of ATP facilitating DNA rotation during the strand exchange
reaction.



By X-ray crystallography and atomic force microscopy approaches, Dr. Wangs'
team provided the answer. They reported that archaeal Sulfolobus
solfataricus RadA proteins can also self-polymerize into a 31 right-handed
filament with 3 monomers per helical turn (reported in PLoS ONE) and a 43
right-handed helical filament with 4 monomers per helical turn (reported
in Nucleic Acids Research).



Additional biophysical and biochemical analyses revealed that RecA family
proteins may couple ATP binding and hydrolysis to the DNA strand exchange
reaction in a manner that promotes clockwise axial rotation of
nucleoprotein filaments. Specially, the 61 RadA helical filament undergoes
clockwise
axial rotation in 2 discrete 120° steps to the 31 extended right-handed
filament and then to the 43 left-handed filament. As a result, all the
DNA-binding motifs (denoted L1, L2 and HhH) in the RadA proteins move
concurrently to mediate DNA binding, homology pairing, and strand
exchange,
respectively. Therefore, the energy of ATP is used to rotate not only DNA
substrates but also the RecA family protein filaments.



This new model is in contrast to all current hypotheses, which overlooks
the fact that RecA family proteins are flexible enough to form both
right-handed and left-handed helical filaments. From this perspective,
these researchers in Taiwan have opened a new avenue for understanding the
molecular mechanisms of RecA family proteins.





Article 1. Citation: Chen L-T, Ko T-P, Chang Y-W, Lin K-A, Wang AH-J, et
al (2007) Structural and Functional Analyses of Five Conserved Positively
Charged Residues in the L1 and N-Terminal DNA Binding Motifs of Archaeal
RadA Protein. PLoS ONE 2(9): e858. doi:10.1371/journal.pone.0000858


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Article 2. "Crystal structure of the left-handed archaeal RadA helical
filament: identification of a functional motif for controlling quaternary
structures and enzymatic functions of RecA family proteins" (Nuclei Acid
Research 2007. 35: 1787-1801).



Nucleic Acid Research (nar.oxfordjournals/)


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