Human DNA Ligase
IV-Artemis Fragment Complex
Sean Smith '16 and Samantha White '16
DNA ligase IV (LigIV)
is an enzyme that helps to repair double strand breaks
(DSBs) through nonhomologous end joining (NHEJ). This is
accomplished when DNA protein kinase (DNA-PK) binds to the ends of
the DSBs and brings them together. The DNA-PK catalytic subunit
(DNA-PKcs) then recruits the protein Artemis
and this enzyme, along with other proteins, processes the
ends of the DNA to be joined. The NHEJ ligase complex is then
recruited, and LigIV, XRCC4 and XLF-Cernunnos work to connect the
two DNA strands.
II. General Structure
Human LigIV has a catalytic region that includes a
DNA binding domain (DBD), a nucleotidyltransferase
domain (NTD), and an OB-fold domain
It also has repeats of a BRCT domain at the carboxy terminus
which interact with XRCC4. In comparison to other human DNA ligases,
there are unique inserts between
alpha chain 5 and alpha chain 6 of the
and between alpha chain 15 and alpha
chain 16 of the NTD
When LigIV is not in complex with Artemis, the conformation
of the loop between beta sheet 12 and beta
sheet 13 is altered.
III. Artemis-LigIV Interactions
It has been found that LigIV makes contact with Artemis
through its first
and second alpha helices, found in
the DBD domain.
This interaction buries 436 Å2 of the surfaces of the two
proteins. A kink in
alpha helix 1
exists due to the proline P15
, and this distortion produces a hydrophobic pocket in which
the tryptophan W489 from Artemis
associates (residues from Artemis italicized).
In this pocket, W489 engages
in van der Waals interactions with V14
This hydrophobic interaction is stabilized by the
interactions between F492 of Artemis
and F49 of LigIV, and the
interactions between F493 of Artemis
and F42 of LigIV.
The Nsigma of W489
also makes a hydrogen bond with the side chain of D18
It was found that when W489,
F492 or F493
of Artemis was mutated to alanine, the LigIV-Artemis complex could
no longer form.
IV. LIG 4 Syndrome Mutations
LIG4 syndrome is a hereditary disorder that
results from the disruption of normal NHEJ due to mutations in
the LigIV enzyme. This disorder is characterized by
microcephaly, growth and developmental delay, and
pancytopenia, among other conditions. One of the residues
whose mutation causes LIG4 syndrome is T9.
This residue interacts in a polar way with S12
and when it is mutated, the flexibility of some of the
residues in the DBD may be increased and DNA binding may be
inhibited. This syndrome also occurs when the M249
residue in the NTD region is mutated.
M249 stabilizes W447,
which interacts with K432,
which are part of the catalytic pocket. Therefore it is
probable that the mutation of M249
leads to destabilization of the catalytic pocket due to the
decreased rigidity of W447 and K432. The residue G469
interacts with two tryptophan side chains, W471
When this guanine is mutated, the structure of the OBD
region may change, and its adenylation and ligation functions
may be inhibited.
Ochi, T., Gu, X., and Blundell, T. (2013).
Structure of the catalytic region of DNA Ligase IV in complex
with an artemis fragment sheds light on double-strand break
repair. Structure, 21(4):672-679.
Malu, S., De Ioannes, P., Kozlov,
M., Greene, M., Francis, D., Hanna, M., Pena, J.,
Escalante, C. R., Kurosawa, A., Erdjument-Bromage, H.,
Tempst, P., Adachi, N., Vezzoni, P., Villa, A.,
Aggarwal, A. K., and Cortes, P. (2012). Artemis
C-terminal region facilitates V(D)J recombination
through its interactions with DNA Ligase IV and
DNA-PKcs. JEM, 209(5):955-963.
(July 2012). LIG4 syndrome.
Orphanet. [Online]. Available:
[2013, December 9].
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