Biology Reference
In-Depth Information
Holliday junction as a dimer and unfolds the antiparallel stacked-X structure
(observed in the presence of divalent cation Mg
รพรพ
) into an open-planar struc-
ture (observed in the absence of divalent ions) and nicks strands of the
same polarity.
430-436
RuvC activity in bacterial cells are closely associated with
the RuvA, the tetrameric protein that binds to Holliday junctions and unfolds it
into an open-planar structure and RuvB, an hexameric ATP hydrolysis-driven
translocase which promotes branch migration.
89,437-441
Holliday junction resolvase activity in mammalian systems was first ob-
served in extracts prepared from homogenized calf thymus tissues.
442
Similar
resolvase activity was later observed in extracts prepared from cultured
cells.
443,444
Given the complexity of eukaryotic genomes and the stringency of
maintaining its integrity, eukaryotic cells have evolved multiple pathways and
resolvases to process Holliday junctions.
445
This in turn made identification of
single mutants defective in Holliday junction resolution challenging in
eukaryotic model organisms.
446
In 2001, the Mus81-Eme1 (Mus81-Mms4 in
budding yeast) heterodimer was identified in fission yeast as an endonuclease
capable of cleaving Holliday junctions as well as branched DNA structures
(
Table I
; Refs.
447-449
). Mus81 is homologous to the XPF subunit of
the ERCC1-XPF nucleotide excision repair endonuclease.
448,449
Eme1 is a
noncatalytic subunit.
420,448,449
Mus81-Eme1 depletion caused some meiotic
defects and stalled replication forks.
448,449
Meiotic defective cells could be
rescued by ectopic expression of bacterial Holliday junction resolvases.
449
In
2003, human MUS81-EME1 was characterized as a replication fork/flap en-
donuclease that is essential to maintain the integrity of replication, even though
it possessed inefficient Holliday junction resolvases
in vitro
.
450
In the case of
budding yeast,
Mus81
deletion only exhibited modest decrease in crossover
formation during meiosis, implying that Mus81-Eme1 is not the sole resolvase
complex of Holliday junctions.
451,452
Similar minor meiotic defects were
observed in
MUS81
knockout studies in mice.
453,454
Search for a RuvC-type
Holliday junction resolvase in eukaryotic cells had been quite challenging by
conventional sequence homology-based queries due to the absence of conser-
vation of primary amino acid sequences.
421
However, tertiary structure-level
conservation was seen among most of the identified Holliday junction
resolvases, categorizing them into two main superfamilies of integrase and
nuclease.
421
Identification of a Holliday junction resolvase in eukaryotes that
resembled bacterial RuvC was inadvertently assigned to the RAD51C-XRCC3
heterodimer, which was isolated from mammalian cells.
446,455
However, the
absence of an apparent nuclease domain and the inability of recombinant
RAD51C-XRCC3 to recapitulate the Holliday junction resolvase activity
led to questions regarding the assignment of the Holliday junction resolvase
to the RAD51C-XRCC3 heterodimer.
420,456
In 2008, after a tedious effort by
the West laboratory (the group that suggested that RAD51C-XRCC3 was a
